Epithelial tissue: structural features, functions and types. Concept of epithelial tissue
Epithelial tissue or epithelium, lining the surface of the body, serous membranes, inner surface hollow organs (stomach, intestines, bladder) and form most of the glands of the body. They originate from all three germ layers - ectoderm, endoderm, mesoderm.
Epithelium are layers of cells located on basement membrane, under which lies loose connective tissue. There is almost no intermediate substance in the epithelium and the cells are in close contact with each other. Epithelial tissues do not have blood vessels and are nourished through the basement membrane from the underlying connective tissue. Fabrics have a high regenerative ability.
The epithelium has a number of functions:
· Protective - protects other fabrics from environmental influences. This function is characteristic of the skin epithelium;
· Nutritional (trophic) - absorption of nutrients. This function is performed, for example, by the epithelium of the gastrointestinal tract;
A - single-layer cylindrical, B - single-layer cubic, C - single-layer flat, D - multi-row, D - multi-layer flat non-keratinizing, E - multi-layer flat keratinizing, G1 - transitional epithelium with a stretched organ wall, G2 - with a collapsed organ wall
· Excretory - removal of unnecessary substances from the body (CO 2, urea);
· Secretory - most glands are built from epithelial cells.
Epithelial tissues can be classified in a diagram. Single-layer and multilayer epithelia differ in cell shape.
Single-layer squamous epithelium consists of flat cells located on the basement membrane. This epithelium is called mesothelium and lines the surface of the pleura, pericardial sac and peritoneum.
Endothelium is a derivative of mesenchyme and is a continuous layer of flat cells covering the inner surface of the blood and lymphatic vessels.
Single layer cuboidal epithelium lines the kidney tubules that excrete the ducts of the glands.
Single layer columnar epithelium consists of prismatic cells. This epithelium lines the inner surface of the stomach, intestines, uterus, oviducts, renal tubules. Goblet cells are found in the intestinal epithelium. These are single-celled glands that secrete mucus.
In the small intestine, epithelial cells have a special formation on the surface - a border. It consists of large number microvilli, which increases the surface of the cell and promotes better absorption of nutrients and other substances. The epithelial cells lining the uterus have ciliated cilia and are called ciliated epithelium.
Single layer multirow epithelium differs in that cells have it different shape and as a result of this, their nuclei lie at different levels. This epithelium has ciliated cilia and is also called ciliated. It lines the airways and some parts of the reproductive system. The movements of the cilia remove dust particles from the upper respiratory tract.
Stratified squamous epithelium is a relatively thick layer consisting of many layers of cells. Only the deepest layer is in contact with the basement membrane. Multilayer epithelium performs a protective function and is divided into keratinizing and non-keratinizing.
Non-keratinizing epithelium lines the surface of the cornea of the eye, oral cavity and esophagus. Consists of cells different shapes. The basal layer consists of cylindrical cells; then cells of various shapes with short thick processes are located - a layer of spinous cells. The topmost layer consists of flat cells that gradually die and fall off.
keratinizing The epithelium covers the surface of the skin and is called the epidermis. It consists of 4-5 layers of cells of different shapes and functions. The inner layer, the basal layer, consists of cylindrical cells capable of reproduction. The spinous cell layer consists of cells with cytoplasmic islands, with the help of which the cells come into contact with each other. The granular layer consists of flattened cells containing grains. The stratum pellucida, in the form of a shiny ribbon, consists of cells, the boundaries of which are not visible due to the shiny substance - eleidin. The stratum corneum consists of flat scales filled with keratin. The most superficial scales of the stratum corneum gradually fall off, but are replenished by the multiplying cells of the basal layer. The stratum corneum is resistant to external and chemical influences, elasticity and low thermal conductivity, which ensures the protective function of the epidermis.
Transitional epithelium characterized by the fact that its appearance changes depending on the state of the organ. It consists of two layers - the basal layer - in the form of small flattened cells and the integumentary layer - large, slightly flattened cells. Epithelium lines bladder, ureters, pelvis, renal calyces. When the organ wall contracts, the transitional epithelium takes the form of a thick layer in which the basal layer becomes multirowed. If the organ is stretched, the epithelium becomes thin and the shape of the cells changes.
Epithelial tissue — outer surface human skin, as well as the lining surface of the mucous membranes internal organs, gastrointestinal tract, lungs, most glands.
The epithelium is devoid of blood vessels, so nutrition occurs due to the adjacent connective tissues, which are nourished by the bloodstream.
Functions of epithelial tissue
Main function skin epithelial tissue - protective, that is, limiting exposure external factors to internal organs. Epithelial tissue has a multilayer structure, so keratinized (dead) cells are quickly replaced by new ones. It is known that epithelial tissue has increased restorative properties, which is why human skin is quickly renewed.
There is also intestinal epithelial tissue with a single-layer structure, which has absorptive properties, due to which digestion occurs. In addition, the intestinal epithelium tends to secrete chemicals, in particular sulfuric acid.
Human epithelial tissue covers almost all organs from the cornea of the eye to the respiratory and genitourinary system. Some types of epithelial tissue are involved in protein and gas metabolism.
The structure of epithelial tissue
Single-layer epithelial cells are located on the basement membrane and form one layer with it. Stratified epithelial cells are formed from several layers and only the lowest layer is the basement membrane.
According to the shape of the structure, epithelial tissue can be: cubic, flat, cylindrical, ciliated, transitional, glandular, etc.
Glandular epithelial tissue has secretory functions, that is, the ability to secrete secretions. The glandular epithelium is located in the intestines and makes up the sweat and salivary glands, endocrine glands, etc.
The role of epithelial tissue in the human body
The epithelium plays a barrier role, protecting internal tissues, and also promotes the absorption of nutrients. When eating hot food, part of the intestinal epithelium dies and is completely restored overnight.
Connective tissue
Connective tissue– building matter that unites and fills the entire organism.
Connective tissue is presented in nature in several states at once: liquid, gel-like, solid and fibrous.
In accordance with this, they distinguish between blood and lymph, fat and cartilage, bones, ligaments and tendons, as well as various intermediate body fluids. The peculiarity of connective tissue is that there is much more intercellular substance in it than there are cells themselves.
Types of connective tissue
— Cartilaginous, there are three types:
a) Hyaline cartilage;
b) Elastic;
c) Fibrous.
— Bone(consists of forming cells - osteoblast, and destroying cells - osteoclast);
— Fibrous, in turn happens:
a) Loose (creates a frame for organs);
b) Formed dense (forms tendons and ligaments);
c) Unformed dense (the perichondrium and periosteum are built from it).
— Trophic(blood and lymph);
— Specialized:
a) Reticular (from it the tonsils, bone marrow, lymph nodes, kidneys and liver);
b) Fat (subcutaneous energy reservoir, heat regulator);
c) Pigment (iris, nipple halo, anus circumference);
d) Intermediate (synovial, cerebrospinal and other auxiliary fluids).
Functions of connective tissue
These structural features allow connective tissue to perform various functions:
- Mechanical The (supporting) function is performed by bone and cartilage tissue, as well as fibrous connective tissue of tendons;
- Protective the function is performed by adipose tissue;
- Transport The function is performed by liquid connective tissues: blood and lymph.
Blood ensures the transport of oxygen and carbon dioxide, nutrients, and metabolic products. Thus, connective tissue connects parts of the body to each other.
The structure of connective tissue
Most of the connective tissue is an intercellular matrix of collagen and non-collagen proteins.
In addition to it - naturally, cells, as well as a number of fibrous structures. The most important cells Fibroblasts can be called fibroblasts, which produce intercellular fluid substances (elastin, collagen, etc.).
Basophils are also important in the structure ( immune function), macrophages (exterminators of pathogens) and melanocytes (responsible for pigmentation).
Peculiarities epithelium: 1) absence of blood vessels (exception: stria vascularis - multilayered epithelium with capillaries); nutrition - diffuse from the lower layers. 2) poor development of the intercellular substance. 3) high ability to regenerate due to cambial cells, which often divide by mitosis. (2 types: physiological - natural renewal of the structure, reparative - the formation of new structures at the site of damage, with the formation of numerous poorly differentiated cells similar to embryonic ones) 4) polarity is expressed in the cells (basal and apical poles, the nucleus is located in the basal, and the apical – secretory granules and organelles special meaning- flickering eyelashes). 5) located on the basement membrane (it is non-cellular, permeable, has an amorphous substance and fibrils). 6) the presence of intercellular contacts: desmosomes - mechanical contact, connects cells; hemidesmosomes – attaches epithelial cells to the BM; encircling desmosome - tight junction, chemically insulating; nexuses – gap junctions. 7) are always located on the border of 2 environments. They form a layer even in cell culture.
Functions epithelium: 1) Integumentary: delimiting the body from external and internal environments, the connection between them. 2) Barrier (protective). Mechanical protection against damage, chemical influences and microorganisms. 3) Homeostatic, thermoregulation, water-salt metabolism etc. 4) Absorption: epithelium of the gastrointestinal tract, kidneys 5) Release of metabolic products, such as urea. 6) Gas exchange: lung epithelium, skin. 7) secretory – epithelium of liver cells, secretory glands. 8) transport – movement along the surface of the mucosa.
Basement membrane. In addition to epithelia in muscle and fatty tissues. This is a homogeneous layer (50 - 100 nm.) Underneath it is a layer of reticular fibers. BM is synthesized by epithelial cells and connective tissue cells and contains type 4 collagen. Epithelial cells are connected to the BM by semidesmosomes. Functions of BM: binding and separation of epithelium and connective tissue, providing nutrition to the epithelium, support for cells, and promotes their organization into a layer.
Single layer:
Multi-layer:
By location the epithelium is divided into: integumentary glandular– forms the parenchyma of the glands.
Single layer epithelium. All cells with their basal parts lie on the BM. The apical parts form a free surface.
Single layer flat The epithelium is represented in the body by mesothelium and, according to some data, by endothelium. Mesothelium (serosa) covers the serous membranes (leaves of the pleura, visceral and parietal peritoneum, pericardial sac, etc.). Mesothelial cells - mesotheliocytes are flat, have a polygonal shape and uneven edges. In the part where the nucleus is located in them, the cells are thicker. Some of them contain not one, but two or even three cores. There are microvilli on the free surface of the cell. Serous fluid is released and absorbed through the mesothelium. Thanks to its smooth surface, internal organs can glide easily. The mesothelium prevents the formation of connective tissue adhesions between the organs of the abdominal and thoracic cavities, the development of which is possible if its integrity is violated. The endothelium lines blood and lymphatic vessels, as well as the chambers of the heart. It is a layer of flat cells - endothelial cells, lying in one layer on the basement membrane. Endotheliocytes are distinguished by a relative paucity of organelles and the presence of pinocytotic vesicles in the cytoplasm.
The endothelium, located in the vessels at the border with lymph and blood, participates in the exchange of substances and gases (02, CO2) between them and other tissues. If it is damaged, a change in blood flow in the vessels and the formation of blood clots - thrombi - in their lumen are possible.
Single layer cubic epithelium (epithelium simplex cuboideum) lines part of the renal tubules (proximal and distal). Proximal tubule cells have a brush border and basal striations. The brush border consists of a large number of microvilli . The striation is due to the presence in basal sections cells of the deep folds of the plasmalemma and mitochondria located between them. The epithelium of the renal tubules performs the function of reverse absorption (reabsorption) of a number of substances from the primary urine flowing through the tubules into the blood of the intertubular vessels.
Single layer prismatic epithelium. This type of epithelium is characteristic of the middle section digestive system. It lines the inner surface of the stomach, small and large intestines, gallbladder, a number of ducts of the liver and pancreas. Epithelial cells are connected to each other using desmosomes, gap communication junctions, lock-type junctions, and tight junctions (see Chapter IV). Thanks to the latter, the contents of the stomach, intestines and other hollow organs cannot penetrate into the intercellular gaps of the epithelium.
Epithelia develop from all three germ layers, starting from the 3rd-4th week embryonic development person. Depending on the embryonic source, epithelia of ectodermal, mesodermal and endodermal origin are distinguished. Related types of epithelia, developing from the same germ layer, under pathological conditions can undergo metaplasia, i.e. pass from one species to another, for example, in the respiratory tract, ectodermal epithelium when chronic bronchitis from a single-layer ciliated one can turn into a multi-layered flat one, which is normally characteristic of the oral cavity and is also of ectodermal origin.
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Classification of epithelial tissues
There are two types of classification of epithelial tissues: morphological and genetic.
Morphological classification epithelial tissues.
1.Single layer epithelium- All cells of this epithelium lie on the basement membrane.
A) Single row- all cells have the same height, so the nuclei of epithelial cells lie in one row.
— Flat.
The height of epithelial cells is less than their width. (endothelium of blood vessels)
— Cubic.The height and width of the epithelial cells are the same.(covers distal sections nephron tubules)
— Cylindrical(Prismatic). The height of epithelial cells is greater than their width. (Covers the mucous membrane of the stomach, small and large intestines).
b) Multi-row- Cells have different heights, so their nuclei form rows. Moreover, all cells lie on basement membrane.
2.Multilayer epithelium. Cells, having the same size, form a layer. In stratified epithelium, only the bottom layer lies on the basement membrane. All other layers are not in contact with the basement membrane. The name of the stratified epithelium is formed according to the shape of the topmost layer.
A) Multilayered squamous non-keratinizing epithelium.B This epithelium does not undergo the upper layers of keratinization. Covers the cornea of the eye, the mucous membrane of the oral cavity and esophagus
b) Stratified squamous keratinizing epithelium.B The human body is represented by the epidermis and its derivatives (nails, hair).
V) Multilayer transitional epithelium. Covers mucous membrane of the urinary tract. It has the ability to transform from a two-layer to a pseudo-multilayer.
Genetic classification:
Epidermal type. Formed from ectoderm. Represented by multilayer and multirow epithelium. Performs integumentary and protective functions.
2.Endodermal type. Formed from the endoderm. Represented by single-layer prismatic epithelium. Performs the function of absorption.
3.Coelonephrodermal type. Formed from the mesoderm. Represented by single-layer epithelia. Performs barrier and excretory functions.
4.Ependymoglial type. Formed from the neural tube. Lines the spinal canal and ventricles of the brain.
5.Angiodermal type. from mesenchyme (extraembryonic mesoderm). Represented by vascular endothelium.
Olfactory organ . General morphofunctional characteristics. Cellular composition of the olfactory epithelium. Organ of taste. General morphofunctional characteristics. Taste buds, their cellular composition.
Olfactory organ is a chemoreceptor. It perceives the action of molecules odorous substances. This is the most ancient look reception. The olfactory analyzer consists of three parts: the olfactory region of the nasal cavity (peripheral part), the olfactory bulb (intermediate part), and also the olfactory centers in the cerebral cortex.
The source of formation of all parts of the olfactory organ is the neural tube.
The olfactory lining of the peripheral part of the olfactory analyzer is located on the superior and partially middle concha of the nasal cavity.
The general olfactory region has an epithelial-like structure. Olfactory neurosensory cells are spindle-shaped with two processes. Based on their shape, they are divided into rod-shaped and cone-shaped. The total number of olfactory cells in humans reaches 400 million, with a significant predominance of rod-shaped cells.
Organum gustus located in the initial department digestive tract and serves to perceive the quality of food.
Taste receptors are small neuroepithelial structures called taste buds (gemmae gustatoriae). They are located in stratified epithelium mushroom-shaped(papillae fungiformes), leaf-shaped(papillae foliatae) and grooved(papillae vallatae) of the papillae of the tongue and in small quantities - in the mucous membrane of the soft palate, epiglottis and back wall throats.
In humans, the number of taste buds reaches 2000 - 3000, of which more than half are located in the grooved papillae.
Each taste bud has the shape of an ellipse and consists of 40 - 60 cells tightly adjacent to each other. among which there are receptor, supporting and basal cells. The apex of the kidney communicates with the oral cavity through an opening - taste pore(porus gustatorius), which leads into a small depression formed by the apical surfaces of taste sensory cells - the taste pit.
TICKET No. 6
- Structural and functional characteristics of membrane organelles.
Membrane organelles come in two varieties: double-membrane and single-membrane. The double-membrane components are plastids, mitochondria and the cell nucleus.
Single-membrane organelles include the organelles of the vacuolar system - the endoplasmic reticulum, the Golgi complex, lysosomes, vacuoles of plant and fungal cells, pulsating vacuoles, etc.
A common property of membrane organelles is that they are all built from lipoprotein films (biological membranes), which close on themselves so that closed cavities, or compartments, are formed.
The internal contents of these compartments are always different from the hyaloplasm.
General morphofunctional characteristics and classification of cartilaginous tissues. Cellular composition cartilage tissue. The structure of hyaline, fibrous and elastic cartilage. Perichondrium. Chondrogenesis and age-related changes cartilaginous tissues.
Cartilaginous tissue (textus cartilaginus) forms articular cartilage, intervertebral discs, cartilages of the larynx, trachea, bronchi, external nose.
Cartilage tissue consists of cartilage cells (chondroblasts and chondrocytes) and dense, elastic intercellular substance.
Cartilage tissue contains about 70-80% water, 10-15% organic substances and 4-7% salts. About 50-70% of the dry matter of cartilage tissue is collagen.
The intercellular substance (matrix), produced by cartilage cells, consists of complex compounds that include proteoglycans, hyaluronic acid, and glycosaminopican molecules.
Cartilage tissue contains two types of cells: chondroblasts (from the Greek chondros - cartilage) and chondrocytes.
Chondroblasts are young round or ovoid cells capable of mitotic division.
Chondrocytes are mature large cells of cartilage tissue.
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They are round, oval or polygonal, with processes and developed organelles.
Structural and functional unit cartilage is chondron, cell-derived or isogenic group of cells, pericellular matrix and lacuna capsule.
In accordance with the structural features of cartilage tissue, three types of cartilage are distinguished: hyaline, fibrous and elastic cartilage.
Hyaline cartilage (from the Greek hyalos - glass) has a bluish color. Its main substance contains thin collagen fibers. Articular, costal cartilages and most of the cartilages of the larynx are built from hyaline cartilage.
Fibrous cartilage, the main substance of which contains large number thick collagen fibers, has increased strength.
The cells located between the collagen fibers have an elongated shape, they have a long rod-shaped nucleus and a narrow rim of basophilic cytoplasm. Fibrous rings of intervertebral discs, intra-articular discs and menisci are built from fibrous cartilage. This cartilage covers the articular surfaces of the temporomandibular and sternoclavicular joints.
Elastic cartilage is elastic and flexible.
The matrix of elastic cartilage, along with collagen, contains a large number of complexly intertwined elastic fibers. The epiglottis, wedge-shaped and corniculate cartilages of the larynx, the vocal process of the arytenoid cartilages, the cartilage of the auricle, and the cartilaginous part of the auditory tube are built from elastic cartilage.
Perichondrium (perichondrium) - a dense vascularized connective tissue membrane covering the cartilage of growing bones, costal hyaline cartilage, laryngeal cartilages, etc.
Articular cartilage lacks perichondrium. The perichondrium serves for the growth and repair of cartilage tissue. It consists of two layers - external (fibrous) and internal (chondrogenic, cambial). The fibrous layer contains fibroblasts that produce collagen fibers and passes into the surrounding connective tissue without sharp boundaries.
The chondrogenic layer contains immature chondrogenic cells and chondroblasts. During the process of ossification, the perichondrium is transformed into periosteum.
Chondrogenesis is the process of formation of cartilage tissue.
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Epithelial cells are epithelial cells. Peculiarities epithelium: 1) absence of blood vessels (exception: stria vascularis - multilayered epithelium with capillaries); nutrition - diffuse from the lower layers. 2) poor development of the intercellular substance. 3) high ability to regenerate due to cambial cells, which often divide by mitosis.
(2 types: physiological - natural renewal of the structure, reparative - the formation of new structures at the site of damage, with the formation of numerous poorly differentiated cells similar to embryonic ones) 4) polarity is expressed in the cells (basal and apical poles, the nucleus is located in the basal, and the apical - secretory granules and organelles of special importance - ciliated cilia).
5) located on the basement membrane (it is non-cellular, permeable, has an amorphous substance and fibrils). 6) the presence of intercellular contacts: desmosomes - mechanical contact, connects cells; hemidesmosomes – attaches epithelial cells to the BM; encircling desmosome - tight junction, chemically insulating; nexuses – gap junctions. 7) are always located on the border of 2 environments.
They form a layer even in cell culture.
Functions epithelium: 1) Integumentary: delimiting the body from external and internal environments, the connection between them. 2) Barrier (protective). Mechanical protection against damage, chemical influences and microorganisms. 3) Homeostatic, thermoregulation, water-salt metabolism, etc.
4) Absorption: epithelium of the gastrointestinal tract, kidneys 5) Release of metabolic products, such as urea. 6) Gas exchange: lung epithelium, skin. 7) secretory – epithelium of liver cells, secretory glands. 8) transport – movement along the surface of the mucosa.
Basement membrane. In addition to epithelia in muscle and fatty tissues.
This is a homogeneous layer (50 - 100 nm.) Underneath it is a layer of reticular fibers. BM is synthesized by epithelial cells and connective tissue cells and contains type 4 collagen. Epithelial cells are connected to the BM by semidesmosomes. Functions of BM: binding and separation of epithelium and connective tissue, providing nutrition to the epithelium, support for cells, and promotes their organization into a layer.
Classification. Morphofunctional:
Single layer: Single-row (flat, cubic, cylindrical), multi-row.
Multi-layer: Non-keratinizing (flat, transitional), Keratinizing
By location the epithelium is divided into: integumentary– covers or lines organs (digestive tube, respiratory tract) and glandular– forms the parenchyma of the glands.
Single layer epithelium. All cells with their basal parts lie on the BM.
The apical parts form a free surface.
Single layer flat The epithelium is represented in the body by mesothelium and, according to some data, by endothelium.
Mesothelium (serosa) covers the serous membranes (leaves of the pleura, visceral and parietal peritoneum, pericardial sac, etc.). Mesothelial cells - mesotheliocytes are flat, have a polygonal shape and uneven edges.
In the part where the nucleus is located in them, the cells are thicker. Some of them contain not one, but two or even three cores. There are microvilli on the free surface of the cell. Serous fluid is released and absorbed through the mesothelium.
Thanks to its smooth surface, internal organs can glide easily. The mesothelium prevents the formation of connective tissue adhesions between the organs of the abdominal and thoracic cavities, the development of which is possible if its integrity is violated. The endothelium lines blood and lymphatic vessels, as well as the chambers of the heart. It is a layer of flat cells - endothelial cells, lying in one layer on the basement membrane. Endotheliocytes are distinguished by a relative paucity of organelles and the presence of pinocytotic vesicles in the cytoplasm.
The endothelium, located in the vessels at the border with lymph and blood, participates in the exchange of substances and gases (02, CO2) between them and other tissues.
If it is damaged, a change in blood flow in the vessels and the formation of blood clots - thrombi - in their lumen are possible.
Single layer cubic epithelium (epithelium simplex cuboideum) lines part of the renal tubules (proximal and distal).
Proximal tubule cells have a brush border and basal striations. The brush border consists of a large number of microvilli . The striation is due to the presence in the basal parts of the cells of deep folds of the plasmalemma and mitochondria located between them.
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The epithelium of the renal tubules performs the function of reverse absorption (reabsorption) of a number of substances from the primary urine flowing through the tubules into the blood of the intertubular vessels.
Single layer prismatic epithelium. This type of epithelium is characteristic of the middle section of the digestive system. It lines the inner surface of the stomach, small and large intestines, gallbladder, a number of ducts of the liver and pancreas. Epithelial cells are connected to each other using desmosomes, gap communication junctions, lock-type junctions, and tight junctions (see.
Chapter IV). Thanks to the latter, the contents of the stomach, intestines and other hollow organs cannot penetrate into the intercellular gaps of the epithelium.
Sources of development of epithelial tissues. Epithelia develop from all three germ layers, starting from the 3rd-4th week of human embryonic development. Depending on the embryonic source, epithelia of ectodermal, mesodermal and endodermal origin are distinguished.
Related types of epithelia, developing from the same germ layer, under pathological conditions can undergo metaplasia, i.e. transition from one type to another, for example, in the respiratory tract, the ectodermal epithelium in chronic bronchitis from a single-layer ciliated one can turn into a multilayer flat one, which is normally characteristic of the oral cavity and is also of ectodermal origin.
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Epithelial tissue
Histology(histos - fabric, logos - teaching) - teaching on textiles. Textile is a historically established system of histological elements (cells and intercellular substance), united on the basis of the similarity of morphological characteristics, functions performed and sources of development. The process of tissue formation is called histogenesis.
Fabrics have many characteristics by which they can be distinguished from one another.
These may be features of structure, function, origin, nature of renewal, differentiation. There are various classifications of tissues, but the most common is the classification based on morphofunctional characteristics that provide the most general and significant characteristics of tissues.
In accordance with this, four types of tissues are distinguished: integumentary (epithelial), internal environment (support-trophic), muscle and nervous.
Epithelia- a group of tissues widely distributed in the body. They have different origins(their ectoderm, mesoderm and endoderm develop) and perform various functions (protective, trophic, secretory, excretory, etc.).
Epithelia are one of the most ancient types of tissue in origin. Their primary function is borderline—separating the organism from its environment.
Epithelia share common morphofunctional characteristics:
1. All types of epithelial tissues consist only of cells - epithelial cells. Between the cells there are thin intermembrane gaps in which there is no intercellular substance. They contain a supra-membrane complex - the glycocalyx; substances entering and secreted by the cells arrive here.
The cells of all epithelia are located tightly to each other, forming layers. Only in the form of layers can epithelium function.
Cells connect to each other in various ways(desmosomes, gap junctions or tight junctions).
3. Epithelia are located on a basement membrane that separates them from the underlying connective tissue. The basement membrane is 100 nm-1 µm thick and consists of proteins and carbohydrates. Blood vessels do not penetrate the epithelium, so their nutrition occurs diffusely through the basement membrane.
4. Epithelial cells have morphofunctional polarity.
They distinguish two poles: basal and apical. The nucleus of epithelial cells is shifted to the basal pole, and almost all of the cytoplasm is located at the apical pole. Cilia and microvilli may be located here.
Epithelia have a well-expressed ability to regenerate; they contain stem, cambial and differentiated cells.
Depending on the function performed, the epithelium is divided into integumentary, absorptive, excretory, secretory and others. Morphological classification divides epithelia depending on the shape of epithelial cells and the number of their layers in the layer. There are single-layer and multilayer epithelia.
The structure and distribution of single-layer epithelia in the body
Single-layer epithelia form a layer one cell thick.
If all the cells in the epithelial layer are the same height, they speak of single-layer single-row epithelium. Depending on the height of the epithelial cells, single-row epithelium is flat, cubic and cylindrical (prismatic). If the cells in a layer of single-layer epithelium are of different heights, then they speak of multirow epithelium.
Without exception, all epithelial cells of any single-layer epithelium are located on the basement membrane.
Single-layer squamous epithelium. Lines the respiratory sections of the lungs (alveoli), small gland ducts, testicular network, middle ear cavity, serous membranes (mesothelium).
Originates from mesoderm. Single-layer squamous epithelium consists of one row of cells, the height of which is less than their width, the nuclei are flattened. The mesothelium covering the serous membranes is capable of producing serous fluid and takes part in the transport of substances.
Single-layer cubic epithelium. Lines the ducts of the glands and tubules of the kidneys. All cells lie on the basement membrane. Their height is approximately equal to their width, the nuclei are round, located in the center of the cells. Has various origins.
Single-layer cylindrical (prismatic) epithelium. Lines the gastrointestinal tract, gland ducts, and collecting ducts of the kidneys.
All its cells lie on the basement membrane and have morphological polarity. Their height is much greater than their width. The cylindrical epithelium in the intestine has microvilli (brush border) at the apical pole, which increase the area of parietal digestion and absorption of nutrients. Has various origins.
Single-layer multirow ciliated (ciliated) epithelium. Lines the airways and some parts of the reproductive system (vas deferens and oviducts).
It consists of three types of cells: short intercalary, long ciliated and goblet. All cells are located in one layer on the basement membrane, but the intercalary cells do not reach the upper edge of the layer. These cells differentiate during growth and become ciliated or goblet-shaped. Ciliated cells bear a large number of cilia at the apical pole. Goblet cells produce mucus.
The structure and distribution of multilayered epithelia in the body
Multilayer epithelia are formed by several layers of cells lying on top of each other, so that only the deepest, basal layer of epithelial cells comes into contact with the basement membrane.
In it, as a rule. stem and cambial cells lie. During the process of differentiation, cells move outward. Depending on the shape of the cells of the surface layer, stratified squamous keratinizing, stratified squamous non-keratinizing and transitional epithelia are distinguished.
Stratified squamous keratinizing epithelium. Originates from the ectoderm.
Forms the surface layer of the skin - the epidermis, the final part of the rectum. It has five layers: basal, spinous, granular, shiny and horny. Basal layer consists of a single row of tall cylindrical cells, tightly connected to the basement membrane and capable of reproduction.
Layer spinosum has a thickness of 4-8 rows of spinous cells. Spine cells retain a relative ability to reproduce. Basal and spinous cells together form germinal zone. Granular layer 2-3 cells thick. Epithelial cells are flattened in shape with dense nuclei and grains of keratohyalin, stained basophilically (dark blue).
Shiny layer consists of 2-3 rows of dying cells. Keratohyalin grains merge with each other, the nuclei disintegrate, keratohyalin turns into eleidin, which is stained oxyphilic ( pink), strongly refracts light. Most superficial layer horny.
It is formed by many rows (up to 100) of flat dead cells, which are horny scales filled with the horny substance keratin. The skin with hair has a thin layer of horny scales. The stratified squamous keratinizing epithelium performs a border function and protects deep-lying tissues from external influences.
Multilayered squamous non-keratinizing (weakly keratinizing) epithelium. Originates from the ectoderm, covers the cornea of the eye, oral cavity, esophagus and part of the stomach of some animals.
It has three layers: basal, spinous and flat. Basal layer lies on the basement membrane, formed by prismatic cells with large oval nuclei, somewhat shifted to the apical pole. The cells of the basal layer divide and move upward. They lose contact with the basement membrane, differentiate and become part of the spinous layer. Layer spinosum formed by several layers of cells of irregular polygonal shape with oval or round nuclei.
The cells have small processes in the form of plates and spines that penetrate between the cells and hold them close to each other.
2 Classification, structure and functional significance of single-layer epithelium
Cells move from the stratum spinosum to the superficial layer - flat layer, 2-3 cells thick. The shape of cells and their nuclei is flattened. The connections between cells weaken, the cells die and slough off from the surface of the epithelium. In ruminants, the surface cells of this epithelium in the oral cavity, esophagus and proventriculus become keratinized.
Transitional epithelium. Originates from mesoderm. It lines the renal pelvis, ureters and bladder - organs that are subject to significant stretching when filled with urine.
Consists of three layers: basal, intermediate and integumentary. Cells basal layer small, of different shapes, are cambial, lie on the basement membrane. Intermediate layer consists of light large cells, the number of rows of which varies greatly depending on the degree of filling of the organ.
Cells cover layer very large, multinucleate or polyploid, often secrete mucus, which protects the surface of the epithelial layer from the action of urine.
Glandular epithelium
Glandular epithelium is a widespread type of epithelial tissue, the cells of which produce and secrete substances of various natures called secrets.
Glandular cells are very diverse in size, shape, and structure, as are the secretions they produce. The process of secretion formation occurs in several stages and is called secretory cycle.
First phase- accumulation of initial products by the cell.
Through the basal pole, various substances of organic and inorganic nature enter the cell, which are used in the process of secretion synthesis.
Second phase- synthesis of secretion from incoming products in the cytoplasmic reticulum. The synthesis of protein secretions occurs in the granular endoplasmic reticulum, and non-protein secretions in the agranular reticulum. Third phase- formation of secretions into granules and their accumulation in the cytoplasm of the cell. Through the cisterns of the cytoplasmic reticulum, the synthesized product enters the Golgi apparatus, where it is condensed and packaged in the form of granules, grains and vacuoles.
After this, the vacuole with a portion of the secretion is detached from the Golgi apparatus and moves to the apical pole of the cell. Fourth phase- secretion removal (extrusion).
Depending on the nature of secretion, three types of secretion are distinguished.
1. Merocrine type. The secretion is removed without violating the integrity of the cytolemma. The secretory vacuole approaches the apical pole of the cell, merges with it with its membrane, and a pore is formed through which the contents of the vacuole flow outside the cell.
Apocrine type. Partial destruction of the glandular cell occurs. Distinguish macroapocrine secretion when the apical part of the cell cytoplasm is rejected along with the secretory granule, and microapocrine secretion when the tips of the microvilli are torn off.
Holocrine type. There is complete destruction of the glandular cell and its transformation into a secretion.
Fifth phase- restoration of the original state of the glandular cell, observed with the apocrine type of secretion.
Organs are formed from the glandular epithelium, the main function of which is to produce secretions.
These organs are called glands. They are of external secretion, or exocrine, and internal secretion, or endocrine. Exocrine glands have excretory ducts that open on the surface of the body or into the cavity of a tubular organ (for example, sweat, lacrimal or salivary glands).
Endocrine glands do not have excretory ducts; their secretions are called hormones. Hormones enter directly into the blood. Endocrine glands are the thyroid gland, adrenal glands, etc.
Depending on the structure of the gland, there are unicellular (goblet cells) and multicellular.
Multicellular glands have two components: the terminal section, where secretion is produced, and the excretory duct, through which the secretion is removed from the gland. Depending on the structure of the end section, the glands are distinguished as alveolar, tubular and alveolar-tubular.
Excretory ducts can be simple or complex. Depending on the chemical composition of the secreted secretion, glands are distinguished into serous, mucous and serous-mucous.
Based on their location in the body, glands are classified into wall glands (liver, pancreas) and wall glands (gastric, uterine, etc.).
Characteristic morphological features of epithelial tissues
Epithelial tissues are a collection of polarly differentiated cells, closely adjacent to each other, located in the form of a layer on the basement membrane; they lack blood vessels and have very little or no intercellular substance.
Functions. Epithelia cover the surface of the body, secondary body cavities, the internal and external surfaces of hollow internal organs, and form the secretory sections and excretory ducts of the exocrine glands. Their main functions are: delimiting, protective, suction, secretory, excretory.
Histogenesis. Epithelial tissues develop from all three germ layers. Epithelia of ectodermal origin are predominantly multilayered, while those developing from endoderm are always single-layered. Both single-layer and multilayer epithelia develop from the mesoderm.
Classification of epithelial tissues
1. Morphofunctional classification takes into account the structural features and functions performed by one or another type of epithelium.
Based on their structure, epithelia are divided into single-layer and multilayer. The main principle of this classification is the ratio of cells to the basement membrane (Table 1). The functional specificity of single-layer epithelia is usually determined by the presence of specialized organelles. For example, in the stomach the epithelium is single-layered, prismatic, single-row glandular. The first three definitions characterize the structural features, and the last indicates that gastric epithelial cells perform secretory function. In the intestine, the epithelium is single-layered, prismatic, single-row, bordered. The presence of a brush border in epithelial cells suggests an absorptive function. In the airways, in particular in the trachea, the epithelium is single-layered, prismatic, multirow ciliated (or ciliated). It is known that cilia in in this case play a protective function. Multilayer epithelia perform protective and glandular functions.
Table 1. Comparative characteristics of single-layer and multilayer epithelia.
|
SINGLE LAYER EPITHELIAS |
MULTILAYERED EPITHELIA |
|
All epithelial cells are in contact with the basement membrane: |
Not all epithelial cells are in contact with the basement membrane: |
|
1) single-layer flat; 2) single-layer cubic (low prismatic); 3) single-layer prismatic (cylindrical, columnar) Happens: |
1) multilayer flat non-keratinizing contains three layers various cells: basal, intermediate (spinous) and superficial; 5 layers: basal, spinous, granular, shiny and horny; The basal and spinous layers constitute the germinal layer of the epithelium, since the cells of these layers are capable of division. |
Ontophylogenetic classification (according to N. G. Khlopin). This classification takes into account from which embryonic rudiment a particular epithelium developed. According to this classification, epidermal (skin), enterodermal (intestinal), coelonephrodermal, ependymoglial and angiodermal types of epithelium are distinguished.
For example, cutaneous epithelium covers the skin, lines the oral cavity, esophagus, glandular chambers of the multichamber stomach, vagina, urethra, border department anal canal; intestinal-type epithelium lines the single-chamber stomach, abomasum, and intestines; epithelium of the coelonephrodermal type lines the body cavities (mesothelium of the serous membranes), forms renal tubules; ependymoglial type of epithelium lines the ventricles of the brain and the central canal of the spinal cord; angiodermal epithelium lines the cavities of the heart and blood vessels.
Single-layer and multilayer epithelia are characterized by the presence of special organelles - desmosomes, hemidesmosomes, tonofilaments and tonofibrils. In addition, single-layer epithelia may have cilia and microvilli on the free surface of cells (see section “Cytology”).
All types of epithelia are located on the basement membrane (Fig. 7). The basement membrane consists of fibrillar structures and an amorphous matrix containing complex proteins - glycoproteins, proteoglycans and polysaccharides (glycosaminoglycans).
Rice. 7. Scheme of the structure of the basement membrane (according to Yu. K. Kotovsky).
BM – basement membrane; WITH - Light plate; T – dark plate. 1 – cytoplasm of epithelial cells; 2 – core; 3 – hemidesmosomes; 4 – keratin tonofilaments; 5 – anchor filaments; 6 – plasmalemma of epithelial cells; 7 – anchoring filaments; 8 – loose connective tissue; 9 - Hemocapillary.
The basement membrane regulates the permeability of substances (barrier and trophic function) and prevents invasion of the epithelium into the connective tissue. The glycoproteins it contains (fibronectin and laminin) promote the adhesion of epithelial cells to the membrane and induce their proliferation and differentiation during the regeneration process.
By location and function of the epithelium are divided into: superficial (cover organs from the outside and inside) and glandular (form the secretory sections and excretory ducts of the exocrine glands).
Surface epithelia are border tissues that separate the body from the external environment and participate in the exchange of substances and energy between the body and the external environment. They are located on the surface of the body (integumentary), the mucous membranes of internal organs (stomach, intestines, lungs, heart, etc.) and secondary cavities (lining).
Glandular epithelia have pronounced secretory activity. Glandular cells - glandulocytes are characterized by a polar arrangement of organelles general meaning, well-developed ER and Golgi complex, the presence of secretory granules in the cytoplasm.
The process of functional activity of a glandular cell associated with the formation, accumulation and release of secretions beyond its boundaries, as well as the restoration of the cell after secretion is released, is called Secretory cycle.
During the secretory cycle, initial products (water, various inorganic substances and low molecular weight organic compounds: amino acids, monosaccharides, fatty acids etc.), from which, with the participation of organelles of general importance, a secret is synthesized and accumulated in cells, and then, through exocytosis, is released into the external ( Exocrine glands ) or internal ( Endocrine glands ) Wednesday.
Secretion is released (extrusion) by diffusion or in the form of granules, but can also be by converting the entire cell into a common secretory mass.
Regulation of the secretory cycle is carried out with the participation of humoral and nervous mechanisms.
Epithelial regeneration
Various types of epithelium are characterized by high regenerative activity. It is carried out due to the cambial elements, which divide by mitosis, constantly replenishing the loss of worn-out cells. Glandular cells, which secrete according to the merocrine and apocrine type, are, in addition, capable of maintaining their vital functions not only through reproduction, but also due to intracellular regeneration. In holocrine glands, constantly dying glandulocytes are replaced during the secretory cycle due to the division of stem cells located on the basement membrane (cellular regeneration).
Animal epithelial tissue forms single-layer or multilayer layers covering the internal and external surfaces of any organism.
Epithelial cells connected with each other a small amount cementing substance, consisting mainly of carbohydrates, and special binders - intercellular contacts. The epithelium is underlain by a basement membrane consisting of interwoven collagen fibers enclosed in a matrix. The term membrane should not be confused with cell membranes, which we discussed in Chapter. 5; here it simply means a thin layer. The matrix does not interfere with diffusion. Since epithelial cells are not supplied by blood vessels, oxygen and nutrients reach them by diffusion from lymphatic vessels located in the intercellular spaces. Nerve endings can penetrate the epithelium.
Function of epithelial tissue is to protect underlying structures from mechanical damage and infection. At constant mechanical influences this tissue thickens and the keratin is shed, and in those areas where the cells are sloughed off due to constant pressure or friction, cell division occurs at a very high rate, so that lost cells are quickly replaced. The free surface of the epithelium is often highly differentiated and performs absorption, secretory or excretory functions, or contains sensory cells and nerve endings specialized to perceive irritations.
Epithelial tissue divides into several types depending on the number of cell layers and the shape of individual cells. In many parts of the body, cells of different types are mixed together, and then epithelial tissue can be difficult to classify as any specific type.
Simple epithelia
Flat epithelium
Squamous epithelial cells thin and flattened. They are so flattened that the core forms a bulge. The edges of the cells are uneven.
As can be clearly seen in the image cell surface. Neighboring cells are tightly connected to each other by special contacts. Flat epithelium is present in Bowman's capsules of the kidneys, in the lining of the alveoli of the lungs and in the walls of capillaries, where, due to its thinness, it allows the diffusion of various substances. It also forms the lining of hollow structures such as blood vessels and heart chambers, where it reduces friction when fluids flow.
Cuboidal epithelium
It is the least specialized of all epithelia. His cells have a cubic shape and contain a centrally located spherical core. If you look at these cells from above, you can see that they have a pentagonal or hexagonal outline. Cuboidal epithelium lines the ducts of many glands, such as the salivary glands and pancreas, as well as the proximal and distal renal tubules and collecting ducts of the kidney in areas where they are not secretory.
Cuboidal epithelium It is also found in many glands - salivary, mucous, sweat, thyroid - where it performs secretory functions.
Columnar epithelium
Cells of this epithelium tall and rather narrow; due to this shape, there is more cytoplasm per unit area of the epithelium. Each cell has a nucleus located at its basal end. Goblet cells are often scattered among the epithelial cells; According to its functions, the cylindrical epithelium can be secretory and (or) absorptive. Often on the free surface of each cell there is a well-defined brush border formed by microvilli, which increase the absorptive and secretory surface of the cell. Columnar epithelium lines the stomach; mucus secreted by goblet cells protects the gastric mucosa from the effects of acidic contents and from digestion by enzymes. It also lines the intestines, where again mucus protects the intestinal walls from self-digestion and at the same time creates a lubricant that facilitates the passage of food. IN small intestine Digested food is absorbed through this epithelium into the bloodstream. Columnar epithelium lines and protects many of the renal tubules; it is also available in thyroid gland and gallbladder.
Ciliated epithelium
Cells of this epithelium usually have a cylindrical shape, but bear numerous cilia on their free surfaces. They are always associated with goblet cells that secrete mucus that flows due to the beating of cilia. The ciliated epithelium lines the inside of the oviducts, ventricles of the brain, the spinal canal and the respiratory tract (trachea, bronchi and bronchioles), ensuring the movement of various substances through them. For example, in the inhalation tract, cilia move mucus up into the throat, which makes it easier to swallow solid food. Mucus traps bacteria, dust and other fine particles, preventing them from entering the lungs.
Pseudostratified (multi-row) epithelium
When examining histological sections of this epithelium it seems that the cell nuclei lie at different levels, because not all cells reach the free surface of the tissue. However, this epithelium consists of only a single layer of cells, each of which is attached to a basement membrane. Pseudostratified epithelium lines urinary tract and the respiratory tract (trachea, bronchi, bronchioles, where it is covered with cilia and consists of cylindrical cells).
