Epithelial Tissue
Covering and lining epithelia - forms outer covering and inner lining
Glandular epithelia - forms the secretory portion of glands
Endocrine glands
Exocrine glands
Secrete directly into blood, usually via traversing interstitial fluid
e.g. pituitary gland
Secrete into ducts that empty onto the surface of a covering or lining epithelium
e.g sweat and salivary glands
Single cell glands
Some glands such as the pancreas are mixed secreting into ducts and directly into the blood
individual secretory cells called mucous cells - secrete mucin; apical cytoplasm is filled with (large) secretory vesicles that look clear/foamy in a light micrograph
Multicellular glands
Simple glands - single duct that does not divide on its way to the gland cells
Simple tubular - glandular cells form straight tubes e.g. intestinal glands
Simple coiled tubular - glandular cells form coiled tubes e.g. merocrine sweat glands
Simple branched tubular - glandular cells form tubes and several secretory areas share a common duct e.g. gastric glands
Simple alveolar (acinar) - glandular cells from sac-like pockets e.g. embryonic stage in development of simple branched glands
Simple branched alveolar - glandular cells form sac-like pockets and share a common duct e.g. sebaceous (oil) glands
Compound glands - duct divides one or more times on its way to gland cells
Compound tubular - glandular cells form tubes e.g. mucous glands in mouth
Compound alveolar (acinar) - glandular cells form sac-like pockets e.g. mammary glands
Compound tubuloalveolar - secretory cells form both tubes and sacs e.g. salivary glands and pancreas
Simple epithelium - single layer of cells all in contact with basement membrane; secretion, absorption, filtration
Simple squamous - flat and thin; irregular like jigsaw; most delicate epithelium; filtration (kidney), diffusion (lung), secretion (serous membranes)
Stratified epithelium - two or more layers of cells
Simple cuboidal - cells about as tall as they are wide; cuboidal/hexagonal boxes; secretion and absorption
Endothelium lining blood vessels
Mesothelium lines pericardial, pleural and peritoneal cavities
Lines kidney tubules and ducts of glands
Simple columnar - rectangular, nuclei often elongated; more cytoplasm = more organelles
Ciliated - have cilia and goblet cells; assist motility of mucus, foreign objects oocytes
Stratified squamous - located where mechanical/chemical stresses are severe; microbe protection;
Non-ciliated - have microvilli on apical surface, which increases the surface area for absorption
Line gastrointestinal tract from stomach to anus
Line parts of the upper respiratory tract, Fallopian tubes and brain ventricles
Apical layer only determines the subtype
Keratinised - apical cells are packed with keratin, making surface tough and waterproof e.g. skin
Non-keratinised - no keratin in apical cells; protect from abrasion, defense from microbes, require secretions e.g. lining of mouth and oesophagus
Stratified cuboidal - ducts of adult sweat glands, oesphageal glands, male urerhtra
Stratified columnar - rare; part of conjunctiva of the eye
Stratified transitional - bladder
Pseduostratified epithelium - appears to have multiple layers as judged by positions of nuclei but all cells in contact with basement membrane and no all reach apical surface
Pseduostratified columnar - two main sub-types
Ciliated - goblet cells secrete mucus and cilia move it; lines airways of most of the upper respiratory tract
Non-ciliated - no goblet cells so functions in absorption and protection; larger ducts of sweat glands
Basement Membrane
All epithelia overlay a basement membrane composed of two parts
Reticular lamina - produced by cells of underlying connective tissue, known as fibroblasts
Functions
Provides a surface along which epithelial cells migrate during growth and wound healing
Acts as a physical barrier
Supports overlying epithelium
The basement membrane is thus found between the epithelium and connective tissue
Basal lamina - secreted by basal layer of epithelial cells
Cell Junctions
Lateral junctions
Basal junction
Hemidesmosome
Connects epithelia to basement membrane
Integrin linker protein binds to laminin in the basement membrane and keratin intermediate filament in cytoplasm
Adherens junctions
Gap junctions
Tight junctions
Desmosome
Transmembrane glycoprotein cadherin links to intermediate filaments containing keratin in adjacent cells
6 connexin molecules form connexons that connect neighbouring cells
Narrow intercellular gap - small molecules/ions diffuse through - allow cell communication and nerve impulses to spread
Cadherins span gap/space between cells; catenins link cadherins to actin in microfilaments in cell cytoplasm
Electrically tight; transmembrane proteins fuse adjacent plasma membranes together - claudins and occludins