Epithelial Tissue PDF

Epithelial tissue Definition: The epithelium is a sheet of closely aggregated cells ❑ that covers body surfaces and lines body cavities or modified to constitute glands, myoepithelium or sensory epithelium. ...

Epithelial tissue Definition: The epithelium is a sheet of closely aggregated cells ❑ that covers body surfaces and lines body cavities or modified to constitute glands, myoepithelium or sensory epithelium.  Most characteristic features: 1) They arising from ectoderm, mesoderm, or endoderm. 2) Cells are close to each other with very little intercellular space. 3) Epithelial cells are tightly bound together by junctional complexes and show polarity. 4) An epithelial layer sits on a basal lamina that separates the epithelium from the underlying tissue. 5) Epithelia lack a direct blood supply (avascular) and are fed via diffusion from blood vessels in the underlying connective tissue. 6) Epithelia are very rich in nerve endings. 7) Epithelia have high mitotic index with high ability to regeneration— an advantage because cells undergo mechanical stress and trauma.  Functions: 1) Protective barrier because of covering and ling of surfaces (eg. Skin). 2) Absorption (eg. Intestine). 3) Secretion (eg. Sweat glands). 4) Sensation (eg. gustative and olfactory neuroepithelium). 5) Contractility (eg. Myoepithelial cells). 6) Filtration (eg. Epithelium of renal corpuscle). 7) Diffusion of gases or fluids (eg. lung alveoli or blood vessels).  Classification: The epithelium is classified into four types; I. Epithelial membranes (covering or lining epithelia) II. Glandular epithelium III. Myoepithelium IV. Sensory or neuroepithelium  Epithelial membranes Classification of epithelial membranes based on two factors: 1. The number of cell layers (simple or stratified) 2. The shape of the surface cells (squamous, cuboidal, or columnar). Simple Epithelium It consists of only one layer of epithelial cells, all of which are in contact with the basal lamina. Depending on the shape of the cells, simple epithelia can be further divided into 4 types: 1. Simple squamous epithelium 2. Simple cuboidal epithelium 3. Simple columnar epithelium 4. Pseudostratified columnar epithelium  1. Simple squamous epithelium: ❑ It is composed of one layer of thin flat, scale-like cells. ❑ On surface view, irregular, serrated outlines with a centrally, bulging nucleus. ❑ In a side view, spindles with tapering ends. The cytoplasm is scanty and has sparse organelles. ❑ Location: Pulmonary alveoli, blood and lymphatic vessels (endothelium), pleural and peritoneal cavities (mesothelium).  2. Simple cuboidal epithelium: ❑ It consists of one layer of cells whose height roughly equals their width, cells resemble small box-like cubes. ❑ has one spherical, centrally placed nucleus. ❑ Locations: Thyroid follicles, Ducts of many glands, covering of ovary, kidney tubules  3. Simple columnar epithelium: ❑ It consists of one layer of cells that are taller than they are wide and look like closely packed, slender columns. ❑ The ovoid nucleus is centrally or basally placed. ❑ Locations: Simple columnar epithelium may be: 1. Absorptive with microvilli (small intestine, gall bladder) 2. Secretory (stomach) 3.Ciliated (oviducts, uterus and bronchioles).  4. Pseudostratified columnar epithelium ❑ It consists of more than one type of epithelial cell. ❑ All cells contact an underlying basement membrane, but only some reach the free surface. ❑ In sections’, the nuclei appear at different levels, two or three layers of crowded nuclei are seen. These features give the epithelium a false impression of stratification— thus, its name ❑ Locations: it may be: ❑ 1. Pseudostratified columnar epithelium ciliated with goblet cells as in respiratory systems. This tissue is called respiratory epithelium. ❑ 2. Pseudostratified columnar epithelium with stereocilia as in male reproductive tract. This tissue lacks goblet cells.  B- Stratified Epithelium This epithelium consists of: 1. more than one layer of epithelial cells. 2. only one of which (basal cell layer) is attached to the basal lamina. 3. Depending on the shape of the cells at the uppermost layer, stratified epithelia can be also divided into 4 types: 1) Stratified squamous epithelium (non-keratinized or keratinized epithelium) 2) Stratified cuboidal epithelium 3) Stratified columnar epithelium 4) Transitional epithelium  Stratified squamous non-keratinized epithelium ❑ the epithelium is thick, as it was composed of several layers of cells: 1. The basal cells: cuboidal in shape. 2. the middle of the epithelium: polymorphous cells 3. the superficial layer: flattened (squamous). ❑ Basal cells: are mitotically active and continuously divide into daughter cells that mature and are pushed toward the surface to die and slough off. This process is called desquamation. ❑ Locations: It is usually wet and is found lining the oral cavity, pharynx, esophagus, anal canal, and vagina.  Stratified squamous keratinized epithelium ❑ It is similar to stratified squamous nonkeratinized epithelium, except that the superficial layers: dead cells whose nuclei and cytoplasm have been replaced with keratin. ❑ Locations: This epithelium is impermeable to water and constitutes the epidermis of skin, hoof and horns.  2. Stratified cuboidal epithelium ❑ It contains only two layers of cuboidal cells. ❑ Locations: In the ducts of sweat glands. 3. Stratified columnar epithelium. ❑ It comprises more than one layer of cells. 1. basal cells: cuboidal 2.intermediate cells: polyhedral 3. superficial cells: columnar ❑ Locations: In the conjunctiva of the eyelids, major ducts of exocrine glands, and parts of the male urethra.  4. Transitional epithelium (Urothelium). ❑ Its appearance: not static, as it rapidly adapts to contraction and distention. ❑ In relaxed state (empty bladder): it consists of 5-7 cell layers, with a thin basement membrane. 1. The basal layer: cuboidal to columnar cells. 2. intermediate cells: polyhedral. 3. The superficial layer (umbrella cells): 4. relatively large, often binucleate cells. In stretched state (full bladder): it appears thinner with only 2- 3 cell layers.  Epithelial cell polarity (specializations of the cell surface) Each cell has 3 domains: 1. an apical or free domain, 2. a lateral domain, 3. a basal domain. The apical domain: directed toward the lumen of an enclosed cavity or tube. The lateral domain: communicates with adjacent cells and is characterized by specialized attachment areas. The basal domain: rests on the basal lamina anchoring the cell to underlying connective tissue.  Apical domain Cilia ❑ Cilia are motile extensions of the cell surface Cilia are found in the respiratory and female reproductive tracts. ❑ with LM: ▪ are visible. ▪ They originate from basal bodies, which are identical to centrioles. ❑ With EM, cilia are surrounded by a plasma membrane and consist of a central core (axoneme) with a “9 + 2” microtubule arrangement (an axoneme consists of two central single microtubules surrounded by nine peripheral doublets). ❑ Functions ❑ Cilia of the respiratory tree, move mucus and debris toward the oropharynx, where it may be swallowed or expectorated. ❑ 2. Cilia of the oviduct move the fertilized ovum toward the uterus.  Microvilli ❖ Microvilli are small finger-like cytoplasmic projections. ❖ With LM: 1. They are too small. 2. The microvilli may be so numerous and regularly arranged. ❖ In the EM: they are unbranched and contain a core of actin filaments that insert into the terminal web (specialization of the cytoskeleton lying immediately beneath the cell surface). ❖ Function: their major role is to increase surface area for absorption.  Stereocilia ❑ 1.They are unusually long, branched microvilli. ❑ With LM: They are non-motile and readily visible. ❑ Function: 1. In the epididymis, they probably function in increasing the surface area. Flagella ❑ Flagella, found in sperm cells, are longer than cilia. ❑ Their structure is similar to that of cilia, but their beating patterns are different.  Lateral domain (Junctional complex and gap junctions)  Basal domain ❖ Basal surface specializations include: 1. the basement membrane, 2. hemidesmosomes, 3. Basal enfoldings:ion-transporting epithelia (e.g., distal convoluted tubule of the kidney, striated ducts in salivary glands) and is associated with vertically oriented mitochondria. Basement membrane: ❖ the boundary between epithelium and underlying connective tissue. ❖ With LM: poorly visualized by H&E but stains intensely with PAS and silver. ❖ with the EM, a basement membrane is seen to have: 1. a basal lamina 2. a reticular lamina.  Glandular epithelium Def: 1- Glandular epithelia are formed by cells specialized to produce secretion. 2- The molecules to be secreted are generally stored in the cells in small membrane-bound vesicles called secretory granules (zymogen granules). 3- Glandular epithelial cells may: a) synthesize, store, and secrete proteins (e.g., pancreas), b) lipids (e.g., adrenal, sebaceous glands) c) complexes of carbohydrates and proteins (e.g., salivary glands).  Classification of glands: ❑ Glands are classified according to the site of secretion: 1. Exocrine glands: ▪ Secrete into a duct or onto a surface (salivary glands). ▪ These glands have a secretory portion, which contains the cells responsible for the secretory process, and duct system, which transport the secretion to the exterior of the gland. 2. Endocrine glands: secrete into the bloodstream (pituitary gland).  Classification of exocrine glands: Exocrine glands are classified according to: A. The number of secretory cells: 1- Unicellular glands (single-celled glands) ▪ Mucus-secreting goblet cells. easily visualized in small intestine. 2. Multicellular glands ▪ It is composed of more than one cell (e.g., salivary gland).  C. According to the type of secretion: 1. Mucous glands: ▪ They produce thick, viscous secretions (e.g., palatine glands and glands of the tongue). ▪ LM (HE): a. The cells are cuboidal or columnar in shape. b. The cytoplasm appears to be empty or vacuolated. c. The nucleus is usually flattened against the base of the cell by accumulated secretory product. 2. Serous glands ▪ They produce thin watery, protein-rich secretion (e.g. the pancreas and parotid salivary glands). LM: 1. The cells are polyhedral or pyramidal 2. The nucleus is round, centrally located. 3. Seromucous or mixed glands: ▪ They produce mixed secretions. ▪ They consist primarily of acini (secretory units) that produce mucous secretions as well as acini that produce serous secretions. ▪ some of the mucous acini possess serous demilunes, a group of cells located at the periphery of the mucous units and secrete a serous fluid (e.g., submandibular and sublingual salivary glands).  C. According to the mechanism of secretion: 1. Merocrine glands ▪ is the most common mode. ▪ The secretory product is released by exocytosis ▪ An example: pancreatic acinar cells. 2. Apocrine glands ▪ The secretory product is released together with part of the apical cytoplasm of the secretory cell. ▪ An example: Epithelial cells of the mammary gland (lipid secretion). 3. Holocrine glands ▪ The secretory product is released by rupture of the entire cell. ▪ Cells lost are replaced by mitosis in basal layer of acinus ▪ An example: sebaceous glands.  Myoepithelium (basket cells) ▪ They are contractile cells. ▪ They are spindle-shaped with long cytoplasmic embrace basket-like network. ▪ lie between the secretory cells and the basal lamina). ▪ cytoplasm is rich in actin and myosin filament.. help propelation of secretory products into and up the duct system. Neuroepithelium It is a special type of epithelium specialized for the reception of external stimuli (e.g., hair cells of the inner ear, the receptor cells of the taste buds and olfactory cells of the nose). 1-Which of the following types of epithelium lines the respiratory system? A) Ciliated pseudostratified columnar with goblet cells B) Keratinized stratified squamous C) Nonkeratinized stratified squamous D) Simple columnar with goblet cells 2-The epithelial lining of the serous membranes is called A) Transitional epithelium B) Mesothelium C) Endothelium D) Mesenchymal epithelium 3-Which type of epithelium consists of more than one layer of cells? A) Simple B) Stratified C) Pseudostratified D) Mesothelium The epithelium of the small intestine is Simple columnar B Simple cuboidal Simple squamous D Pseudostratified columnar The epitheliums whose main functions is to prevent penetration Stratified cuboidal B Stratified squamous keratinized Simple squamous D Simple columnar Glands are classified according to the type of secretion into Exocrine and endocrine glands B Serous and mucous glands Unicellular and multi-cellular glands D None of the above Apocrine glands characterized by some cells are destroyed, to form the secretion B no change occur in the secretory cells the apex of the cell is destroyed during secretion D sebaceous glands In simple cuboidal epithelium, the nuclei (at L/m level) rounded and central B flat and central rounded and apical D oval and basal Taste bud is an example of neuro-epithelium B myoepithelium merocrine gland D sero-mucous gland The epithelium of the trachea is Simple columnar B Simple cuboidal Pseudostratified columnar D Simple squamous