Epithelial Tissue PDF

Epithelial Tissue D/ sahar fayez  Definition The epithelium is one of the four basic tissues forming the body; the others being called connective, muscular and nervous tissues. It consists of a sheet of aggregated cells of similar type that cove...

Epithelial Tissue D/ sahar fayez  Definition The epithelium is one of the four basic tissues forming the body; the others being called connective, muscular and nervous tissues. It consists of a sheet of aggregated cells of similar type that covers body surfaces, lines hollow organs or modified to form glands. D/ sahar 2  General Features 1. May originate from any of the three embryonic germ layers (ectoderm, mesoderm and endoderm). 2. Composed of large number of similar cells with very little intercellular substances (very close to each other) adhere to one another by specialized cell junctions D/ sahar 3  3- All epithelia have one free surface, called the apical surface, which is exposed at the body surface or at the lumen of the body cavity, duct, tube or vessel. 4. Separated from the underlying connective tissue by a basement membrane. D/ sahar 4  5. It is avascular, blood and lymph vessels do not penetrate the basement membrane into the epithelium. Thus, it receives its nutritional support by diffusion. Even though it is highly innervated. 6. Has high capacity for regeneration (renew). D/ sahar 5  7.Can change from one form to another in a process called metaplasia. 8. It performs many functions such as protection, secretion, excretion, absorption, sensory reception and contraction. D/ sahar 6 D/ sahar 7  The epithelial tissue is classified into four types. 1- Epithelial membranes 2- Glandular epithelium 3- Myoepithelium 4- Neuroepithelium D/ sahar 8  I. Epithelial Membranes CLASSIFICATION 1) According to the number of cell layers, the epithelial membranes are classified into: A. Simple Epithelium: Consists of a single layer of cells. B. Compound or Stratified Epithelium: Consists of two or more than two layers of cell. D/ sahar 9 D/ sahar 10  2- According to the shape of the most superficial cell layer A. Squamous; scale-like. B. Cuboidal; cub-like. C. Columnar; pillar-like. D/ sahar 11  A. Simple epithelium Consists of single (one) layer of epithelial cells resting on basement membrane 1. Simple Squamous Epithelium It consists of a single layer of thin flat, scale- like cells D/ sahar 12 - On surface view, the cells have an irregular shape with a slightly serrated border. Each cell has a centrally located spherical or oval nucleus. - In a side view, the cells are so flat that they can only be recognized by their elongated nuclei that bulge into the lumen. The cytoplasm is scanty and has sparse organelles. D/ sahar 13  Location Lung alveoli glomerular capsule of the kidneys. Lines the blood and lymph vessels (endothelium) Lines the serous membranes (peritoneum, pleura and pericardium) and is called mesothelium. Lines the subarachnoid and subdural spaces (mesenchymal epithelium), the anterior chamber of the eye and perilymphatic spaces of the ear. D/ sahar 14 D/ sahar 15 D/ sahar 16  2. Simple Cuboidal Epithelium It consists of a single layer of cube-like cells whose width and heights are nearly equal. In cross sections, the cells are squares with spherical centrally located nuclei. It is usually located in organs that have secretory or absorptive functions. D/ sahar 17  Locations It is found in thyroid follicles, glands' ducts and kidney tubules D/ sahar 18 D/ sahar 19  3. Simple Columnar Epithelium It consists of a single layer of tall, narrow cells having greater height than width (column or pillar-like). The nuclei are oval and are located near the base of the cells. D/ sahar 20  Locations Simple columnar secretory is found in stomach. Ciliated columnar epithelium is found in oviducts and bronchioles. Found in small intestine and gall bladder with absorptive function D/ sahar 21 D/ sahar 22 D/ sahar 23  4. Pseudostratified Columnar Epithelium It is composed of single layer of irregular cells. All the cells rest on the basement membrane but not all the cells reach to the luminal surface. The nuclei are located at different levels, thus creating the false impression of cellular stratification. D/ sahar 24 - The cells reaching the surface are either ciliated or goblet cells. The short cell acts as progenitors for the tall cells. The pseudostratified columnar ciliated epithelium may be differentiated from the stratified epithelium by: 1. Cilia are never existing on stratified epithelium. 2. The apical cytoplasm of its cell does not contain nuclei. D/ sahar 25  Locations Non-ciliated pseudostratified columnar epithelium is found in some parts of the auditory tube, and the male urethra. Ciliated pseudostratified columnar epithelium is seen in the trachea. Pseudostratified columnar epithelium with stereocilia (long microvilli) is seen in epididymis. D/ sahar 26 D/ sahar 27 D/ sahar 28  B. Stratified epithelium It consists of two or more than two layers of cells. 1. Stratified Squamous Epithelium It consists of several layers of cells with only the superficial cells having squamous shape. D/ sahar 29  a. Stratified Squamous Non- Keratinized In situations where the surface of the squamous epithelium remains moist, the most superficial cells are living and nuclei can be seen in them. It consists of three layers: 1. Stratum basale; A single layer of cuboidal to columnar cell rests on a wavy basement membrane. D/ sahar 30  2- Stratum spinosum Composed of several layers of polyhedral cells tightly adhere to each other by numerous desmosomes. In H&E sections, the desmosomal attachments appear as small spiny processes, hence the name of this layer (spiny layer or stratum spinosum). D/ sahar 31  The stratum basale and the deep layer of stratum spinosum are involved in active mitosis; therefore, this region is referred to as stratum germinativum. 3. Stratum squamosum The superficial layer and is made up of few layers of flat squamous cells with small ovoid nuclei. D/ sahar 32  Locations It is found in oral cavity, pharynx, esophagus, anal canal, and vagina. Such sites are normally subjected to moderate mechanical abrasion and are kept moist by local glandular secretions. D/ sahar 33 D/ sahar 34 D/ sahar 35  b. Stratified Squamous Keratinized At places where the epithelial surface is dry (as in the skin) the most superficial cells die and lose their nuclei. These cells contain a substance called keratin, which forms a non-living covering over the epithelium. It consists of five to six cell layers: D/ sahar 36 1. Stratum basale Consists of single layer of cuboidal to columnar cells resting on a wavy basement membrane. 2. Stratum spinosum This has the same structure like that of the stratified squamous non-keratinized epithelium. D/ sahar 37 3. Stratum granulosum Consists of few layers of flattened cells having small pyknotic nuclei and rich in keratohyaline granules. 4. Stratum lucidum (found only in non-hairy thick skin) It is a layer of flattened, keratinized cells between the stratum granulosum and stratum corneum. It has a translucent or shiny appearance because the cytoplasm of these cells is rich in proteinaceous materials called eleidin. D/ sahar 38  5- Stratum corneum The outermost layer and consists of dead, keratinized cells. The cells have lost their nuclei and their cytoplasm filled with keratin that is a water-resistant protein. 6. Stratum disjunctum Formed of groups of cells in the outermost layer of the stratum corneum that become loose and detach to constitute this layer. D/ sahar 39  Locations : Found in epidermis of thick skin (palm and sole of hands and foot), hooves, horns, and feathers. D/ sahar 40 D/ sahar 41 D/ sahar 42  2. Stratified Cuboidal Epithelium It consists of two or more layers of cells, with only the superficial cells having a cuboidal shape. It frequently occurs as two-layered epithelium. Location: Found in the large ducts of sweat and salivary glands. D/ sahar 43 D/ sahar 44 D/ sahar 45  3. Stratified Columnar Epithelium It consists of several layers of cells with only the superficial layer having tall columnar cells. Location: Found in distal portion of the urethra, ducts of parotid and mandibular glands, lacrimal sac and lacrimal duct. D/ sahar 46 D/ sahar 47  4. Transitional Epithelium Itis a form of stratified epithelium found only in the urinary tract (lines the ureter and urinary bladder), so it is called urothelium. It is highly specialized to resist a great degree of stretch and to withstand the toxicity of urine. It is a multilayered epithelium and is 4 to 6 cells thick. D/ sahar 48  It differs from stratified squamous epithelium in that the cells at the surface are not squamous. The deepest cells are columnar or cuboidal. The middle layers are made up of polyhedral or pear- shaped cells. The cells of the surface layer are large and often shaped like an umbrella. In relaxed state (empty bladder): It consists of 4-5 layers of cells. The basal layer is cuboidal in shape rests on thin basal lamina. D/ sahar 49  The intermediate layer consists of several layers of polyhedral or pear-shaped cells. The surface cells are large cuboidal or dome- shaped (Facet cells) with convex outer surface and concave inner surface. Their nuclei are large, spherical with prominent nucleoli; some cells are binucleated. D/ sahar 50 D/ sahar 51 D/ sahar 52  In stretched state (full bladder): it appears only as two or three cell layers thickness. The intermediate and surface layers are extremely flattened. The superficial cells have a thicker plasmalemma that acts as a barrier against diffusion of fluid from the subepithelial tissue to the hypertonic urine. D/ sahar 53  II. Glandular Epithelium It may be classified as follow… 1) According to the presence or absence of ducts A- Exocrine glands (Gr. exo, outside, + krinein, to separate). They are glands that have a duct system to convey their secretory products to the sites of utilization (e.g., salivary glands). D/ sahar 54  B. Endocrine glands: (Gr. endon, within, + krinein, to separate). It secretes its products (hormones) into the bloodstream with no duct system e.g. thyroid glands. D. Paracrine glands: (Para= beside or nearby). Secretes its products into the local extracellular space affecting the surrounding cells e.g. enteroendocrine cells of gastrointestinal tract (GIT). D/ sahar 55  2) According to the number of cells forming the gland A. Unicellular glands Consist of a single secretory cell in a non-secretory epithelium. An example of a unicellular gland is the goblet cell of the lining of the small intestine or of the respiratory tract. B. Multicellular glands Formed from clusters of cells e.g., salivary gland and pancreas. D/ sahar 56 D/ sahar 57  3) According to the morphology of duct and the secretory parts A. Simple tubular where the duct is not branched and the secretory part is in the form of tubule (e.g., glands of the large intestine). B. Simple acinar or alveolar where the duct is not branched and the secretory part is in the form of alveolus or acinus (e.g., sebaceous gland and the glands of skin of amphibians). D/ sahar 58  C. Simple tubuloalveolar where the duct system is not branched and the secretory part is tubular and alveolar (are rare). D. Simple branched tubular where the duct is not branched while the tubular secretory part is branched (e.g., glands of the stomach). E. Simple branched alveolar where the duct is not branched while the alveolar secretory part is branched (e.g., sebaceous glands). D/ sahar 59 F. Simplebranched tubuloalveolar where the duct is not branched while the tubular and alveolar secretory part is branched (e.g., minor salivary glands). G. Compound tubular where the duct is branched and the secretory parts are tubular (e.g., liver and male accessory genital glands). D/ sahar 60  H. Compound alveolar where the duct is branched and the secretory parts are alveolar (e.g., mammary glands). I. Compound tubuloalveolar where the duct is branched and the secretory parts are tubular and alveolar (e.g., salivary glands and pancreas). D/ sahar 61 D/ sahar 62 D/ sahar 63  4) According to the nature of secretion A. Mucous glands They produce thick, viscous secretions (mucus). The cells of the mucous secretory units are cuboidal in shape and filled with mucinogen, the precursor of mucus that stain light (foamy or vacuolated). D/ sahar 64  In H&E. The nuclei are flattened and rest on the basement membrane and myoepithelial cells enclosed within the same basement membrane. The lumen is wide (e.g., palatine glands and lingual glands). D/ sahar 65 D/ sahar 66 D/ sahar 67 D/ sahar 68  B. Serous glands They produce thin watery secretion. The cells of the secretory units are pyramidal in shape. The nuclei are spherical and situated near the center of the cells. The cytoplasm has two zones, basal zone that appears basophilic due to the presence of rER and apical eosinophilic zone due to the presence of zymogen granules (e.g., parotid glands and pancreas). D/ sahar 69 D/ sahar 70 D/ sahar 71 D/ sahar 72  C. Seromucous (mixed) glands They produce mixed secretions. Consist primarily of mucous secretory units with crescent-shaped clusters of serous cells (serous demilunes) located at the periphery of the mucous units. The serous secretion reaches the lumen through intercellular canaliculi located between the mucous cells (e.g., submandibular and sublingual salivary glands). D/ sahar 73 D/ sahar 74  5) According to the mode of secretion A. Merocrine glands (secretion without loss) The cells of which remain intact (not destroyed) during the process of secretion. The secretory granules are discharged by exocytosis (e.g., salivary glands). D/ sahar 75  b. Apocrine glands (secretion with apical loss)  The apical parts of the cells are lost during the secretory process (e.g., some sweat glands and mammary glands). D/ sahar 76  c. Holocrine glands (secretion with whole loss)  The product of secretion is shed with the whole cell in a process that involves destruction of the secretion-filled cells (e.g., sebaceous glands). D/ sahar 77 D/ sahar 78 D/ sahar 79 D/ sahar 80 D/ sahar 81  III. Myoepithelium The myoepithelial cells are flattened cells with long cytoplasmic processes lie between the secretory cells and the basement membrane. The processes form a basket-like network (hence the name basket cells) around the secretory units. D/ sahar 82  Their cytoplasm is rich in contractile actin and myosin filaments. Contraction of these cells forces the secretory product into the duct system. The myoepithelial cells are found in salivary, sweat and mammary glands. D/ sahar 83  IV. Neuroepithelium It is a special type of epithelium designed to receive sensory stimuli (e.g., taste buds and olfactory epithelium). It will be considered with the digestive and the respiratory systems. D/ sahar 84  Thank you D/ sahar 85