Epithelial Tissue (epithelium)
Classification: First name indicates number of cell layers, second name indicates shape of cells
Main functions: Protection, absorption, filtration, excretion, secretion and sensory reception
Physical
Physiological
Characteristics: sheet of cells that cover body surfaces or cavities. With PSSAR
Covering/lining epithelia: Found on external/internal surfaces like skin. First names inclu ...
Glandular epithelia: secretory tissue in glands (salivary glands). Gland: One or more cells that makes and secretes an aqueous fluid called a secretion
Simple epithelia: single layer thick, involved in absorption, secretion or filtration processes
Stratified epithelia: two or more layers thick and involved in protection (more durable than simple epithelia) i.e. skin (shape can vary in each layer, so cell named according to shape in apical layer), new cells regenerate from below (basal cells divide and migrate toward surface)
Exocrine: externally secreting (sweat)
Endocrine: internally secreting (hormones)
Supported by connective tissues: all epithelial sheets supported by connective tissue
Avascular, but innervated: no blood vessels are found in epithelial tissue (must be nourished by diffusion from underlying connective tissues) and epithelia are supplied by nerve fibers
Specialized Contacts: Epithelial tissues need to fit closely together (many form continuous sheets) and bind adjacent epithelial cells together via specialized contact points (lateral contacts include tight junctions (cells tightly tied together by protein conglomerates - helps epithelial polarity by preventing proteins in apical region from diffusing into basal region) and desmosomes (blocks large molecules from passing through adjacent cells and connects adjacent cells cytoskeleton))
Regeneration: epithelial cells have high regenerative capacities - stimulated by loss of apical=basal polarity and broken lateral contacts
Polarity: Cells have polarity (top and bottom) which differ in structure and function
Basal: Lower attached side facing inwards towards body. Attaches to basal lamina (adhesive sheet that holds basal surface of epithelial cells to underlying cells)
Apical: upper free side exposed to surface or cavity. Most are smooth but some have microvilli
Reticualr lamina: deep to basal lamina, consists of network of collagen fibers
Basement membrane: made up of basal and reticular lamina, reinforces epithelial sheet and resists stretching and tearing, defines epithelial boundary
Cancerous epithelial cells are not contained by basement membrane boundary as they penetrate and invade underlying tissues, resulting in spread of cancer
Some cells are exposed to friction/hostile substances, resulting in damage and must be replaced, requiring adequate nutrients and cell division
Cuboidal: box-like, cube
Columnar: tall, column-like
Squamous: flattened and scale-like
simple squamous epithelium (air sacs) : cells flattened laterally and cytoplasm is sparse, function where rapid diffusion is priority like kidney/lungs
Endothelium: lining of lymphatic vessels, blood vessels and heart
Mesothelium: Serous membranes in the ventral body cavity
simple cuboidal epithelium (kidney tubules): involved in secretion and absorption, forms walls of smallest ducts of glands and many kidney tubules
simple columnar epithelium (stomach/intestines): involved in absorption and secretion of mucus, enzymes and other substances (ciliated cells move mucus), found in digestive tract, gallbladder,uterine tubes, bronchi and ducts of some glands
Some have microvilli, cilia and or mucus-secreting goblet cells
Pseudostratified ("false" stratified) columnar epithelium (trachea): cells vary in height and appear to be multilayered (many cells are ciliated), involved in secretion, particularly of mucus, and movement of mucus via ciliary sweeping action. Located mostly in upper respiratory tract, ducts of large glands and tubulules in testes
Stratified squamous epithelium (esophagus): most widespread of stratified epithelia, free surface is squamous with deeper cuboidal or columnar layer, located in areas of high wear and tear (i.e. skin), keratinized cells found in skin/nonkeratinized cells are found in most linings
stratified cuboidal epithelium (sweat/mammary glands): quite rare, typically only two cell layers thick
stratified columnar epithelium (pharynx, male urethra, glandular ducts): also rare, only apical layer is columnar, occurs at transition areas between two other types of epithelia
Transitional epithelium (bladders,ureters,urethra): basal layer cells are cuboidal or columnar. They can change shape when stretched, allowing for increased flow of urine and more storage space
Unicellular (i.e goblet cells)
multicellular (i.e. salivary)
Ductless glands: secretions are released into surrounding interstitial fluid (which is picked up by circulatory system) instead of a duct
Secrete hormones (messenger chemicals that travel through lymph or blood to their specific target organs) via exocytosis
Secretions are released onto body surfaces (i,e, skin) or into body cavities. Products are secreted into ducts
More numerous than endocrine glands and can be unicellular, multicellular
Multicellular exocrine glands: composed of a duct and secretory unit, usually surrounded by supportive connective tissue that supplies blood and nerve fibers to gland (connective tissue can form capsule around gland and extend into gland, dividing it into lobes). Further classified by structure and mode of secretion
Unicellular exocrine glands (mucous cells and goblet cells): found in epithelial linings of intestinal and respiratory tracts. All produce mucin ( water soluble, mucus forming/slimy protective/lubricating coating sugar-protein)
Mode of secretion
structure
simple exocrine glands have unbranched ducts vs compound glands that have branched ducts
tubular glands have secretory cells that form a duct vs alveolar glands that have secretory cells that form sacs. tubuloalveolar glands have both types
Holocrine (sebaceous oil glands): accumulate products within, then rupture
Merocrine (sweat pancreas): most secrete products by exocytosis as secretions are produced (?)
Apocrine (mammary cells?): accumulate products within, but only apex ruptures; existence in humans=controversial