Medical Biology. Epithelial tissue

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1 Lec: five Dr:Fadia Al-kahyat Medical Biology Epithelial tissue The human body consists of many interacting systems. Each system contributes to the maintenance of homeostasis, of itself, other systems, and the entire body. A system consists of two or more organs, which are functional collections of tissue. Each tissue composed from cells, extracellular substances. Although all cells of the body share common structures such as nuclei, centrosomes, Golgi apparatus...etc., they differ in size, shape and function. The human body has approximately 5x10 cells. Histology: The science which deals with form, structure and function of tissues. Histology bridges the gap between cytology and gross anatomy and provides the bases for understanding system physiology. Tissue: A group of closely associated cells that are similar in structure and perform a common or related function. Primary tissues: four basic tissues interweave to form the "fabric" of the body. These are: Epithelial tissues. Connective tissues. Muscular tissues. Nervous tissues. Epithelial tissue or epithelium is a sheet of cells that covers a body surface, lines a body cavity or form glands. Location of Epithelia: Epithelia are found in form of sheets or membranes lining or covering surfaces and also present as masses in many glands Epithelia perform the following functions: protection, absorption, filtration, excretion, secretion and sensation Characteristics of Epithelia: Epithelial tissues have the following characteristics that distinguish them from other tissue types 1-Cellularity: Epithelial tissue is composed of closely packed cells with a tiny amount of extracellular material lies in the narrow spaces between cells. 2-Specialized contacts: Epithelial cells fit close together at many points to form a continuous sheet. Adjacent cells are bound together at many points by lateral contacts such as tight junctions, desmosomes...etc. 1 13

2 3-Polarity: All epithelia have a free surface (apical surface) "-and an attached surface (basal surface). All epithelia exhibit polarity, meaning that cells near the apical surface differ from those at the basal surface. Although some apical surfaces are smooth and slick, most have microvilli (fingerlike extensions of plasma membrane which increase the exposed area). In epithelia that absorb or secret substances (lining the intestine or kidney tubules), the microvilli are often so dense that the cell apices have a fuzzy appearance called a brush border. Some epithelia, such as those lining the trachea, have motile cilia that propel substances along their free surfaces. 4-Innervated but avascular: Although epithelium is innervated (supplied by nerve fibers), it is avascular (contains no blood vessels). Epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue. 5-Rest on a basal lamina: A thin supporting sheet called the basal separates the epithelium from the connective tissue deep to it. Basal lamina is noncellular adhesive sheet consisting largely of glycoproteins secreted by the epithelial cells. Directly deep to the basal lamina, is the reticular lamina which is a similar layer of extracellular material containing a fine network of collagen protein fibers. The two lamina (basal lamina and reticular lamina) form the basement membrane. The basal lamina acts as a selective barrier between connective tissue and other cells, contains information necessary for cell-cell interaction and orients the epithelial cell location of movement. 6-Regeneration: Epithelium has a high regenerative capacity. Some epithelia are exposed to friction and their surface cells are removed by abrasion. Others are damaged by hostile substances in the external environment (bacteria, acids, smoke... etc.). As long as epithelial cells receive adequate nutrition, they can replace lost cells rapidly by mitosis. Classification of Epithelia: Epithelial tissues are classified according to the number of their layers into: simple, stratified and pseudostratified. Simple tissues are composed of only one layer of cells. Stratified tissues are composed of two or more layers of cells. Pseudostratified tissues present a superficial stratification appearance. Epithelial tissues are also classified according to the shape of the cells (in simple tissue) and shape of apical cells (in stratified tissues) as: squamous (flattened and scale like), cuboidal (approximately as tall as they are wide) and columnar (tall and column shaped). By mixing the above criteria of classification, epithelial tissues are classified into the following groups: 1-Simple E.P ( squamous, cuboidal and columnar) 2-Pseudostratified (ciliated, plain) 3-Stratifid E.P (squamous, cuboidal, columnar and transitional) 2

3 Simple Squamous Epithelial Tissue The cells are very thin, flat and irregular in outline, with rounded, central nuclei and wavy edges. In longitudinal section, cells are spindle in shape with oval central nuclei Located in air sacks of the lungs, Bowman's capsule (kidney glomeruli), endothelium that lines blood vessels and lymphatic vessels and mesothelium that lines the peritoneal cavity (serosa). Allows passage of materials by diffusion and filtration in sites where protection is not important; secretes lubricating substances in serosa. Simple Cuboidal Epithelial Tissue The cells are block like in cross section and hexagonal from a surface view with spherical central nuclei Found in many glands such as thyroid gland, salivary gland, pancreas, ovary surface, collecting tubes of the kidney and capsule surrounding the eye lens. Functions as secretion and absorption. 3

4 Simple Columnar Epithelial Tissue Cells are elongated between their apical and basal surfaces with oval basal nuclei. When the free surfaces are observed, the cells have a prolonged configuration. Some secretory cells (goblet cells) may be present. They produce mucus (a protective glycoprotein). The cells of this tissue may have cilia on their free surfaces and hence they are known as ciliated. In other cases,-free surfaces have no cilia and hence they are known as plain or non-ciliated. Another modification of the free surface of the columnar cells is the presence of microvilli. These structures are seen as brush border when observed with light microscope under oil immersion. Columnar cells that line the small intestine and make up the wall of kidney collecting tubules exhibit distinct brush border. Function: Absorption, secretion of mucus enzymes and other substances; ciliated type propels mucus (or reproductive cells) by cilia action. Pseudostratified Columnar Epithelial Tissue Cells are varied in height. All cells rest on the basal lamina, but only the tallest reach the apical surface of the epithelium. Cell nuclei are located at different levels above the basal lamina, giving the false(pseudo) impression that several cell layers are present also Goblet cells are found. Two types are present: Plain type is found in parts of the male urethra and parotid gland. The ciliated type is more common and found in trachea, bronchi, auditory tube and part of middle ear. Function: secretion, particularly of mucus 4

5 Stratified epithelia Stratified epithelia consist of two (or more) cell layers: basal layer, (intermediate layer or layers) and superficial or apical layer. The basal cells divide by mitosis and push apically to replace the older apical cells. The nomination of stratified epithelia depends on shape of the cells in the apical layer. The following tissues are known: - Stratified squamous epithelial tissues (non-keratinized and keratinized) Stratified cuboidal epithelial tissue Stratified columnar epithelial tissue Transitional epithelial tissue The following is a brief account on description, function and location of the different stratified epithelia. Stratified Squamous Epithelial Tissues These include non-keratinized and keratinized types. a) Non-keratinized Stratified Squamous Epithelial Tissue composed of several layers: Basal cells are cuboidal or columnar and metabolically active, intermediate cells are polyhedral and the apical cells are flattened (squamous). Location: Forms the moist linings of the mouth, tongue, pharynx, esophagus, vagina and cornea. Function: Protects underlying tissues in areas subjected to abrasion. b) Keratinized Stratified Squamous Epithelial Tissue The keratinized epithelium is a dry membrane represented by the epidermis of the skin, which is keratinized (its surface cells contain keratin, a tough protective protein). The epidermis consists of an outer region (many layers of scaly remains of dead epithelial cells known as stratum corneum), thin translucent layer (stratum lucidum), a darkly stained layer (stratum granulosum) and a mulitlayerd stratum spinosum. All these four layers of epidermis originate from the deepest layer of cells of the stratum spinosum. which is known as stratum basale or stratum germinativum. Function: Protects underlying tissue in area subjected to abrasion. 5

6 Non-keratinized S.S.E Keratinized S.S.E the epithelial cells lack protein keratin serves as a wet lining capable of withstanding moderate abrasive stresses. the epithelial cells contain keratin serves as a dry covering capable of withstanding severe abrasive stresses. epithelium is bathed by mucus the surface is free of mucus and dry 6

7 Stratified Cuboidal Epithelial Tissue This consists of two layers of cuboidal cells. Location: Large ducts of sweat glands, mammary glands and Salivary glands. Function: Protection Cuboidal layers nucleus lumen Stratified Columnar Epithelial Tissue This consists of several cell layers. The basal layer is usually cuboidal while the superficial columnar cells are variable in height. Location: Rare in the body; small amount in male urethra and large ducts of some glands. Function: Protection and secretion 7

8 Transitional Epithelial Tissue This tissue contains many layers of cells; the basal layer consists of cuboidal or columnar. The apical cells vary in appearance, depending on the degree of distension of the organ. When the organ is not stretched, the membrane is many layered and the superficial cells are rounded and dome like.when the organ is distended with urine, the epithelium thins (undergoes a transition) from about six cell layers to three, and it's apical cells flatten and become squamous like. The ability of transitional cells to change their shape allows a greater volume of urine to flow through a tube like organ. Location: Lines the ureter, urinary bladder and part of the urethra. Function: Stretches readily and permits distension of urinary organ by contained urine. Glandular epithelium A gland consists of one or more cells that make and secrete a particular product. This product, called a secretion, is an water-based fluid that usually contains proteins. The term secretion can refer to the gland's product and the process of making and releasing that product. Through the embryonic development, glands develop by an invagination of the epithelial cells into connective tissue underlying an epithelial membrane. In exocrine glands (glands that secret their products through a duct), the site of the original invagination persists as the duct system, while in endocrine (ductless) glands; connection with the epithelial membrane is lost, so the secretion passes into a vascular system. Classification of Glands Glands can be classified according to the following criteria: 1- According to the number of their cells: - 8

9 A-Unicellular glands: Glands composed of one cell such as the goblet cells that scattered between simple epithelial cells or pseudostratified cells. Goblet cell is a cupshaped with flattened basal nucleus. It also has abundant rough endoplasmic reticulum and large Golgi apparatus.goblet cells produce mucin (a complex glycoprotein). Upon dissolving in water after its secretion, mucin forms mucus which is a slimy coating that protects and lubricates surfaces. B-Multicellular glands: Glands which are composed of many cells as in the majority of glands in human body. 2- According to the route of their product: Glands are either endocrine or exocrine. Endocrine glands: Also known as ductless glands. As these glands lose their ducts, they produce hormones directly into the blood or lymph. These include thyroid, adrenal...etc. Exocrine glands: These glands are composed of epithelial-derived duct and a secretory unit consisting of secretory cells. They secrete their products through their ducts onto body surface or into body cavity. These include sweat glands, sebaceous gland, salivary glands... etc. 3-According to structure of their ducts: compound. Exocrine glands are either simple or - Simple glands: Glands with single unbranched ducts as in sweat and sebaceous glands. - Compound glands: Glands with branched duct as in salivary and mammary glands 4-According to the structure of their secretory parts: The secretory-cells form one of the following structures :- 1-Tubular: The secretory cells form a tube. 2-Alveolar (acinar): The secretory cells form small, flask-like sac. 3-Tubuloalveolar: The glands contain both tubular and alveolar secretory units. 5- According to the structure of their ducts and secretory parts: The terms denoting duct and secretory part structure are combined to describe the whole gland in the following way Simple tubular Simple coiled tubular Simple branched tubular Simple alveolar Simple branched alveolar Compound tubular Compound alveolar Compound Tubuloalveolar 9

10 6- According to their mode of secretion: Glands are either: - A-Merocrine glands: Glands which secrete their products by exocytosis as they are produced. The secretory cells are not altered. These include pancreas, most sweat glands and salivary glands. B- Holocrine glands: Glands accumulate their products within them until they rupture. These cells are replaced by the division of the underlying cells as in the sebaceous glands. C- Apocrine glands: Glands of this type also accumulate their products, but in this case accumulation occurs only just beneath the free surface. Eventually, the apex of the cell pinches off and secretion is released. The cell repairs its damage and repeats the process again and again, as in the mammary glands. According to the nature of their secretion: Glands fall in three different types in this respect. 11

11 Serous cell glands: Serous alveoli are composed of serous cells which are pyramidal cells arranged around a small visible lumen. These cells have a small, round nucleus located in the basal zone of deeply basophilic cytoplasm and a lighter apical cytoplasm. Secretory granules (zymogen) are seen in the apical area. Their secretion is watery filled with zymogen. Their numbers vary with the state of cell activity. Myoepithelial (basket) cells lie between the basement lamina and the epithelial cells; usually the nucleus of basket cell is visible. Mucous cell glands: Mucous alveoli are larger than serous alveoli and more variable in size and shape. They are composed of often more columnar form of mucous cells,.with pale (almost colorless staining reaction), oval or flattened nuclei at the bases of the cells and somewhat larger, more apparent lumen. Sero-mucous cell glands (Mixed glands): Mixed alveoli are typically mucous alveoli surrounded by one or more groups of serous cells known as serous demilune or serous crescent. Basket cells are present, especially around the serous cells. In addition to the glandular epithelia, two other types of epithelial cells are known: - Myoepithelial cells: Branched cells that contain myosin and large number of actin microfilaments. They are specialized for contraction, mainly of the alveoli of mammary, sweat and salivary glands. Neuroepithelial cells: Cells of epithelial origin with specialized sensory function due to the presence of nerve endings in these cells as in the case of taste buds in the tongue. 11