Book Recommended: Basic HistologyLec. By Dr. Habib Qureshi
Epithelium is the collection of cells with very little intercellular substance lining internal and external surfaces of body and performing similar function
EPI = UPON THELE = NIPPLE DERIVED FROM ALL THREE EMBRYONIC GERM LAYERS Basal layer of epithelium rest on BASEMENT MEMBRANE
Basement membrane consists of Basal lamina proteoglycans, type 1V collagen, electron microscopic - Forms limit of cell - Secreted by epithelial cells Reticular lamina - Consist of fine collagen fibers
Single layer of flat cells. Rest on basement membrane Nucleus bulging Function: gas exchange, diffusion of fluids Endothelium = blood vessels Mesothelium = peritoneal, pleural and pericardial cavities Sites= bowman’s capsule, alveoli of lungs, mesothelium, endothelium, loop of Henle of kidney
SINGLE LAYER OF CUBOIDAL CELLS For secretion Sites=thyroid follicles, tubules of kidney, surface of ovary All cells of same height and width and rest on basement membrane
Tall cells on basement membrane Nucleus near base For absorption and secretion ciliated = uterus Non ciliated = GIT, (has brush border – microvilli, folds of cell membrane to increase surface area
S. sq. keratinized Surface layer is squamous In between 3-7 layers are polygonal to flat Basal layer is cuboidal There is layer of keratin at the the top Sites. skin S. Sq. Non-keratinized No keratin layer Same as other sites= esophagus, anal canal, vagina
Stratified Cuboidal epithelium Two to three layers of cells Mostly present in ducts of salivary glands. Stratified Columnar epithelium Two to three layers of cells Present in ducts of salivary glands, Urethra.
ALL LAYERS REACH BASEMENT MEMBRANE Nuclei at different levels Trachea Has goblet cells
Empty urinary bladder Basal layer columnar Intermediate layers polygonal 2-3 layers Flask shaped layers Dome shaped( umbrella) at top Stretched urinary bladder 2-3 layers of flat cells
Needed for cell aggregation and cohesion Marked in epithelial tissue Due to binding action of glycoproteins Lateral membranes have special junctions For adhesion Act as seals for flow of materials Present in definite order from apex to base
Tight junctions – ZONULA OCCLUDENS Most apical Belt like , totally occluding (Closing) Have ridges and grooves forming net like structure Form tight seal (e.g Urinary bladder) to control paracellular pathway ZONULA ADHERENS Belt like Encircles cell For adhesion of neighbouring cells Forms terminal web on cytoplasmic surface of cell membrane
Gap junctions –NEXUS Present anywhere Present in all tissues except skeletal muscle There is gap of 2 nm in between opposing surfaces Protiens of junction make connexons which control the flow Permits molecules less than 1500 Da. Eg. Heart muscle.
DESMOSOMES:MACULA ADHERENS Disc like structure Membrane > 30 nm apart Dense material in intracellular space DESMOSIN for adhesion Attechment plaque protiens inside the cell membrane HEMIDESMOSOMES (Half Desmosomes) Are between basal surface of epithelial cells and basement membrane.
For specific activities at the cell mambrane Microvilli Fingerlike folds of cell membrane Increase surface area Present in absorptive cells e. g. small intestine where the make brush or striated border 1 um in height and 0.08 um wide Within mv are clusters of actin microfilaments forming terminal web at the base
STEREOCILIA Long nonmotile processes of cell Long microvill Present in epididymis Increase surface area for molecular movement CILIA AND FLAGELLA Long, motile hair like structures 5-10 um long and 0.2 in dia Surrounded by the cell membrane Contain central pair of microtubule surrounded by9 pairs of microtubules Inserted into basal body, inside the cell Cilia eg. In trachea for movement of mucous Flagela eg in sperms. Tail like movement