L2 - Epithelial | Histo Lec

• epithelial tissue is avascular, meaning it has no blood vessels in its cell
• epithelial tissue - covers the body surfaces and those found as sheets lining the organ cavities
• epithelial tissue is usually wet or moist (except the epidermis of the skin), attached to extracellular matrix and made up of polyhedral cells
• epithelial tissue functions mainly for absorption, secretion and covering
• Examples of Functions per Distribution

  Absorption - small intestine

  secretion - glands

  excretion - kidney tubules

  protection - skin or epidermis

  sensory reception - neuroepithelium

  lubrication - sebaceous glands

  reproduction - ovary lining
• Structures Associated with Epithelial Tissues

  1. Basal lamina or basement membrane
  2. Tunica propria or lamina propria
• basement membrane or basal lamina

  • where all epithelial tissue cells are attached
  • provides anchorage for the cells
  • barrier limiting or regulating exchanges of macromolecules between CT and other tissues
• Components Contribute to Image of Basement Membrane: basal lamina, reticular fibers and ground substances
• Passage of tumor cells across basal lamina indicate invasive quality of these cells and is important due to pathologist in evaluating degree of malignancy
• Tunica propria or lamina propria - support to the epithelium and vascularized CT bed providing nourishment for epithelial tissue cells
• all epithelia has tunica propria
• polarity - in columnar epithelium, structural and functional differences between two ends
• cell axis - imaginary line passing through the centrosome and center of the nucleus that is perpendicular to basal lamina
• terminal web - made of feltwork of fine filaments beneath the surface that provide mechanical support for ciliated or striated borders
• Zonula Occludens or Tight junctions

  • serves as tight seal that prevents flow of materials between epithelial cells
  • belt-like barrier junction around apex of the cell
  • provides close apposition of adjacent plasma membranes and occludes the intercellular space
  • prevents diffusion of material between the intercellular space and the lumen of the organ
• zonula occludens is a transporting epithelium as in that of a gall bladder
• zonula occludens is the most apical junction of the epithelial tissue
• Tight junctions serves a mechanical role in maintenance of structural integrity of the cells
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  Zonula adherens or intermediate junctions

  • the region where adjacent cells are firmly held together that is also responsible for terminal bar
  • hold together epithelial cell
  • lies below the tight junction
• desmosome or macula adherens - a disk-like structure at the surface of one cell that is match with identical structure at surface of adjacent cell
• macula adherens only functions as special adhesion of cells
• gap junction or nexus - permit transfer of small molecules such as ions, sugar, amino acids and some hormones
• nexus can be mistaken for zonula occludens but it has opposing membrane that are not fused
• nexus occur almost anywhere along lateral membranes of most epithelial cells but absent in skeletal muscle and blood cells
• gap junctions are well developed in tissues where cells are electrically coupled such as cardiac and smooth muscles
• microvilli

  • striated or long brush border
  • short or long finger-like extensions or folds
  • prominent in cells that function in absorption
• brush border in microvilli serve as adaptation to enhance the surface area of membrane exposed to substances to be absorbed
• stereocilia

  • long non-motile processes of cells projecting in the lumen from each cell in the epithelium of epidydimis
  • like hair of a water color brush
  • longer branched microvilli
  • function is not well-established but epithelium is absorptive
  • assumed that it promotes this function by amplifying the cell surface
• cilia

  • numerous motile processes larger than microvilli
  • arranged in parallel rows projecting from surfaces of some epithelial cells
  • at the base is dense elongated granule called basal body
  • moves in waves and sweep over epithelium
  • surfaces serve to propel fluid or coating of mucus towards the exterior
• flagella

  • elongated motile structure on surface of epithelial cells, longer than cilia
  • differ with cilia in movements and number of cells
  • function is not apparent
• largest flagella is the mammalian spermatozoa
• polyhedral form is the common form due to their position in cellular layers
• long axis of the nucleus is always parallel to the main axis of the cell
• simple epithelium - with single layer
• stratified epithelium - with more than one layer
• simple squamous

  • thin plate-like cells arranged in mosaic pattern
  • with central, round and bulging nucleus
  • cells with serrated borders/outlines
  • forms barriers in regions of filtration and diffusion
• simple squamous are found in the lining walls of blood vessels, lymph vessels, heart cavities, lining serous cavities, lining interior chamber of eye, perilymph spaces, subarachnoid spaces, and air sacs of lungs
• simple cuboidal

  • made up of cube-like cells with central and round nucleus
  • in kidneys, it performs an excretory function
  • found in non-ciliated kidney tubules, thyroid follicles, secretory ducts of glands and bronchioles
• simple columnar non-ciliated

  • made up of columnar cells
  • cells with oval nucleus, located near the basement membrane
  • no cilia attached on the free surface of the cell
  • found in gallbladder, pyloric end of stomach, small and large intestine, and rectum
• simple columnar ciliated

  • with cilia
  • found in fallopian tubes, lungs and uterine tube