راما ملحم

• Epithelial tissue (1)
• Epithelial tissue


  • Closely packed cells form a sheetlike or tubular structure
  • Have strong connections (cell junctions)
  • Have little amount of extracellular matrix (ECM)
  • Avascular (no blood vessels): nutrients diffuse in from underlying CT
  • Innervated tissue
  • Rest on basement membrane (ECM)
  • Have high mitotic activity (continuous renewal)
• Basement membrane


  A thin extracellular sheet of specialized proteins, with TEM has 2 layers
• Basal lamina


  • Secreted by epithelial cells
  • Contains type IV collagen
• Reticular lamina


  • Secreted by connective tissue cells
  • Contains reticular fibers & type VII collagen
• Basement membrane is a semipermeable filter for substances reaching epithelial cells from CT
• Basement membrane provides structural support for epithelial cells and attach epithelia to underlying CT
• Basement membrane maintains cell polarity and helps to localize endocytosis
• Basement membrane serves as a scaffold that allows rapid epithelial repair and regeneration
• Microvilli


  Fingerlike cytoplasmic projections from the apical cell surface supported by microfilaments (actin)
• Microvilli are found in absorptive cells (intestine, proximal convoluted tubule of kidney)
• Microvilli increase membrane surface area for greater absorption
• The average microvillus is about 1 μm long and 0.1 μm wide, but with hundreds or thousands present on the end of each absorptive cell
• The thick glycocalyx covering microvilli of the intestinal brush border
• The total surface area can be increased by 20- or 30-fold
• The actin filaments insert into the terminal web of cortical microfilaments at the base of the microvilli
• Stereocilia


  Less common type of apical process, restricted to absorptive epithelial cells lining the male reproductive system epididymis and the proximal part of ductus deferens
• Stereocilia increase the cells' surface area, facilitating absorption
• More specialized stereocilia with a motion-detecting function are important components of inner ear sensory cells
• Stereocilia are much longer and less motile than microvilli, branching along their length
• Cilia


  Long, highly motile apical structures, larger than microvilli, and containing internal arrays of microtubules not microfilaments
• Motile cilia are abundant on cuboidal or columnar cells of many epithelia
• Typical cilia are 5-10 μm long and 0.2 μm in diameter, which is much longer and two times wider than a typical microvillus
• Cilia are found in the respiratory tract, uterine tube
• Cilia exhibit rapid beating patterns that move a current of fluid and suspended matter in one direction along the epithelium
• The long flagellum that extends from each fully differentiated sperm cell has an axonemal structure like that of a cilium and moves with a similar mechanism