Lecture 7 GenMed_2nd semester Epithelial tissue – definition, classification and histogenesis Overview of covering and glandular epithelia. Characteristics of glandular cells Absorptive, respiratory, and sensory epithelia Epithelial tissue is composed of cells that are in close apposition with one another; among cells only a small amount of intercellular substance is present epithelial cells are usually of regular form without extensive cytoplasmic processes adhesion between cells is very strong epithelia derive from the all germ layers epithelial tissue is avascular - it contains no blood capillaries it exhibits a remarkable degree of physiologic regeneration Classification of the epithelial tissue a) according to the arrangement of cells -2 forms - epithelial membranes either sheets - composed of one or more cell layers in thickness or - solid cords or tubules (rarely follicles) that have developed as out- growths from an epithelial sheet cords and tubulus especially occur in glands b) according to the function of cells covering or protective - cells cover external and internal surfaces of human body and protect underlying tissues against loss moisture and mechanical damage, secretory or glandular - cells are engaged in synthetic processes and product substances with defined functional destination, absorptive - cells transport substances from the alimentary canal and renal tubules into the systemic circulation, respiratory - cells take part in the transport of oxygen and carbon dioxide from alveoli into the blood, sensory - cells are specialized for reception of stimuli (olfactory epithelium, taste buds etc.) a given epithelium may serve one or more functions, depending upon the cell types presented, eg. protective and secretory functions of the epithelium in the stomach or protective and absorptive Covering epithelia /epithelial membranes/ common features: cells always rest upon a basement membrane it attaches them to underlying connective tissue the basemenet membrane is thin and composed of delicate reticular fibers that are embedded in a gel-like mucopolysaccharide substance in ordinary staining procedures (eg. using hematoxyline and eosin), the membrane is not visible but it stains with PAS technique epithelial cells are mostly polarized lateral and apical aspects of cells are subjects of various modifications: microvilli, motile cilia interdigitations and intercellular junctions spot and belt desmosome, gap junctions and occluding junctions) two types: simple and stratified epithelia the simple epithelia consist of single layer of cells, all of which are in contact with the basement membrane the stratified epithelia are composed of two or more layers of cells, of which only basal is in contact with the basement membrane and remainder layer superimposed one upon other the both are further subdivided according to the shape of cells: the component cells may be squamous - the width and depth of the cell is greater than its height; cuboidal - the width, depth and height are approximately the same; and columnar - the height of the cell appreciably exceeds the width and depth the classification based on both criteria is: Simple epithelia: - simple squamous epithelium - simple cuboidal epithelium - simple columnar epithelium - pseudostratified columnar epithelium Stratified epithelia: - stratified squamous epithelium - stratified columnar epithelium - transitional epithelium Simple squamous epithelium cells are flattened viewed from the surface they appear as fairly large cells with clear cytoplasm and an oval nucleus; cell boundaries are not visible in ordinary preparations but may be demonstrated by the use of impregnation technique in sections the cytoplasm is barely visible, but there is an enlargement of the cell at center where the nucleus is located chief distribution: the parietal layer of capsule of Bowman and descending limb of the loop of Henle in the kidney inner surface of the wall of the membranous labyrinth and inner surface of the tympanic membrane the smallest excretory ducts of many glands in two locations, simple squamous epithelium is modified and has special functions there is a mesothelium - lines serous cavities and is deriving from the mesoderm and the endothelium - lines the lumina of blood and lymph vessels and arises from the mesenchyme Simple cuboidal epithelium cells of cuboidal epithelium are smaller in surface view than those of the simple squamous type and are roughly hexagonal cell boundaries are often clearly visible in vertical sections the cells are square with a spherical nucleus in the center of each the square shape is modified to that of truncated prism when cells line small ducts chief distribution: free surface of the ovary, the choroid plexus surface, the inner surface of the lens, the excretory ducts of many glands Simple columnar epithelium the surface view of columnar epithelium is like that of the cuboidal epithelium in sections, the cells are seen to be taller than they are broad; their nucleus is near the base of the cell as in the case of the cuboidal epithelium, the rectangular shape of columnar cells is changed to pyramidal when they are grouped around small lumen in order to right diagnosis of this epithelium it is important to select a region, in which the section passes through the tissue in a plane perpendicular to the surface chief distribution: the stomach, larger excretory ducts, and uterus; in the case of oviduct and small bronchi, apices of some epithelial cells are ciliated simple columnar epithelium with striated border (microvilli) a simple columnar ciliated epithelium (with kinocilia) Pseudostratified columnar epithelium all cells are in contact with the basement membrane but only a part of them extends to the free surface in pseudostratified epithelium cells are variable in shape and their nuclei lie at different levels; according to cell height, three different cell types are distinguished: small basal or supporting cells spindle-shaped cells tall superficial cells that are connected with the basement membrane via slender processes, apices of tall superficial cells are often ciliated goblet cells producing mucous secretion are usually present in this epithelium chief distribution: the respiratory passages Stratified epithelia small cuboidal or columnar cells lie next to the basement membrane above them one or more layers of polygonal cells are the free surface is covered by a layer of cells that are important for diagmosis Stratified squamous epithelium the thickness of the epithelium and number of cells vary in different parts of the body the shape and arrangement of component cells, however, follow the same general plan: consists of small cuboidal or even columnar cells are are covered by a varying number of layers of more or less polygonal (polyhedral) cells they gradually begin to flatten out toward to the epithelium surface they become scale-like form, their nuclei are pyknotic but they may be yet considered as viable such epithelium is found in the mouth, the esophagus, the vagina, and on the surface of the cornea on the exposed outer surfaces, the stratified squamous epithelium is modified as cornified or keratinized the surface cells loose their nuclei and undergo a transformation into dry and dead scales the keratinized stratified squamous epithelium covers the skin as a epidermis stratified squamous epithelium stratified squamous epithelium (cornified) = epidermis Stratified columnar epithelium it differs from the pseudostratified epithelium in having a continuous layer of small, rounded cells next to the basement membrane, a varying number (3-4) of more or less polyhedral cells and by presence of superficial cells that are tall and prismatic the epithelium is truly stratified chief distribution: the fornix of the conjunctiva, the cavernous part of the male urethra, large excretory ducts of some glands Transitional epithelium it consists of several layers of cells the basal cells are like those of stratified columnar epithelium above them there is a varying number of layers of polygonal or pear-shaped cells, the cell layer at the free surface is composed of large, rather flattened cells, usually described as dome shaped one of dome shaped cell often covers two or three pear-shaped cells that underlie it cells show unusual degree the ability to change their position, sliding over each other when an organ is distended, the epithelium is reduced to three or four layers and vice versa (by organ contraction the cells heap up forming several layers between the basal and surface cells) chief distribution: it covers the mucosa of the excretory passages from the renal calyces to the urethra transitional epithelium Secretory /glandular/epithelium the epithelium whose cells are able to synthesize special products - secretions substances that are for organism and right function of many organs very important in many cases they cannot be substituted secretory products are released either upon free surfaces or into blood (rarely lymphatic) vessels for distribution /hormones/ Histophysiology of secretion secretory process involves 3 phases as follows: ingestion of single substances or precursors from the surrounding or blood synthesis of secretory products including their segregation and accumulation release or extrusion of secretory products Ingestion of precursors is usually realized by 3 ways: - passive diffusion down a concentration gradient, - active transport against a concentration gradient using ATP as an energy source, - pinocytosis - the way