Tissue is a system of cells and intercellular substance, united by the unity of structure, function and origin. There are 4 types of tissues in the human body: epithelial, connective, muscle, and nervous. Tissues consist of cells and intercellular substance, the ratio of which is different. The intercellular substance is usually gel-like and may contain fibers.
epithelial tissue (Fig. 2.2) It is represented by epithelial cells, forming continuous layers in which there are no vessels. The nutrition of the epithelium occurs by diffusion of nutrients through the supporting basement membrane that separates the epithelium from the underlying loose connective tissue.
The integumentary epithelium is single-layered (squamous, cuboidal, multi-row ciliated, cylindrical) and multi-layered (keratinizing, non-keratinizing, transitional).
A single layer of squamous epithelium lines the serous membranes, the alveoli of the lungs. In the chambers of the heart, blood vessels, it reduces the friction of flowing fluids and is called the endothelium. Multi-row ciliated epithelium covers the mucous membranes of the respiratory tract, fallopian tubes and consists of ciliary and goblet mucous cells, the nuclei of which are located at different levels. Cilia are outgrowths of the cytoplasm at the free end of the columnar cells of this epithelium. They constantly fluctuate, preventing any foreign particles from entering the lungs, promoting the egg in the fallopian tubes. The cuboidal epithelium is found in the collecting ducts of the kidneys and lines the ducts of the pancreas. The cylindrical epithelium is represented by tall narrow cells with the functions of secretion and absorption. Sometimes on the free surface of the cells there is a brush border, consisting of microvilli that increase the absorption surface (in the small intestine). Goblet cells located between cylindrical epithelial cells secrete mucus that protects the gastric mucosa from the harmful effects of gastric juice and facilitates the passage of food in the intestine.
The glandular epithelium forms glands (sweat, sebaceous, etc.) that perform the function of excretion. Glands are multicellular (liver, pituitary gland) and unicellular (goblet cell of the ciliated epithelium that secretes mucus). Exocrine glands are located in the skin or hollow organs. They usually have excretory ducts and bring the secret out either (sweat, sebum, milk) or into the organ cavity (bronchial mucus, saliva). Their secrets have a local impact. Exocrine glands are divided into simple and complex depending on whether their excretory duct branches or not. Endocrine glands do not have excretory ducts; they secrete their hormones (adrenaline, etc.) into the blood and lymph, affecting the entire body.
Stratified epithelium consists of several rows of cells. Only the bottom layer of cells is located on the basement membrane. The epidermis (stratified squamous keratinized epithelium) covers the skin. Its lower layer is represented by germ cells, among which are melanocyte pigment cells with the black pigment melanin, which gives color to the skin. The mucous membranes are lined with stratified squamous non-keratinized epithelium (oral cavity, pharynx, esophagus, etc.). The transitional epithelium can have a different number of layers depending on the degree of filling of the organ with urine (urinary tract).
Connective tissue makes up 50% of body weight, is diverse in structure and function, and is widely distributed in the body.
The connective tissue itself forms the stroma and capsules of the internal organs, is located in the skin, ligaments, tendons, fascia, vascular walls, sheaths of muscles and nerves. In the body, this tissue performs plastic, protective, supporting and trophic functions. It consists of cells and intercellular substance containing fibers and ground substance. The main cell - a mobile fibroblast - forms the main substance and secretes fibers: collagen, elastic, reticulin. There are proper connective tissue, cartilage and bone.
The connective tissue itself is represented by loose and dense fibrous connective tissue with the functions of musculoskeletal, protective (dense fibrous connective tissue, cartilage, bone). The trophic (nutritional) function is performed by loose fibrous and reticular connective tissue, blood and lymph.
Loose fibrous connective tissue (Fig. 2.3.) contains fibroblasts, fibrocytes, and other cells and fibers, differently located in the ground substance, depending on the structure and function of the organ. This tissue makes up the stroma of parenchymal organs, accompanies blood vessels, participates in immune, inflammatory reactions, and wound healing.
Dense fibrous connective tissue can be unformed and formed, depending on the ordering of its fibers. In the reticular layer of the skin, connective tissue fibers are randomly intertwined. In tendons, ligaments, fascia, these fibers form bundles located in a certain direction and give strength to these formations. (fig.2.4).
Reticular connective tissue, consisting of reticular cells and fibers, forms the basis of hematopoietic and immune organs (red bone marrow, lymph nodes and follicles, spleen, thymus). Its main cell is a multipronged reticulocyte that secretes thin reticulin fibers. The processes of cells are connected to each other to form a network, in the loops of which hematopoietic cells and blood cells are located.
Adipose connective tissue forms a subcutaneous fat layer, located under the peritoneum, in the omentums. Its cells - spherical lipocytes - accumulate fat droplets. Adipose tissue is a depot of the most important energy source of fat and associated water, has good thermal insulation properties.
Cartilage tissue consists of chondrocytes, forming groups of two or three cells, and the main substance is a dense, elastic gel. The cartilage does not have vessels, nutrition is carried out from the capillaries of the perichondrium covering it. There are three types of cartilage. Hyaline cartilage is translucent, smooth, dense, shiny. It contains almost no fibers, forms articular, costal cartilages, cartilages of the larynx, trachea, bronchi. Fibrous (fibrous) cartilage has many strong collagen fibers and forms fibrous rings of intervertebral discs, intraarticular discs, menisci, and pubic symphysis. Elastic cartilage is yellowish, contains many helical elastic fibers that cause elasticity. It consists of some cartilages of the larynx, auricle, etc.
Bone tissue is hard and strong, forms a skeleton. It consists of mature multi-pronged cells - osteocytes, young ones - osteoblasts, embedded in a solid intercellular substance containing mineral salts. When bone is damaged, osteoblasts are involved in regeneration processes. The third type of bone tissue cells - multinuclear osteoclasts are able to phagocytize (absorb) the intercellular substance of bone and cartilage tissue in the process of bone growth and remodeling.
Muscle tissue has excitability, conductivity and contractility. The main cell is the myocyte. There are three types of muscle tissue (Fig. 2.5). Striated skeletal muscle tissue forms skeletal muscles and some internal organs (tongue, pharynx, larynx, etc.). The striated cardiac muscle tissue forms the heart. Smooth muscle tissue is located in the eyeball, the walls of blood vessels and hollow internal organs (in the stomach, intestines, trachea, bronchi, etc.).
Skeletal muscle tissue consists of multinuclear, transversely striated muscle fibers up to 4-10 cm long, the sheath of which is similar in electrical properties to the membrane of nerve cells. The fibers contain special contractile organelles, myofibrils are longitudinal filaments that can shorten when excited. Myofibrils are formed by contractile proteins - actin and myosin with different light-refracting and physico-chemical properties, which causes the alternation of dark and light transverse stripes (discs) during microscopy of this muscle tissue. The cytoplasm of the muscle fiber contains the endoplasmic reticulum. Its membranes are associated with the cell membrane and actively transport Ca + from the cytoplasm to the tubules of the endoplasmic reticulum. Skeletal muscle under short-term loads covers its energy needs through both aerobic and anaerobic oxidation. Skeletal muscle contraction is fast, consciously controlled, and regulated by the somatic nervous system.
Cardiac muscle tissue, the myocardium, consists of cells - transversely striated cardiomyocytes, which, with the help of intercalated discs, are connected into a functionally unified network. Excitation that occurs in any part of the heart extends to all muscle fibers of the myocardium. The myocardium is extremely sensitive to a lack of oxygen: it covers its energy needs only through aerobic oxidation. The myocardium contracts involuntarily and is regulated by the autonomic nervous system.
Smooth muscle tissue consists of thin single-nuclear, striated, spindle-shaped myocytes up to 0.5 cm long, collected in bundles or layers. Their actin and myosin filaments are randomly arranged without forming myofibrils. The contraction of smooth muscle tissue occurs slowly (except for the muscles that regulate the width of the pupil), involuntarily and is controlled by the autonomic nervous system.
Nervous tissue consists of nerve cells - neurons and neuroglia. Neurons produce nerve impulses, neurohormones and neurotransmitters. Neurons and neuroglia form a single nervous system that regulates the relationship of the body with the external environment, coordinating the functions of internal organs and ensuring the integrity of the body.
The neuron has a body, processes and end devices. By the number of processes, neurons with one, two and several processes are distinguished (unipolar, bipolar and multipolar - the latter prevail in humans). Short branching processes - dendrites - a neuron can have up to 15. They connect neurons to each other, transmitting nerve impulses. Along a single long (up to 1.5 m), thin, non-branching process - an axon - a nerve impulse travels from the body of a neuron to a muscle, gland or other neuron (fig.2.6)
Nerve fibers end in terminal apparatus - nerve endings. Axons end on muscles and glands with effectors - motor nerve endings. Receptors are sensitive nerve endings. In response to irritation, an excitation process occurs in the receptors, which is recorded as a very weak alternating electric current (nerve impulses, biocurrents). Information about the stimulus is encoded in nerve impulses. Synapses are contacts between nerve cells and their processes. The transfer of excitation in synapses and effectors occurs with the help of biologically active substances - mediators (aceticholine, norepinephrine, etc.).
Neurons do not divide by mitosis under normal conditions. Restorative functions belong to neuroglia. Neuroglial cells line the cavities of the brain and spinal cord (ventricles, canals), serve as a support for neurons, surrounding their bodies and processes, carry out phagocytosis and metabolism, and secrete some mediators.
Histology refers to the morphological sciences. Unlike anatomy, which studies the structure of organs at the macroscopic level, histology studies the structure of organs and tissues at the microscopic and electron microscopic levels. At the same time, the approach to the study of various elements is made taking into account the function they perform. This method of studying the structures of living matter is called histophysiological, and histology is often referred to as histophysiology. When studying living matter at the cellular, tissue and organ levels, not only the shape, size and location of the structures of interest are considered, but the chemical composition of the substances that form these structures is determined by the methods of cyto- and histochemistry. The studied structures are also considered taking into account their development both in the prenatal period and during the initial ontogenesis. It is with this that the need to include embryology in histology is connected.
The main object of histology in the system of medical education is the body of a healthy person, and therefore this academic discipline is referred to as human histology. The main task of histology as an academic subject is the presentation of knowledge about the microscopic and ultramicroscopic (electron-microscopic) structure of cells, tissues of organs and systems of a healthy person in close connection with their development and functions. This is necessary for further study of human physiology, pathological anatomy, pathological physiology and pharmacology. Knowledge of these disciplines shapes clinical thinking. The task of histology as a science is to elucidate the patterns of structure of various tissues and organs in order to understand the physiological processes occurring in them and the possibility of controlling these processes.
Tissue is a historically established system of cells and non-cellular structures that has a common structure, and often origin, and specializes in performing certain functions. Tissues are formed from germ layers. This process is called histogenesis. The tissue is formed from stem cells. These are pluripotent cells with great potential. They are resistant to harmful environmental factors. Stem cells can become semi-stem cells and even multiply (proliferate). Proliferation - an increase in the number of cells and an increase in tissue in volume. These cells are able to differentiate, i.e. acquire the property of mature cells. Only mature cells perform a specialized function, thus. cells in a tissue are characterized by specialization.
The rate of cell development is genetically predetermined; tissue is determined. Cell specialization must occur in the microenvironment. Differon is a collection of all cells developed from a single stem cell. Tissues are characterized by regeneration. It is of two types: physiological and reparative.
Physiological regeneration is carried out by two mechanisms. Cellular proceeds by dividing stem cells. In this way, ancient tissues are regenerated - epithelial, connective. Intracellular is based on increased intracellular metabolism, as a result of which the intracellular matrix is restored. With further intracellular hypertrophy, hyperplasia (increase in the number of organelles) and hypertrophy (increase in cell volume) occur. Reparative regeneration is the restoration of a cell after damage. It is carried out by the same methods as the physiological one, but in contrast it proceeds several times faster.
Fabric classificationFrom the position of phylogenesis, it is assumed that in the process of evolution of organisms, both invertebrates and vertebrates, 4 tissue systems are formed that provide the main functions of the body: integumentary, delimiting from the external environment; internal environment - supporting homeostasis; muscular - responsible for movement, and nervous - for reactivity and irritability. The explanation for this phenomenon was given by A.A. Zavarzin and N.G. Khlopin, who laid the foundations for the theory of evolutionary and ontogenetic determination of tissues. Thus, the position was put forward that tissues are formed in connection with the main functions that ensure the existence of the organism in the external environment. Therefore, tissue changes in evolution follow parallel paths (A.A. Zavarzin’s theory of parallelisms).
However, the divergent path of evolution of organisms leads to the emergence of an increasing variety of tissues (the theory of divergent evolution of tissues by N.G. Khlopin). It follows from this that tissues in phylogeny develop both in parallel rows and divergently. Divergent differentiation of cells in each of the four tissue systems eventually led to a wide variety of tissue types, which histologists subsequently began to combine into systems or groups of tissues. However, it became clear that in the course of divergent evolution, tissue can develop not from one, but from several sources. Isolation of the main source of tissue development, giving rise to the leading cell type in its composition, creates opportunities for classifying tissues according to a genetic trait, and the unity of structure and function - according to morphophysiological. However, it does not follow from this that it was possible to construct a perfect classification that would be universally recognized.
Most histologists in their work rely on the morphofunctional classification of A.A. Zavarzin, combining it with the genetic system of N.G. Khlopin. The well-known classification of A.A. Klishova (1984) postulated the evolutionary determination of four tissue systems developing in animals of different types in parallel rows, together with the organ-specific determination of specific types of tissues that form divergently in ontogenesis. The author identifies 34 tissues in the epithelial tissue system, 21 tissues in the blood system, connective and skeletal tissues, 4 tissues in the muscle tissue system, and 4 tissues in the nervous and neuroglial tissue system. This classification includes almost all specific human tissues.
As a general scheme, a variant of the classification of tissues according to the morphophysiological principle (horizontal arrangement) is given, taking into account the source of development of the leading cellular differon of a particular tissue (vertical arrangement). Here, ideas about the germ layer, embryonic germ, tissue type of most known tissues of vertebrates are given in accordance with the ideas about four tissue systems. The above classification does not reflect the tissues of extra-embryonic organs, which have a number of features. Thus, the hierarchical relationships of living systems in an organism are extremely complex. Cells, as first-order systems, form differons. The latter form tissues as mosaic structures or are the only differon of a given tissue. In the case of a polydifferential tissue structure, it is necessary to identify the leading (main) cellular differon, which largely determines the morphophysiological and reactive properties of the tissue.
Tissues form systems of the next order - organs. They also highlight the leading tissue that provides the main functions of this organ. The architectonics of an organ is determined by its morphofunctional units and histions. Organ systems are formations that include all lower levels with their own laws of development, interaction and functioning. All the listed structural components of the living are in close relationship, the boundaries are conditional, the underlying level is a part of the overlying one, and so on, making up the corresponding integral systems, the highest form of organization of which is the body of animals and humans.
epithelial tissues. Epithelium
Epithelial tissues are the oldest histological structures that appear first in phylo- and ontogenesis. The main property of the epithelium is borderline. Epithelial tissues (from the Greek epi - over and thele - skin) are located at the boundaries of two environments, separating the body or organs from the environment. Epithelia, as a rule, have the form of cell layers and form the outer cover of the body, the lining of the serous membranes, the lumens of organs that communicate with the external environment in adulthood or in embryogenesis. Through the epithelium, the exchange of substances between the body and the environment is carried out. An important function of epithelial tissues is to protect the underlying tissues of the body from mechanical, physical, chemical and other damaging effects. Some epithelia are specialized in the production of specific substances - regulators of the activity of other body tissues. Derivatives of integumentary epithelium are glandular epithelium.
A special type of epithelium is the epithelium of the sense organs. Epithelia develop from the 3rd-4th week of human embryogenesis from the material of all germ layers. Some epithelia, such as the epidermis, are formed as polydifferential tissues, since they include cellular differons that develop from different embryonic sources (Langerhans cells, melanocytes, etc.). In the classifications of the epithelium by origin, as a rule, the source of development of the leading cellular differon, the differon of epithelial cells, is taken as the basis. Cytochemical markers of epitheliocytes are proteins - cytokeratins, forming tonofilaments. Cytokeratins are characterized by great diversity and serve as a diagnostic marker for a specific type of epithelium.
There are ectodermal, endodermal and mesodermal epithelium. Depending on the embryonic rudiment, which serves as a source of development of the leading cellular differon, epithelia are divided into types: epidermal, enterodermal, whole nephrodermal, ependymoglial and angiodermal. According to the histological features of the structure of the leading (epithelial) cell differon, single-layer and multilayer epithelia are distinguished. Monolayer epithelium in the form of their constituent cells are flat, cubic, prismatic or cylindrical. Single-layer epithelium is divided into single-row, if the nuclei of all cells lie at the same level, and multi-row, in which the nuclei are located at different levels, that is, in several rows.
Stratified epithelium is divided into keratinized and non-keratinized. Stratified epithelium is called squamous, given the shape of the cells of the outer layer. The cells of the basal and other layers may have a cylindrical or irregular shape. In addition to those mentioned, there is also a transitional epithelium, the structure of which varies depending on the degree of its stretching. Based on data on organ-specific determination, the epithelium is divided into the following types: skin, intestinal, renal, coelomic, and neuroglial. Within each type, several types of epithelium are distinguished, taking into account their structure and functions. The epithelia of the listed types are firmly determined. However, in pathology, it is possible to transform one type of epithelium into another, but only within one tissue type. For example, among dermal type epithelium, the stratified ciliated epithelium of the airways can become stratified squamous. This phenomenon is called metaplasia. Despite the diversity of structure, functions performed and origin from different sources, all epithelia have a number of common features, on the basis of which they are combined into a system or group of epithelial tissues. These general morphofunctional features of the epithelium are as follows.
Most epithelia in their cytoarchitectonics are single-layer or multi-layer layers of tightly closed cells. Cells are connected by intercellular contacts. The epithelium is in close interaction with the underlying connective tissue. At the border between these tissues there is a basement membrane (plate). This structure is involved in the formation of epithelial-connective tissue relationships, performs the functions of attachment with the help of epithelial cell hemidesmosomes, trophic and barrier. The thickness of the basement membrane usually does not exceed 1 micron. Although in some organs its thickness increases significantly. Electron-microscopically, light (located closer to the epithelium) and dark plates are isolated in the membrane. The latter contains type IV collagen, which provides the mechanical properties of the membrane. With the help of adhesive proteins - fibronectin and laminin, epitheliocytes are attached to the membrane.
The epithelium is nourished through the basement membrane by diffusion of substances. The basement membrane is considered as a barrier to the growth of the epithelium in depth. With tumor growths of the epithelium, it is destroyed, which allows the altered cancer cells to grow into the underlying connective tissue. Epithelial cells are heteropolar. The structure of the apical and basal parts of the cell is different. In multilayer layers, cells of different layers differ from each other in structure and function. This is called vertical anisomorphy. Epithelia have a high ability to regenerate due to mitoses of cambial cells. Depending on the location of cambial cells in epithelial tissues, diffuse and localized cambium are distinguished.
Multilayer fabrics
Thick, function - protective. All stratified epithelia are of ectodermal origin. They form integuments of the skin (epidermis) lining the mucous membrane of the oral cavity, esophagus, final section of the rectum, vagina, urinary tract. Due to the fact that these epitheliums are more in contact with the external environment, the cells are arranged in several floors, therefore these epitheliums perform a protective function to a greater extent. If the load increases, then the epithelium undergoes keratinization.
Stratified squamous keratinizing. Skin epidermis (thick - 5 layers and thin) In thick skin, the epidermis contains 5 layers (soles, palms). The basal layer is represented by stem basal and pigment cells (10 to 1), which produce melanin grains, they accumulate in the cells, the excess is secreted, absorbed by the basal, spiny cells and penetrates into the dermis through the basement membrane. In the spinous layer, epidermal macrophages, memory T-lymphocytes are in motion, they support local immunity. In the granular layer, the process of keratinization begins with the formation of keratohyalin. In the brilliant layer, the process of keratinization continues, the protein eleidin is formed. The keratinization in the stratum corneum is completed. Horny scales contain keratin. Cornification is a protective process. Soft keratin is formed in the epidermis. The stratum corneum is impregnated with sebum and moistened with sweat secretion from the surface. These secrets contain bactericidal substances (lysozyme, secretory immunoglobulins, interferon). In thin skin, the granular and shiny layers are absent.
Multilayer flat non-keratinized. On the basement membrane is the basal layer. The cells of this layer are cylindrical. They often divide by mitosis and are stem. Some of them are pushed away from the basement membrane, that is, they are pushed out and enter the path of differentiation. Cells acquire a polygonal shape, can be located in several floors. A layer of spiny cells is formed. The cells are fixed by desmosomes, the thin fibrils of which give the appearance of spines. The cells of this layer can, but rarely, divide by mitosis, so the cells of the first and second layers can be called germ cells. The outer layer of squamous cells gradually flattens, the nucleus shrinks, the cells gradually desquamate from the epithelial layer. In the process of differentiation of these cells, there is a change in the shape of cells, nuclei, the color of the cytoplasm (basophilic - eosinophilic), and a change in the color of the nucleus. Such epithelium is found in the cornea, vagina, esophagus, and oral cavity. With age or under adverse conditions, partial or signs of keratinization are possible.
Stratified transitional uroepithelium. Lines the urinary tract. It has three layers. Basal layer (growth). The cells of this layer have dense nuclei. Intermediate layer - contains three, four or more floors. The outer layer of cells - they are pear-shaped or cylinder-shaped, large in size, stain well with basophilic dyes, can divide, and have the ability to secrete mucins that protect the epithelium from the effects of urine.
glandular epithelium
The ability of body cells to intensively synthesize active substances (secretion, hormone) necessary for the implementation of the functions of other organs is characteristic of epithelial tissue. The epithelium that produces secrets is called glandular, and its cells are called secretory cells, or secretory glandulocytes. Glands are built from secretory cells, which can be designed as an independent organ or be only a part of it. There are endocrine (endo - inside, krio - separate) and exocrine (exo - outside) glands. The exocrine glands consist of two parts: the terminal (secreting) part and the excretory ducts, through which the secret enters the surface of the body or into the cavity of the internal organ. The excretory ducts usually do not take part in the formation of a secret.
Endocrine glands lack excretory ducts. Their active substances (hormones) enter the blood, and therefore the function of the excretory ducts is performed by capillaries, with which the glandular cells are very closely connected. Exocrine glands are diverse in structure and function. They can be unicellular and multicellular. An example of unicellular glands are goblet cells found in simple columnar border and pseudostratified ciliated epithelium. The nonsecretory goblet cell is cylindrical and similar to nonsecretory epithelial cells. The secret (mucin) accumulates in the apical zone, and the nucleus and organelles are displaced to the basal part of the cell. The displaced nucleus takes the form of a crescent, and the cell takes the form of a glass. Then the secret is poured out of the cell, and it again acquires a columnar shape.
Exocrine multicellular glands can be single-layered and multilayered, which is genetically determined. If the gland develops from a multilayered epithelium (sweat, sebaceous, mammary, salivary glands), then the gland is multilayered; if from a single layer (glands of the bottom of the stomach, uterus, pancreas), then they are single layer.
The nature of the branching of the excretory ducts of the exocrine glands is different, so they are divided into simple and complex. Simple glands have a non-branching excretory duct, while complex glands have a branching one.
The terminal sections of simple glands branch and do not branch, in complex glands they branch. In this regard, they have the corresponding names: branched gland and unbranched gland. According to the shape of the terminal sections, the exocrine glands are classified into alveolar, tubular, tubular-alveolar. In the alveolar gland, the cells of the terminal sections form vesicles or sacs, in tubular glands they form the appearance of a tube. The shape of the terminal part of the tubular alveolar gland occupies an intermediate position between the sac and tubule.
The cells of the terminal section are called glandulocytes. The process of secretion synthesis begins from the moment of absorption by glandulocytes from the blood and lymph of the initial components of the secret. With the active participation of organelles synthesizing a secret of a protein or carbohydrate nature, secretory granules are formed in glandulocytes. They accumulate in the apical part of the cell, and then, by reverse pinocytosis, are released into the cavity of the terminal section. The final stage of the secretory cycle is the restoration of cellular structures, if they were destroyed during the secretion process. The structure of the cells of the terminal part of the exocrine glands is determined by the composition of the excreted secret and the method of its formation.
According to the method of secretion formation, the glands are divided into holocrine, apocrine, merocrine (eccrine). With holocrine secretion (holos - whole), glandular metamorphosis of glandulocytes begins from the periphery of the terminal section and proceeds in the direction of the excretory duct.
An example of holocrine secretion is the sebaceous gland. Stem cells with basophilic cytoplasm and a rounded nucleus are located on the periphery of the terminal part. They intensively divide by mitosis, therefore they are small in size. Moving to the center of the gland, the secretory cells increase, as droplets of sebum gradually accumulate in their cytoplasm. The more fat droplets are deposited in the cytoplasm, the more intense the process of destruction of organelles. It ends with the complete destruction of the cell. The plasma membrane breaks, and the content of the glandulocyte enters the lumen of the excretory duct. With apocrine secretion (aro - from, from above), the apical part of the secretory cell is destroyed, then being an integral part of its secret. This type of secretion takes place in the sweat or mammary glands. During merocrine secretion, the cell is not destroyed. This method of secretion formation is typical for many glands of the body: gastric glands, salivary glands, pancreas, endocrine glands.
Thus, the glandular epithelium, like the integumentary one, develops from all three germ layers (ectoderm, mesoderm, endoderm), is located on the connective tissue, is devoid of blood vessels, so nutrition is carried out by diffusion. Cells are characterized by polar differentiation: the secret is localized in the apical pole, the nucleus and organelles are located in the basal pole.
Regeneration. Integumentary epithelium occupy a border position. They are often damaged, therefore they are characterized by a high regenerative capacity. Regeneration is carried out mainly mitomically and very rarely amitotically. The cells of the epithelial layer quickly wear out, age and die. Their restoration is called physiological regeneration. Restoration of epithelial cells lost due to trauma and other pathology is called reparative regeneration. In single-layer epitheliums, either all cells of the epithelial layer have regenerative capacity, or, if epptheliocytes are highly differentiated, then due to their zonal stem cells. In stratified epithelium, stem cells are located on the basement membrane, therefore they lie deep in the epithelial layer. In the glandular epithelium, the nature of regeneration is determined by the method of secretion formation. In holocrine secretion, stem cells are located outside the gland on the basement membrane. Dividing and differentiating, stem cells are converted into glandular cells. In the merocrine and apocrine glands, the restoration of epitheliocytes proceeds mainly by intracellular regeneration.
As a result of evolutionary development, tissues arose in higher multicellular organisms.
Tissues are historically (phylogenetically) formed systems of cells and non-cellular structures that have a common structure, in some cases a common origin, and are specialized in performing certain functions.
In any system, all its elements are ordered in space and function in concert with each other; the system as a whole has properties that are not inherent in any of its elements taken separately. Accordingly, in each tissue, its structure and functions are not reducible to a simple sum of the properties of the individual cells included in it.
The leading elements of the tissue system are cells. In addition to cells, there are cellular derivatives and intercellular substance.
Cell derivatives include symplasts (for example, muscle fibers, the outer part of the trophoblast), syncytium (developing male germ cells, pulp of the enamel organ), as well as post-cellular structures (erythrocytes, platelets, horny epidermal flakes, etc.).
The intercellular substance is subdivided into the main substance and into fibers. It can be presented as a sol, gel or be mineralized.
Among the fibers, there are usually three types: collagen, reticular, elastic.
TISSUE DEVELOPMENT
The properties of any tissue bear the imprint of the entire previous history of its formation. The development of a living system is understood as its transformations both in phylogenesis and ontogenesis. Tissues as systems consisting of cells and their derivatives arose historically with the advent of multicellular organisms.
Already in the lower representatives of the animal world, such as sponges and coelenterates, cells have different functional specializations and, accordingly, different structures, so that they can be combined into different tissues. However, the signs of these tissues are not yet stable, the possibilities for the transformation of cells and, accordingly, some tissues into others are quite wide. With the historical development of the animal world, the properties of individual tissues were consolidated, and the possibilities of their mutual transformations were limited, while the number of tissues at the same time gradually increased in accordance with an ever-increasing specialization.
Ontogenesis. Concepts of determination and committing.
The development of the organism begins with a unicellular stage - the zygote. In the course of crushing, blastomeres appear, but the totality of blastomeres is not yet a tissue. Blastomeres at the initial stages of cleavage are not yet determined (they are totipotent). If you separate them from one another, - each can give rise to a full-fledged independent organism - the mechanism for the emergence of monozygotic twins. Gradually, in the following stages, there is a limitation of potencies. It is based on the processes associated with the blocking of individual components of the cell genome and determination.
Determination is the process of determining the further path of cell development based on the blocking of individual genes.
The concept of "committing" is closely related to cell division (the so-called committing mitosis).
Committing is a limitation of possible development paths due to determination. The commit is done in steps. First, the corresponding transformations of the genome concern its large sections. Then they are more and more detailed, therefore, at first the most general properties of cells are determined, and then more particular ones.
As you know, at the stage of gastrulation, embryonic rudiments appear. The cells that make up their composition are not yet completely determined, so that from one rudiment cell aggregates arise that have different properties. Therefore, one embryonic germ can serve as a source of development for several tissues.
THE THEORY OF TISSUE EVOLUTION
Sequential stepwise determination and commitment of the potencies of homogeneous cell groups is a divergent process. In general, the evolutionary concept of divergent development of tissues in phylogenesis and ontogenesis was formulated by N.G. Khlopin. Modern genetic concepts confirm the correctness of his ideas. It was N.G. Khlopin who introduced the concept of genetic tissue types. Khlopin's concept gives a good answer to the question of how and in what ways the development and formation of tissues took place, but does not dwell on the causes that determine the paths of development.
The causal aspects of tissue development are revealed by A.A. Zavarzin’s theory of parallelisms. He drew attention to the similarity in the structure of tissues that perform the same functions in animals belonging to even very distant evolutionary groups. At the same time, it is known that when the evolutionary branches only diverged, the common ancestors did not yet have such specialized tissues. Consequently, in the course of evolution, in different branches of the phylogenetic tree, equally organized tissues appeared independently, as if in parallel, performing a similar function. The reason for this is natural selection: if some organisms arose in which the correspondence between the structure and function of cells, tissues, organs was violated, they were also less viable. Zavarzin's theory answers the question why the development of tissues went one way and not another, reveals the casual aspects of the evolution of tissues.
The concepts of A.A. Zavarzin and N.G. Khlopin, developed independently from one another, complement each other and were combined by A.A. Brown and V.P. Mikhailov: similar tissue structures arose in parallel with the course of divergent development.
(See Course of histology by A.A. Zavarzin and A.V. Rumyantsev, 1946)
The development of tissues in embryogenesis occurs as a result of cell differentiation. Differentiation is understood as changes in the structure of cells as a result of their functional specialization, due to the activity of their genetic apparatus. There are four main periods of differentiation of embryonic cells - ootypic, blastomeric, rudimentary and tissue differentiation. Passing through these periods, the cells of the embryo form tissues (histogenesis).
CLASSIFICATION OF FABRICS
There are several classifications of fabrics. The most common is the so-called morphofunctional classification, according to which there are four groups of tissues:
epithelial tissues;
tissues of the internal environment;
muscle tissue;
nervous tissue.
The tissues of the internal environment include connective tissues, blood and lymph.
Epithelial tissues are characterized by the association of cells into layers or strands. Through these tissues, the exchange of substances between the body and the external environment takes place. Epithelial tissues perform the functions of protection, absorption and excretion. The sources of the formation of epithelial tissues are all three germ layers - ectoderm, mesoderm and endoderm.
The tissues of the internal environment (connective tissues, including skeletal, blood and lymph) develop from the so-called embryonic connective tissue - mesenchyme. The tissues of the internal environment are characterized by the presence of a large amount of intercellular substance and contain various cells. They specialize in performing trophic, plastic, supporting and protective functions.
Muscle tissues are specialized in performing the function of movement. They develop mainly from the mesoderm (transversely striated tissue) and mesenchyme (smooth muscle tissue).
The nervous tissue develops from the ectoderm and specializes in performing a regulatory function - the perception, conduction and transmission of information.
FOUNDATIONS OF THE KINETICS OF CELL POPULATIONS
Each tissue has or had in embryogenesis stem cells - the least differentiated and least committed. They form a self-sustaining population, their descendants are able to differentiate in several directions under the influence of the microenvironment (differentiation factors), forming progenitor cells and, further, functioning differentiated cells. Thus, stem cells are pluripotent. They rarely divide, replenishment of mature tissue cells, if necessary, is carried out primarily at the expense of cells of the next generations (progenitor cells). Compared to all other cells of this tissue, stem cells are the most resistant to damaging effects.
Although the composition of the tissue includes not only cells, it is the cells that are the leading elements of the system, that is, they determine its main properties. Their destruction leads to the destruction of the system and, as a rule, their death makes the tissue non-viable, especially if stem cells have been affected.
If one of the stem cells enters the path of differentiation, then as a result of a successive series of committing mitoses, first semi-stem cells and then differentiated cells with a specific function arise. The exit of a stem cell from the population serves as a signal for the division of another stem cell according to the type of non-committing mitosis. The total number of stem cells is eventually restored. Under normal conditions, it remains approximately constant.
The totality of cells developing from one type of stem cells constitutes a stem differon. Often, various differons are involved in the formation of tissue. So, in addition to keratinocytes, the composition of the epidermis includes cells that develop in the neural crest and have a different determination (melanocytes), as well as cells that develop by differentiation of blood stem cells, i.e., already belonging to the third differon (intraepidermal macrophages, or cells Langerhans).
Differentiated cells, along with the performance of their specific functions, are able to synthesize special substances - kalons, which inhibit the intensity of reproduction of progenitor cells and stem cells. If for some reason the number of differentiated functioning cells decreases (for example, after an injury), the inhibitory effect of chalons weakens and the population is restored. In addition to chalons (local regulators), cell reproduction is controlled by hormones; at the same time, the waste products of cells regulate the activity of the endocrine glands. If any cells undergo mutations under the influence of external damaging factors, they are eliminated from the tissue system due to immunological reactions.
The choice of cell differentiation path is determined by intercellular interactions. The influence of the microenvironment changes the activity of the genome of a differentiating cell, activating some and blocking other genes. In cells that have already differentiated and have lost the ability to further reproduce, the structure and function can also change (for example, in granulocytes starting from the metamyelocyte stage). Such a process does not lead to differences among the descendants of the cell and is more appropriately called "specialization".
TISSUE REGENERATION
Knowledge of the fundamentals of the kinetics of cell populations is necessary for understanding the theory of regeneration, i.e. restoration of the structure of a biological object after its destruction. According to the levels of organization of the living, cellular (or intracellular), tissue, and organ regeneration are distinguished. The subject of general histology is regeneration at the tissue level.
There are physiological regeneration, which takes place constantly in a healthy organism, and reparative regeneration due to damage. Different tissues have different possibilities of regeneration.
In a number of tissues, cell death is genetically programmed and occurs constantly (in the stratified keratinizing epithelium of the skin, in the single-layered epithelium of the small intestine, in the blood). Due to continuous reproduction, primarily semi-stem progenitor cells, the number of cells in the population is replenished and is constantly in a state of equilibrium. Along with programmed physiological cell death in all tissues, unprogrammed death also occurs - from random causes: trauma, intoxication, exposure to background radiation. Although there is no programmed death in a number of tissues, stem and semi-stem cells remain in them throughout life. In response to an accidental death, their reproduction occurs and the population is restored.
In tissues where there are no stem cells left in an adult, regeneration at the tissue level is impossible, it occurs only at the cellular level.
The organs and systems of the body are multitissue formations in which various tissues are closely interconnected and interdependent in performing a number of characteristic functions. In the process of evolution, higher animals and humans have developed integrating and regulating systems of the body - nervous and endocrine. All multitissue components of the organs and systems of the body are under the control of these regulatory systems and, thus, a high integration of the body as a whole is carried out. In the evolutionary development of the animal world, with the complication of organization, the integrating and regulating role of the nervous system increased, including in the nervous regulation of the activity of the endocrine glands.
Histology (from Greek ίστίομ - tissue and Greek Λόγος - knowledge, word, science) is a branch of biology that studies the structure of tissues of living organisms. This is usually done by dissecting tissue into thin layers and using a microtome. Unlike anatomy, histology studies the structure of the body at the tissue level. Human histology is a branch of medicine that studies the structure of human tissues. Histopathology is a branch of the microscopic examination of diseased tissue and is an important tool in pathomorphology (pathological anatomy), since an accurate diagnosis of cancer and other diseases usually requires histopathological examination of specimens. Forensic histology is a branch of forensic medicine that studies the features of damage at the tissue level.
Histology was born long before the invention of the microscope. The first descriptions of fabrics are found in the works of Aristotle, Galen, Avicenna, Vesalius. In 1665, R. Hooke introduced the concept of a cell and observed the cellular structure of some tissues under a microscope. Histological studies were carried out by M. Malpighi, A. Leeuwenhoek, J. Swammerdam, N. Gru and others. A new stage in the development of science is associated with the names of K. Wolf and K. Baer, the founders of embryology.
In the 19th century, histology was a full-fledged academic discipline. In the middle of the 19th century, A. Kölliker, Leiding, and others created the foundations of the modern theory of fabrics. R. Virchow initiated the development of cellular and tissue pathology. Discoveries in cytology and the creation of cell theory stimulated the development of histology. The works of I. I. Mechnikov and L. Pasteur, who formulated the basic ideas about the immune system, had a great influence on the development of science.
The 1906 Nobel Prize in Physiology or Medicine was awarded to two histologists, Camillo Golgi and Santiago Ramón y Cajal. They had mutually opposite views on the nervous structure of the brain in various examinations of identical images.
In the 20th century, the improvement of methodology continued, which led to the formation of histology in its current form. Modern histology is closely connected with cytology, embryology, medicine and other sciences. Histology develops such issues as patterns of development and differentiation of cells and tissues, adaptation at the cellular and tissue levels, problems of tissue and organ regeneration, etc. Achievements in pathological histology are widely used in medicine, making it possible to understand the mechanism of development of diseases and suggest ways to treat them.
Research methods in histology include the preparation of histological preparations with their subsequent study using a light or electron microscope. Histological preparations are smears, prints of organs, thin sections of pieces of organs, possibly stained with a special dye, placed on a microscope slide, enclosed in a preservative medium and covered with a coverslip.
Tissue histology
A tissue is a phylogenetically formed system of cells and non-cellular structures that have a common structure, often origin, and are specialized in performing specific specific functions. Tissue is laid in embryogenesis from the germ layers. From the ectoderm, the epithelium of the skin (epidermis), the epithelium of the anterior and posterior alimentary canal (including the epithelium of the respiratory tract), the epithelium of the vagina and urinary tract, the parenchyma of the large salivary glands, the outer epithelium of the cornea and nervous tissue are formed.
From the mesoderm, mesenchyme and its derivatives are formed. These are all types of connective tissue, including blood, lymph, smooth muscle tissue, as well as skeletal and cardiac muscle tissue, nephrogenic tissue and mesothelium (serous membranes). From the endoderm - the epithelium of the middle part of the digestive canal and the parenchyma of the digestive glands (liver and pancreas). Tissues contain cells and intercellular substance. At the beginning, stem cells are formed - these are poorly differentiated cells capable of dividing (proliferation), they gradually differentiate, i.e. acquire the features of mature cells, lose the ability to divide and become differentiated and specialized, i.e. capable of performing specific functions.
The direction of development (differentiation of cells) is genetically determined - determination. This orientation is provided by the microenvironment, the function of which is performed by the stroma of organs. A set of cells that are formed from one type of stem cells - differon. Tissues form organs. In the organs, the stroma formed by connective tissues and the parenchyma are isolated. All tissues regenerate. A distinction is made between physiological regeneration, which constantly proceeds under normal conditions, and reparative regeneration, which occurs in response to irritation of tissue cells. The mechanisms of regeneration are the same, only reparative regeneration is several times faster. Regeneration is at the heart of recovery.
Regeneration mechanisms:
By cell division. It is especially developed in the earliest tissues: epithelial and connective, they contain many stem cells, the proliferation of which ensures regeneration.
Intracellular regeneration - it is inherent in all cells, but is the leading mechanism of regeneration in highly specialized cells. This mechanism is based on the enhancement of intracellular metabolic processes, which lead to the restoration of the cell structure, and with further enhancement of individual processes
hypertrophy and hyperplasia of intracellular organelles occurs. which leads to compensatory hypertrophy of cells capable of performing a greater function.
Origin of tissues
The development of an embryo from a fertilized egg occurs in higher animals as a result of multiple cell divisions (crushing); the cells formed in this case are gradually distributed in their places in different parts of the future embryo. Initially, embryonic cells are similar to each other, but as their number increases, they begin to change, acquiring characteristic features and the ability to perform certain specific functions. This process, called differentiation, eventually leads to the formation of different tissues. All tissues of any animal come from three initial germ layers: 1) the outer layer, or ectoderm; 2) the innermost layer, or endoderm; and 3) the middle layer, or mesoderm. So, for example, muscles and blood are derivatives of the mesoderm, the lining of the intestinal tract develops from the endoderm, and the ectoderm forms integumentary tissues and the nervous system.
Fabrics have evolved. There are 4 groups of tissues. The classification is based on two principles: histogenetic, based on origin, and morphofunctional. According to this classification, the structure is determined by the function of the tissue. The first to appear were epithelial or integumentary tissues, the most important functions being protective and trophic. They are rich in stem cells and regenerate through proliferation and differentiation.
Then appeared connective tissues or musculoskeletal, tissues of the internal environment. Leading functions: trophic, supporting, protective and homeostatic - maintaining the constancy of the internal environment. They are characterized by a high content of stem cells and regenerate through proliferation and differentiation. In this tissue, an independent subgroup is distinguished - blood and lymph - liquid tissues.
The following are muscle (contractile) tissues. The main property - contractile - determines the motor activity of organs and the body. Allocate smooth muscle tissue - a moderate ability to regenerate by proliferation and differentiation of stem cells, and striated (striated) muscle tissue. These include cardiac tissue - intracellular regeneration, and skeletal tissue - regenerates due to the proliferation and differentiation of stem cells. The main recovery mechanism is intracellular regeneration.
Then came the nervous tissue. Contains glial cells, they are able to proliferate. but the nerve cells themselves (neurons) are highly differentiated cells. They react to stimuli, form a nerve impulse and transmit this impulse through the processes. Nerve cells have intracellular regeneration. As the tissue differentiates, the leading method of regeneration changes - from cellular to intracellular.
Main types of fabrics
Histologists usually distinguish four main tissues in humans and higher animals: epithelial, muscular, connective (including blood), and nervous. In some tissues, cells have approximately the same shape and size and are so tightly adjacent to one another that there is no or almost no intercellular space between them; such tissues cover the outer surface of the body and line its internal cavities. In other tissues (bone, cartilage), the cells are not so densely packed and are surrounded by the intercellular substance (matrix) that they produce. From the cells of the nervous tissue (neurons) that form the brain and spinal cord, long processes depart, ending very far from the cell body, for example, at the points of contact with muscle cells. Thus, each tissue can be distinguished from others by the nature of the location of the cells. Some tissues have a syncytial structure, in which the cytoplasmic processes of one cell pass into similar processes of neighboring cells; such a structure is observed in the germinal mesenchyme, loose connective tissue, reticular tissue, and can also occur in some diseases.
Many organs are composed of several types of tissues, which can be recognized by their characteristic microscopic structure. Below is a description of the main types of tissues found in all vertebrates. Invertebrates, with the exception of sponges and coelenterates, also have specialized tissues similar to the epithelial, muscular, connective, and nervous tissues of vertebrates.
epithelial tissue. The epithelium may consist of very flat (scaly), cuboidal, or cylindrical cells. Sometimes it is multi-layered, i.e. consisting of several layers of cells; such an epithelium forms, for example, the outer layer of the human skin. In other parts of the body, for example in the gastrointestinal tract, the epithelium is single-layered, i.e. all of its cells are connected to the underlying basement membrane. In some cases, a single-layered epithelium may appear to be multi-layered: if the long axes of its cells are not parallel to each other, then it seems that the cells are at different levels, although in fact they lie on the same basement membrane. Such an epithelium is called multilayered. The free edge of epithelial cells is covered with cilia, i.e. thin hair-like outgrowths of protoplasm (such a ciliary epithelium lines, for example, the trachea), or ends with a “brush border” (the epithelium lining the small intestine); this border consists of ultramicroscopic finger-like outgrowths (so-called microvilli) on the cell surface. In addition to protective functions, the epithelium serves as a living membrane through which gases and solutes are absorbed by cells and released to the outside. In addition, the epithelium forms specialized structures, such as glands that produce substances necessary for the body. Sometimes secretory cells are scattered among other epithelial cells; an example is the mucus-producing goblet cells in the surface layer of the skin in fish or in the intestinal lining in mammals.
Muscle. Muscle tissue differs from the rest in its ability to contract. This property is due to the internal organization of muscle cells containing a large number of submicroscopic contractile structures. There are three types of muscles: skeletal, also called striated or voluntary; smooth, or involuntary; cardiac muscle, which is striated but involuntary. Smooth muscle tissue consists of spindle-shaped mononuclear cells. The striated muscles are formed from multinuclear elongated contractile units with a characteristic transverse striation, i.e. alternating light and dark stripes perpendicular to the long axis. The cardiac muscle consists of mononuclear cells, connected end to end, and has a transverse striation; while the contractile structures of neighboring cells are connected by numerous anastomoses, forming a continuous network.
Connective tissue. There are different types of connective tissue. The most important supporting structures of vertebrates consist of two types of connective tissue - bone and cartilage. Cartilage cells (chondrocytes) secrete around themselves a dense elastic ground substance (matrix). Bone cells (osteoclasts) are surrounded by a ground substance containing salt deposits, mainly calcium phosphate. The consistency of each of these tissues is usually determined by the nature of the basic substance. As the body ages, the content of mineral deposits in the ground substance of the bone increases, and it becomes more brittle. In young children, the main substance of the bone, as well as cartilage, is rich in organic substances; due to this, they usually have not real bone fractures, but the so-called. fractures (fractures of the "green branch" type). Tendons are made up of fibrous connective tissue; its fibers are formed from collagen, a protein secreted by fibrocytes (tendon cells). Adipose tissue is located in different parts of the body; This is a peculiar type of connective tissue, consisting of cells, in the center of which there is a large globule of fat.
Blood. Blood is a very special type of connective tissue; some histologists even distinguish it as an independent type. The blood of vertebrates consists of liquid plasma and formed elements: red blood cells, or erythrocytes containing hemoglobin; a variety of white cells, or leukocytes (neutrophils, eosinophils, basophils, lymphocytes, and monocytes), and platelets, or platelets. In mammals, mature erythrocytes entering the bloodstream do not contain nuclei; in all other vertebrates (fish, amphibians, reptiles, and birds), mature, functioning erythrocytes contain a nucleus. Leukocytes are divided into two groups - granular (granulocytes) and non-granular (agranulocytes) - depending on the presence or absence of granules in their cytoplasm; in addition, they are easy to differentiate using staining with a special mixture of dyes: eosinophil granules acquire a bright pink color with this staining, the cytoplasm of monocytes and lymphocytes - a bluish tint, basophil granules - a purple tint, neutrophil granules - a faint purple tint. In the bloodstream, the cells are surrounded by a transparent liquid (plasma) in which various substances are dissolved. Blood delivers oxygen to tissues, removes carbon dioxide and metabolic products from them, and carries nutrients and secretion products, such as hormones, from one part of the body to another.
nervous tissue. Nervous tissue is made up of highly specialized cells called neurons, which are concentrated mainly in the gray matter of the brain and spinal cord. A long process of a neuron (axon) stretches for long distances from the place where the body of the nerve cell containing the nucleus is located. The axons of many neurons form bundles, which we call nerves. Dendrites also depart from neurons - shorter processes, usually numerous and branched. Many axons are covered by a special myelin sheath, which is made up of Schwann cells containing a fat-like material. Neighboring Schwann cells are separated by small gaps called nodes of Ranvier; they form characteristic depressions on the axon. Nervous tissue is surrounded by a special type of supporting tissue known as neuroglia.
Tissue responses to abnormal conditions
When tissues are damaged, some loss of their typical structure is possible as a reaction to the violation that has occurred.
Mechanical damage. With mechanical damage (cut or fracture), the tissue reaction is aimed at filling the resulting gap and reconnecting the edges of the wound. Weakly differentiated tissue elements, in particular fibroblasts, rush to the rupture site. Sometimes the wound is so large that the surgeon has to insert pieces of tissue into it to stimulate the initial stages of the healing process; for this, fragments or even whole pieces of bone obtained during amputation and stored in the "bank of bones" are used. In cases where the skin surrounding a large wound (for example, with burns) cannot provide healing, transplants of healthy skin flaps taken from other parts of the body are resorted to. Such grafts in some cases do not take root, since the transplanted tissue does not always manage to form contact with those parts of the body to which it is transferred, and it dies or is rejected by the recipient.
Pressure. Calluses occur with constant mechanical damage to the skin as a result of pressure exerted on it. They appear as well-known corns and thickenings of the skin on the soles of the feet, the palms of the hands and on other areas of the body that experience constant pressure. Removal of these thickenings by excision does not help. As long as the pressure continues, the formation of calluses will not stop, and cutting them off, we only expose the sensitive underlying layers, which can lead to the formation of wounds and the development of infection.
Skin - covered with stratified squamous (flat) keratinizing epithelium; The oral cavity, pharynx, esophagus, final section of the rectum are covered with stratified non-keratinized epithelium; The mucous membrane of the urinary tract is covered with transitional epithelium (mesothelium); Stomach, trachea, bronchi - single-layer columnar epithelium; Serous membranes (peritoneum, pleura) - lined with a single layer of squamous epithelium. Sebaceous, sweat, lacrimal, pancreas, thyroid, etc. - are made up of glandular epithelium.
Connective tissue. Connective tissue, or tissues of the internal environment, is represented by a group of tissues that are diverse in structure and functions, which are located inside the body and do not border on either the external environment or organ cavities. Connective tissue protects, insulates and supports parts of the body, and also performs a transport function within the body (blood). For example, ribs protect the organs of the chest, fat is an excellent insulator, the spine supports the head and torso, and blood carries nutrients, gases, hormones, and waste products. In all cases, the connective tissue is characterized by a large amount of intercellular substance. The following subtypes of connective tissue are distinguished: connective tissue proper (loose, fatty, reticular, dense fibrous), cartilage, bone, and blood.
proper connective tissue. The connective tissue itself is represented by loose and dense fibrous connective tissue. Connective tissue performs supporting, protective (mechanical) functions. Loose connective tissue has a network of elastic and elastic (collagen) fibers located in a viscous intercellular substance. This tissue surrounds all blood vessels and most organs, and also underlies the epithelium of the skin.
Fatty. Loose connective tissue containing a large number of fat cells is called adipose tissue; it serves as a place for storing fat and a source of water formation. Some parts of the body are more capable of storing fat than others, such as under the skin or in the omentum. fibrous tissue Loose tissue contains other cells - macrophages and fibroblasts. Macrophages phagocytize and digest microorganisms, destroyed tissue cells, foreign proteins and old blood cells; their function can be called sanitary. Fibroblasts are mainly responsible for the formation of fibers in the connective tissue.
Reticular. Consists of reticular cells and reticular fibers. It forms the backbone of the hematopoietic organs and organs of the immune system (bone marrow, thymus, spleen, lymph nodes, group and single lymphoid nodules). In the loops formed by the reticular tissue, blood-forming and immunocompetent cells are located.
Dense fibrous Irregular connective tissue. Consists of many connective tissue fibers densely intertwined. Dense formed connective tissue is distinguished by an ordered arrangement of fiber bundles, determined by their direction (ligaments, tendons).
cartilaginous. Connective tissue with a dense intercellular substance is represented by either cartilage or bone. Cartilage provides the strong yet flexible backbone of organs. The outer ear, nose and nasal septum, larynx and trachea have a cartilaginous skeleton. The main function of these cartilages is to maintain the shape of various structures. The cartilaginous rings of the trachea prevent its collapse and ensure the movement of air into the lungs. The cartilage between the vertebrae makes them mobile relative to each other.
Bone. Bone is a connective tissue, the intercellular substance of which consists of organic material (ossein) and inorganic salts, mainly calcium and magnesium phosphates. It always contains specialized bone cells - osteocytes (modified fibroblasts), scattered in the intercellular substance. Unlike cartilage, bone is permeated with a large number of blood vessels and a certain number of nerves. From the outside, it is covered with a periosteum (periosteum). The periosteum is a source of osteocyte progenitor cells, and restoration of bone integrity is one of its main functions.
- This is a connective tissue with a liquid intercellular substance, plasma, which makes up a little more than half of the total blood volume. Plasma contains the protein fibrinogen, which, in contact with air or if a blood vessel is damaged, forms a fibrin clot consisting of fibrin filaments in the presence of calcium and blood coagulation factors. The clear yellowish liquid that remains after clot formation is called serum. Plasma contains various proteins (including antibodies), metabolic products, nutrients (glucose, amino acids, fats), gases (oxygen, carbon dioxide and nitrogen), various salts and hormones. On average, an adult male has about 5 liters of blood.
Muscle. Muscles provide movement of the body in space, its posture and contractile activity of internal organs. The ability to contract, to some extent inherent in all cells, is most strongly developed in muscle cells. There are three types of muscles: skeletal (striated, or voluntary), smooth (visceral, or involuntary), and cardiac.
Skeletal muscles. Skeletal muscle cells are long tubular structures, the number of nuclei in them can reach several hundred. Their main structural and functional elements are muscle fibers (myofibrils), which have a transverse striation. Skeletal muscles are stimulated by nerves (end plates of motor nerves); they react quickly and are controlled largely voluntarily. For example, the muscles of the limbs are under voluntary control, while the diaphragm depends on it only indirectly.
Smooth muscles consist of spindle-shaped mononuclear cells with fibrils devoid of transverse bands. These muscles act slowly and contract involuntarily. They line the walls of internal organs (except the heart). Thanks to their synchronous action, food is pushed through the digestive system, urine is excreted from the body, blood flow and blood pressure are regulated, and the egg and sperm move through the appropriate channels.
Nervous tissue is characterized by the maximum development of such properties as irritability and conductivity. Irritability - the ability to respond to physical (heat, cold, light, sound, touch) and chemical (taste, smell) stimuli (irritants). Conductivity - the ability to transmit an impulse (nerve impulse) that has arisen as a result of irritation. The element that perceives irritation and conducts a nerve impulse is a nerve cell (neuron).
A neuron consists of a cell body containing a nucleus, and processes - dendrites and an axon. Each neuron may have many dendrites, but only one axon, which, however, has several branches. Dendrites, perceiving a stimulus from different parts of the brain or from the periphery, transmit a nerve impulse to the body of the neuron.
From the cell body, a nerve impulse is conducted along a single process - an axon - to other neurons or effector organs. The axon of one cell can contact either dendrites, or the axon or bodies of other neurons, or muscle or glandular cells; these specialized contacts are called synapses. The axon extending from the cell body is covered with a sheath formed by specialized (Schwann) cells; the sheathed axon is called a nerve fiber. Bundles of nerve fibers make up nerves. They are covered with a common connective tissue sheath, in which elastic and non-elastic fibers and fibroblasts (loose connective tissue) are interspersed along the entire length. In the brain and spinal cord, there is another type of specialized cells - neuroglial cells. These are auxiliary cells contained in the brain in very large quantities. Their processes braid the nerve fibers and serve as a support for them, as well as, apparently, insulators. In addition, they have secretory, trophic and protective functions. Unlike neurons, glial cells are capable of dividing.
