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BIOL 302 - Epithelium

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BIOL 302
Mungo Marsden

BIOL 302 – Histology - Epithelial Tissues Definition: Membranous tissue composed of one or more layers of cells separated by very little intercellular substance and forming the covering of most internal and external surfaces of the body and its organs. Epithelia are sheets of cells that are linked tightly together by intercellular junctions. The major function of epithelia is to form barriers between two compartments. Distinctive characteristics:  Lies on basement membrane  Apical surface faces outside  Cells readily divide to make more cells; helps for recovery after an abrasion  No blood vessels; epithelial is often damaged when things move against it, it would not make sense for blood vessels to be present since you would bleed every time you scraped your skin or swallowed something rough  Tight junctions are present; to act as a barrier so fluids don’t leak in or out Functions:  Protection: abrasion resistance through dead epithelial cells at surface that can be sloughed off (external) and mucous secreting cells (internal).  Absorption: epithelial facing the digestive tract must be very good at absorbing or else we would not get nutrients we need. Folds in small intestine increase surface area for increased absorption.  Secretion: our bodies sometimes need to release materials, like hormones into the blood, and epithelial cells must be able to accommodate this. Pancreatic cells secrete insulin and glucagon.  Transport: striation ducts in salivary glands allow for transport of IgA antibody to neutralize any pathogens in substances being passed into the body.  Barrier: tight junctions act as a barrier between the inside and outside world. They force trans- membrane movement. Types of Epithelial Layers:  Simple: single layer of cells; each cell is attached to basal lamina and has an apical surface.  Stratified: has two or more layers of cells; the cells on the apical surface and basal lamina are separated by layers of cells.  Pseudostratified: all cells rest on basal lamina, but not all at apical surface. The bottom nuclei look crowded; the more apical nuclei are messy, uneven, and are elongated. Shapes of Epithelial Cells:  Squamous: flat in cross sections, thin, nucleus bulging  Cuboidal: round nucleus in center of cell  Columnar: taller than they are wide, elongated nuclei located at the bottom of cells Types of Epithelial Tissue:  Simple Squamous: one layer, very thin, offers little protection, functions to allow materials to pass through. Ex: endothelium (line blood vessels) in ear, esophagus, stomach, Bowman’s capsule o Keratinized Simple Squamous: superficial layer of dead cells, no nuclei; resists abrasion and prevents moisture loss. Ex: Skin.  Simple Cuboidal: offers some protection, more commonly found where there is a lot of secretion and absorption. Ex: distal and collecting duct of kidneys, thyroid gland  Simple Columnar: offers some protection and where secretion or absorption is necessary. Ex: digestive tract, goblet cells, oviducts, kidneys **It is the top layer that determines classification of stratified tissues!**  Stratified Squamous: uppermost cellular layer, usually made up of many layers, offers most protection to underlying tissue. Keratinized cells have lost their nucleus, ex outermost layer of skin. Ex: tongue, skin, esophagus.  Stratified Cuboidal: found where secretion is important. Ex: sweat ducts, salivary glands  Stratified Columnar: transitional tissue. Ex: lining of large salivary ducts  Transitional tissue: specialized stratified in bladder to accommodate stretch. When unstretched, they appear cuboidal, pale, called pillow cells. When stretched, they become compressed and appear flat. Have prominent nucleus: round with nucleoli visible as a dot in the middle of the nucleus. Apical Specializations Microvilli  Small cylindrical projections, 0.5-1 um  Core contains actin bundles so they cannot actively move – non motile  Present on luminal surface  Increase surface area for increased absorption – commonly found in digestive tract, kidney  Brush border/”striated” border under light microscope – cannot see individual microvilli, length of border <50% of nucleus. Stereocilia  Modified microvilli; specialized  Long, 75% of cell length  Sparse and ragged looking  Found in inner ear  Exact function unknown, possible absorptive role Cilia  Lon
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