Epithelial Tissue PDF

Epithelial Tissue Dr. Yusra Erfifi Tissue: is a group of similarly specialized cells united in performing a specific function Cells Extracellular matrix (ECM)  The Four basic tissue types Epithelial tissue Connective tissue Nervous tissue...

Epithelial Tissue Dr. Yusra Erfifi Tissue: is a group of similarly specialized cells united in performing a specific function Cells Extracellular matrix (ECM)  The Four basic tissue types Epithelial tissue Connective tissue Nervous tissue Muscular tissue  Epithelial tissue ▪ Epithelial tissue covers the body and lines the inner and outer surfaces of the organs and the body cavities.  General characteristic of Epithelial tissues Derived from all three germinal layers Ectoderm Epidermis of skin Endoderm Lining of the gastrointestinal tract  Mesoderm - The endothelial lining of the circulatory system - The mesothelium of the body cavities - Lining of urogenital system including gonads, ducts, and accessory glands.  Cover body surfaces (cutaneous and gastrointestinal) and line most body cavities (peritoneal, pericardium, and pleural cavities). Closely aggregated polyhedral cells with minimal intercellular space Very little extracellular matrix. Epithelial cells are adherent to each other by intercellular junctions The epithelial cells rest on a basement membrane Avascular(absence of blood vessels and lymphatics) Receive nutrition by diffusion from the underlying connective tissues  Rich in sensory nerve endings The epithelial cells show polarity, Apical pole basal pole and lateral surfaces. High power of regeneration (renewal), the epithelial cells undergo mitosis and replace damaged cells with new cells.It can be fast in tissues such as the intestinal epithelium, which is replaced every week, or slow, as in the large glands. Its nuclear shape corresponds roughly to cell shape (nuclei are oval in columnar cell, round in cuboidal cell ,and flat in squamous cell) The nuclei allow to determine the number of cell layers in epithelia The epithelial tissue may modify to perform special function such as Neuroepithelia.  Broad classification of epithelial tissues Surface/lining Glandular or secretory Myoepithelium Germinal epithelium Neuroepithelium (e.g. ovary and testis epithelium epithelium (e.g. myoepithelial (e.g. taste buds) cells) (e.g. Salivary glands) Functions Protection Secretion Sensation Contraction Reproduction Absorption  Surface epithelium Covering or Lining Epithelia Cover the body surface or line the cavities of an organ.  Classification of covering or lining Epithelia Number of cell layers The cell morphology in the surface layer Simple (one cell layer) Squamous Cuboidal Stratified (two or more layers of cells) Columnar Pseudostratified (False stratification) 1) Simple squamous epithelium ▪ Made of one layer of squamous cells ▪ Side view (sectional) - Flattened scale-like cells, the width of the cell is greater than its height - Centrally placed Flat nucleus and very small amount of cytoplasm ▪ Surface view - polygonal cells similar to a fried egg ▪ All the cells touch the basal lamina  Site of distribution Main Function provide a smooth surface, which Lining of vessels and the heart makes it easier for blood or other (endothelium) fluid to flow over them. Active transport (Pinocytosis) lining the serous cavities: Facilitates the movement of the viscera pericardium, pleura, peritoneum (mesothelium) lubrication, reducing friction Lining pulmonary alveoli gaseous exchange(diffusion) (Pneumocytes type I ) Parietal layer of Bowman’ Facilitates filtration and diffusion capsule and the loop of Henle in the kidney lining the inner surface of the cornea Squamous epithelium Simple squamous epithelium 2) Simple cuboidal Epithelia Made of one layer of cuboidal cells. The width, depth, and height are approximately the same. Nuclei are round and centrally located Some cells have microvilli (renal tubules) or cilia (terminal bronchioles of the lung)  Site of distribution Main Function Thyroid follicles Proximal and distal convoluted Covering, secretion, absorption tubules of kidney Collecting tubules of kidney Covering of ovary The ducts of many glands of the body (e.g. intercalated ducts of salivary gland) Simple cuboidal epithelium from renal tubules. Thyroid gland showing the simple cuboidal epithelium 3) Simple columnar Epithelia Made of one layer of columnar cells The height of the cell exceeds the width Basal oval nuclei a) Simple columnar Secretory Absorptive with goblet cells ✓ the cells show striated border, or microvilli b) Simple columnar ciliated epithelia ✓ the cells provided by cilia  Site of distribution Main Function Lines the stomach Protection, lubrication, Simple columnar Lining of small and large intestine absorption, secretion epithelia common bile duct and gallbladder Renal collecting duct o lines the uterus o Simple columnar o Oviducts o Transportation, absorption, ciliated epithelia secretion, protection o ductuli efferentes o small bronchi and bronchioles  Simple columnar epithelia (stomach)  Simple columnar epithelia with goblet cell 4) Pseudostratified Columnar Epithelia All cells rest on basal lamina but not all reach epithelial surface with nuclei at different levels The surface cells are columnar appears to be stratified, but it is actually composed of a single layer  Site of distribution Main Function Pseudostratified Ciliated Lining of nasal cavity , trachea Columnar Epithelia with and primary bronchi goblet cells( respiratory auditory tube epithelium) protection, secretion; cilia mediated transport of particles Pseudostratified columnar Epididymis trapped in mucus out of the air epithelium with stereocilia passages Ductus deferens Pseudostratified Non- ciliated columnar Membranous part of the male urethra, epithelium large excretory ducts of salivary gland Pseudostratified Ciliated Columnar Epithelia with goblet cells Pseudostratified columnar epithelium from the epididymis. Basement membrane (black arrows) and basal cells (yellow arrows) 5) Stratified Squamous Epithelium ▪ Two or more layers of cells (up to 30) Basal layer: cuboidal or low columnar in shape - Mitotically active Middle layer: polygonal cells with central rounded nuclei ,and joined by desmosomes Surface layer: flattened cells (squamous) ▪ Clear and wavy basement membrane a) Stratified Squamous (Nonkeratinized) Epithelium (moist or wet epithelium) Several layers of cells The flattened cells of the surface layer retain their nuclei and most metabolic functions. Distribution Mouth, esophagus, true vocal folds, outer covering of cornea, vagina, anal canal Main Function Protection, secretion; prevents water loss Stratified Squamous (Nonkeratinized) Epithelium b) Stratified Squamous keratinized Epithelium (Cornified or dry epithelium) Several layers of cells Superficial layers of the epithelium are composed of dead cells whose nuclei and cytoplasm have been replaced with keratin Distribution o Epidermis of skin o External ear Main Function o Protection; prevents water loss Stratified Squamous keratinized Epithelium c) stratified squamous parakeratinized epithelium. Surface-most cells retain their nuclei though they are pyknotic Its degree of keratinization is more than that of nonkeratinized and less than that of keratinized stratified squamous epithelium Distribution The epithelium of both the hard palate and gingiva 6) Transitional epithelium (urothelium) It is a stratified epithelium, with capacity to stretch Relaxed state (empty bladder) Number of layers 6-8 Basement membranes non clear non wavey ▪ Basal layer :low columnar or cuboidal cells ▪ Intermediate layer : polyhedral cells with central rounded nuclei and the intercellular space containing mucous like substance which helps gliding of the cells over each other ▪ Superficial layer: large, dome-like cells sometimes called umbrella cells or facet cells , are occasionally binucleated  Distended state (full bladder) The dome-shaped cells become flattened, and the epithelium becomes thinner(2-3) due to gliding of the cells with the help of mucous ▪ Distribution lines the urinary tract from the renal calyces to the proximal part of the urethra ▪ Main Function protect underlying tissues from the hypertonic and potentially cytotoxic effects of urine. Accommodates fluctuations in the volume of fluid in an organ Transitional epithelium from the bladder. Cytoplasmic plaques (black arrows) and a binucleate cell (green arrow) are indicated. 7) Stratified cuboidal epithelia Rare type of epithelium Two layers of cuboidal cells The excretory ducts of salivary and sweat glands 8) Stratified columnar epithelia Rare type of epithelium Basal layer: cuboidal cells Superficial layer : columnar cells. Distribution The conjunctiva lining the eyelids large excretory ducts penile portions of male urethra Function Protection and mucous secreting Stratified columnar epithelium from a salivary gland duct.  Epithelial Tissues as Glands (Glandular epithelia) ▪ Type of epithelium characterized by its secretory function o Pancreas proteins o Sebaceous glands lipids o Salivary glands complexes of carbohydrates and proteins o Thyroid gland a polypeptide hormone o Sweat glands water and electrolytes ▪ Glands develop from covering epithelia by cell proliferation and growth into the underlying connective tissue, followed by further differentiation  Classification of glandular epithelium According to: ❖ The method of distribution of their secretory products. ❖ Number of cells forming the gland. ❖ Branching of the ducts and secretory portion. ❖ Morphology of their secretory units. ❖ Mode of secretion (Mechanism). ❖ Nature of their secretion.  ❖According to the method of distribution of their secretory products 1)Exocrine glands 2)Endocrine glands Secrete their products onto the epithelial surface Ductless, having lost their connections to the directly or via ducts originating epithelium, and thus secrete their products (hormones) into connective tissue ▪ Ducts may convey the secreted material in an from which they enter the bloodstream to reach unaltered form or may modify the secretion by their target cells concentrating it or adding or reabsorbing substances 3)Mixocrine glands Contain both the exocrine and endocrine component (pancreas)  Structural Organization of Exocrine gland Parenchyma Stroma The cells of which have a supporting The cells responsible for the role in the organ. Except in the brain organ’s specialized functions and spinal cord, the stroma is always connective tissue Glands The connective tissue that invade The secretory units, along with and support the parenchyma. their ducts  Exocrine glands Composed of secretory units and duct system ❖ Classified according to the number of cells forming the gland Unicellular Exocrine Glands Multicellular Exocrine Glands The simplest form of exocrine gland (one cell) Organized clusters of secretory cells(acini) Common in simple cuboidal, simple Most glands of the body columnar, and pseudostratified epithelia Goblet cells Sweat glands and major salivary glands  Unicellular Exocrine Glands Goblet cell ▪ It is formed of one cell ▪ Site : the epithelia lining the intestine and respiratory tract. ▪ L/M : o Inverted flask shape ,slender base and expanded apex due to presence of mucin granules which are not stained by H&E o stained by the PAS method o Nucleus is basal and flat o Basophilic foamy cytoplasm  E/M o Thin basal region contains ,the stem, the nucleus, ribosomes, rER and Golgi complex. Sits on the basal lamina o Expanded apical portion, the theca, faces the lumen of the digestive tube or respiratory tract. o The theca is filled with membrane-bound secretory droplets filled with mucinogen  Mucinogen are glycosylated proteins released by exocytosis when stimulated by chemical irritation and parasympathetic innervation. When mucins are released from the cell, they become hydrated and form a layer of mucous. ▪ Merocrine gland. Function lubricating and protecting the epithelial sheet.  ❖ According to branching of the ducts Simple Compound The ducts do not branch The ducts do branch  ❖ According to morphology of their secretory units 1)Tubular The secretory part is tubular in shape 2) Acinar (alveolar) The secretory part is rounded or a grape like 3)Tubulo-acinar The secretory part has the shape of a tubule that terminates by an alveolus Each of these types of secretory units may be branched or non -branched  Simple tubular glands Intestinal gland or crypts Simple alveolar glands mucous gland along the urethra Simple coiled tubular Sweat glands  Simple branched tubular Fundic glands of the stomach Simple branched acinar Sebaceous glands Simple tubuloalveolar glands Rare in man  Simple branched tubularalveolar glands glands of mouth o Compound tubular glands mucous gland of duodenum o Compound acinar glands Exocrine part of pancreas o Compound tubulo-acinar Salivary glands Mammary glands  ❖ According to the mechanisms for releasing the product Merocrine gland The secretory cells release their contents by exocytosis with no loss of the cytoplasm or cell membrane Most exocrine glands are merocrine The most common method of protein or glycoprotein secretion Salivary glands e.g., parotid gland  Apocrine gland Part of the cytoplasm of the secretory cells is released along with the secretory contents lactating mammary glands  Holocrine gland The secretory cells together with its accumulated secretion is released into the duct secretion is produced by the disintegration of the secretory cells themselves as they complete their terminal differentiation Sebaceous glands of the skin  acne vulgaris Excessive holocrine secretion of sebum and keratin Occurs in both genders at puberty Triggered by the surge of the steroid hormone testosterone Holocrine secretion in a sebaceous gland  ❖ According to the type (nature ) of secretion ▪ Mucous gland ▪ Serous gland ▪ Mixed gland (seromucous gland) ▪ Gland of special nature ✓ Waxy secretion (ceruminous gland of external ear) ✓ Watery secretion (lachrymal glands) ✓ Fatty secretion (sebaceous gland) ✓ cellular secretion (ovary ,testes)  Mucous Acinus Serous acinus Size Relatively large in Relatively smaller than diameter Mucinous Acinus lumen wide narrow In a section Formed of few number of Formed of relatively pyramidal or cuboidal larger number of cells that have well defined pyramidal or cuboidal boundaries cells that have ill- defined boundaries Cytoplasm Pale basophilic and Basophilic with vacuolated (due to characteristic basal mucinogen content) striation due to rER.Apical acidophilic secretory(zymogen) granules Nucleus Each cell contains a single Each cell contains a flattened and basal single rounded nucleus nucleus shifted towards the base Secretion Mucous (a viscous thick Watery whey- like secretion) containing enzymes Duodenal (Brunner) gland Parotid gland Mucous gland Secrete viscous thick secretion containing mucin Goblet cells, duodenal (Brunner) gland, and the minor salivary glands of the hard and soft  Serous gland Secrete watery secretion containing proteins and rich with enzymes Parotid gland  Mixed gland (seromucous gland) It is formed of a mucous acinous, surrounded partially by a small cap of serous cells This cap is called a serous demilune or serous crescent or crescent of Gianuzzia The secretions of the serous cells reach the lumen of the mucous acinus through a narrow intercellular canaliculi between the mucous cells submandibular gland In any of the three types (mucous, serous, sero-mucous acini) there is a special type of epithelial cells (myoepithelium) placed between the secretory cells and the basement membrane  Secretes both mucous and serous secretions submandibular gland  Myoepithelial cells Special type of epithelium They have some characteristics of smooth muscle cells, particularly contractility. The cytoplasm contains contractile elements (myosin ,actin and intermediate filaments) so called myoepithelium The cell has a long branching processes, wrapping around the acini and some of the small ducts like a basket ,so it is called basket cells Myoepithelial cell are present between the basal ends of secretory or duct cells and their basement membrane  Their contractions assist in expressing secretions from the acini and from some small ducts Distribution Salivary gland Sweat gland Mammary gland Neuroepithelium Special type of epithelium which acts as a receptor and receives stimuli for sensation It consists of two types of cells: sensory cells and supporting cells Taste buds in the tongue for taste sensation Organ of corti in the ear for hearing Crista ampularis, Macula utriculi and sacculi in the ear for equilibrium Rods and Cons in the retina for vision  Function of epithelium Protection surfaces and barrier (epidermis and the lining epithelium of colon) Absorption (the intestinal epithelium, alveolar epithelium, vascular endothelium) Secretion (parenchymal cells of glands intestinal goblet cell, parotid gland) Selective permeability Sensory perception Excretion