Understanding Epithelial Tissue: Function and Classification

1. Lecture 7 GenMed_2nd semester • Epithelial tissue – definition, classificationandhistogenesis • Overview of covering and glandularepithelia. Characteristics of glandular cells • Absorptive, respiratory, and sensory epithelia

2. Epithelialtissue is composed of cells that are in close apposition with one another; among cells onlya small amount of intercellular substanceis 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

3. Classification of the epithelial tissue a) according to the arrangement of cells -2 forms - epithelial membraneseither 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

4. Coveringepithelia /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 mostlypolarized 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)

5. 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 bothare further subdivided according to the shape of cells: the component cells may be squamous - the width and depth of the cell is greaterthan its height; cuboidal - the width, depth and height are approximately thesame; and columnar - the height of the cell appreciably exceeds the width anddepth the classification based on both criteria is:

6. Simple epithelia: - simple squamous epithelium - simple cuboidal epithelium - simple columnar epithelium - pseudostratified columnar epithelium Stratified epithelia: - stratified squamous epithelium - stratified columnar epithelium - transitional epithelium

7. 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 preparationsbut may bedemonstrated 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 cavitiesand is deriving from the mesoderm and theendothelium- lines the lumina of blood and lymph vessels and arises from themesenchyme

9. Simple cuboidal epithelium cells of cuboidal epithelium are smaller in surface view than those of thesimple 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 ofeach 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 toselect a region, in which the section passes through the tissue in a planeperpendicular 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

10. simple columnar epithelium with striated border (microvilli)

11. a simple columnar ciliated epithelium (with kinocilia)

12. Pseudostratified columnar epithelium all cells are in contact with the basement membrane but only a part of them extendsto the free surface in pseudostratified epithelium cells are variable inshape 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

13. 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

15. stratified squamous epithelium stratified squamous epithelium (cornified) = epidermis

16. 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 orless 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 freesurface 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 cellsshow 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 organcontraction 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

17. transitional epithelium

18. 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 providing of transport of macromolecules across the plasma membrane

19. synthesis of secretory products undercooperationofmany cell organelles - free ribosomes or unbounded polyribosomes and the rough endoplasmic reticulum synthesized products are then transferred from this compartment to the Golgi apparatus, where they become final form and are directed to their final destination in each cell synthetic processes need an energy - is produced by mitochondria all the mentioned cell organelles are typical of glandular cells release or extrusion of secretory products three modeshavebeendistinguished: - merocrine secretion - apocrine secretion - holocrinesecretion

20. Release of secretion products • merocrine secretion • products are released through the cell membrane without the loss any cytoplasm (during the whole secretory cycle) • the cell does not change its shape • goblet cells

21. apocrine secretion • by a loss of the apical parts that containsan accumulated secretory material • glandular cells of follicles in thyroid • apocrine sweet glands

22. holocrine secretion the discharge of product is associated with a destruction of the whole glandular cells cells of sebaceous glands

23. secretory cells constitute special organs called glands Classification of glands glands are classified in several different ways: a) with respect to the mode of secretion: exocrine glands - discharge their product via ducts at an external or internal surface, endocrine glands release their product into the blood or lymph vessels have no ducts (ductless glands) secretions are called hormones and regulate and coordinate the activities of all the cells in the body in cooperation with the nervous system b) with respect to the number of glandular cells: unicellular glands- the glandular components consist of a single cell among other cells that are not glandular multicellular glands - the glandular components consist of more than one cell

24. goblet cells Exocrine glands or glands of external secretion retain connections with surfaces exocrine glands occur as uni - and multicellular ones unicellular glandsare - mucous cells or goblet cells and - enteroendocrine (GEP) cells goblet cells are scattered among the columnar cells of the simple columnar or pseudostratified columnar epithelia cells secrete mucin each cell has an expanded, oval apex filled with pale droplets of mucigen and a slender basal end containing a compressed nucleus and a small amount of deeply staining basophilic cytoplasm enteroendocrine (GEP) cells occur in the wall of the stomach, small and large intestine, bile ducts (hepatic, cystic or common bile) and islets of Langerhans produce catechol amines or peptide hormones; for visualization of endocrine cells were formerly used impregnation techniques, recently is used TEM or immunohistochemistry

25. multicellular glands intraepithelial glands are relative rare and are composed of small accumulations of glandular cells that lie wholly within the thickness of an epithelial membrane chief distribution: mucous glands of lacunae urethrales Morgagni in the male urethra extraepithelial glands are always in the connective tissue of respective organ or often more deeper glnds develop as invaginations of the epithelial membrane into the underlying tissues the extraepithelial glands consist of secretory units and ducts secretory units are of three types: • tubules (unit is shaped like a tube) • alveoli (acini) (unit is shaped like a flask) • tuboalveolar portions (combination of the tube and alveolus which is located at one end of the tube) ducts may be short and unbranched - simple glands or ducts are long a branched - compound glands the shape of secretory units and the length of ducts are used in the anatomical classification of glands as follows

26. Multicellular glands

27. besides the anatomical classification of glands it may be taken in account the chemical properties of secreted product the secretory units are divided into • mucous units - the shape of straight tubules with a small lumina cells are usually of cuboidal form or of truncated pyramids, nuclei are usually small, dark, and are flattened against the plasma membrane of the cells the cytoplasm stains light in HE preparation and may have a foamy appearance (alcian blue, mucicarmine, and the PAS reaction are used to demonstrate mucous tubules ) • serous units- are the form of alveoli and are composed of pyramidal cells whose nuclei are located centrally secretion granules are more or less acidophilic and are primarily protein in character the cytoplasm always stains in pink or pinkish purple, more darkly than that of mucous cells the lumen of alveoli (acini) is definite and and smaller in diameter than that of mucous tubules • seromucous units - mixed units are composed of both mucous and serous cells a mixed unit is composed of mucous cells lining a tubular part (short tubule) to which one end is added a collection of serous cells arranged in a half-moon fashion these serous cells collections are called as demilune complexes or lunulae of Giannuzzi in a great number they are occurred in the human submandibular gland

28. acinar / alveoli /

29. tubuloacinar /tubules with serous demilunes/

30. tubule with serous demilune= demilune of Gianuzzi

31. Absorptiveepithelium cellswhoseapices are providedwithmicrovilli • striatedborder(enterocytes) • brushborder(cellsofproximal tubule ofthenephron)

32. Respiratoryepithelium isextremelythinandlinesalveoli • membranouspneumocytes (type I) • granularpneumocytes (type II)

33. Sensory epithelium sensory organs sensory areas (fields) • supporting cells • sensory cells - receipt stimuli from external environment, stimuli induce a special status in their bodies -irritation sensory cells are of two types: a) primary sensory cells(neuroepithelial cells or unipolar neurons) they consist of receptor area (can be photosensitive or registrate odoriferous substances), cell body and axon or neurite that conducts impulses to the next neuron in the human - olfactory cells in the olfactory epithelium and by rod cells and cone cells in the retina b) secondary sensory cells are only modified cells of covering epithelia cells have spindle-like or flask-like form and consist of only receptor area and body the process corresponding to the axon is always missing (signals from cells are conducted via dendritic processes of sensory nerves that endon the surface of these cells) cells of taste buds, and hair cells in the membranous labyrinth

34. cone cell rod cell

35. taste bud with taste cells hair cells (utricle)