Histology Chapter 4

Main characteristics of the four basic types of tissues.

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Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface. Epithelia (Gr. epi, upon + thele, nipple) line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue.

lamina propria

The connective tissue that underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems
-Most epithelia are adjacent to connective tissue containing blood vessels from which the epithelial cells receive nutrients and O2.
- Even thick epithelia do not themselves normally contain blood vessels.

papillae

The area of contact between the two tissues (epithelia & lamina propria) may be increased by small evaginations called papillae (L. papula, nipple) projecting from the connective tissue into the epithelium. *Papillae occur most frequently in epithelial tissues subject to friction, such as the covering of the skin or tongue.

Epithelial Cell Polarity

- polarity with organelles and membrane proteins distributed unevenly within the cell
- basal pole - the region of the cell contacting the ECM and connective tissue
- apical pole - the opposite end, usually facing a space

basement membrane

Parts of the Basement Membrane

1.) Basal lamina - closest layer to epithelium (type IV collagen, fibers, laminin (major glycoprotein (b)), proteoglycans, glycoproteins)
2.) Reticula layer - Type III collagen fibers secreted by underlying connective tissue. Absent in some epithelial membranes.

ECM components of the Basal Lamina

■ Type IV collagen: Monomers of type IV collagen self- assemble into a two-dimensional network of evenly spaced subunits.
■Laminin: These are large glycoproteins that attach to trans-membrane proteins called integrins at the cells' basal surface and project through the network of type IV collagen.
■ Nidogen and perlecan: Respectively a short, rod-like protein and a proteoglycan, both of these cross-link laminin to the collagen network and help determine the porosity (porous) of the basal lamina and the size of molecules able to filter through it.

external laminae

reticular lamina

The more diffuse meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue

Functions of Basement Membranes

-filters
- structural support for epithelial cells
- attach epithelia to underlying connective tissue
- Basal lamina components help organize integrins and other proteins in the plasma membrane of epithelial cells, maintaining cell polarity and helping to localize endocytosis, signal transduction, and other activities.
- mediate many cell-to-cell interactions involving epithelia and mark routes for certain cell migrations along epithelia.
- serves as a scaffold that allows rapid epithelial repair and regeneration.

Junctional complexes of epithelial cells.

Epithelial cell junctions, their major structural features and functions, and medical significance

Histology Chapter 4

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