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1. Epithelium ("covering")
2. Epithelial tissue occurs in two different forms:
- 1. is a sheet of cells that cover a body surface or lines a body cavity
- 2.
- Covering and lining epithelium covers the outer and inner surfaces of most body organs (Ex. Skin)
- Glandular epithelium forms most of the body glands
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Special Characteristics of Epithelia
- Cellularity
- Specialized contacts
- Polarity
- Support by connective tissue
- Avascular but innervated
- Regeneration
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1. Glands
2. Secretion
- epithelial cells that make and secrete a product
- the products of glands are aqueous fluids that usually contain protein
- 2. is the process whereby gland cells obtain needed substances from the blood and transform them chemically into a product that is then discharged from the cell
- RER creates protein→Golgi Apparatus→exocytosis
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Glands are classified as:
- endocrine (internal secretion)
- exocrine (external secretion)
- and
- unicellular ("one-celled")
- multicellular ("many-celled")
- most multicellular glands develop by invagination of an epithelial sheet into the underlying connective tissue
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Endocrine Glands
- Ductless Glands
- produce messenger molecules called hormones
- ↓ released into the extracellular space
- ↓ enter nearby capillaries and travel through bloodstream to specific target organs
- target organ
and endocrine gland are usually far apart
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Exocrine Glands
- secrete their products onto body surfaces (skin) or into body cavities (digestive tube)
- multicellular have ducts that carry products to the epithelial surfaces
- secretion is local; the secretion acts near the area where it is released
- Ex. sweat and oil glands
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Goblet Cell=Unicellular Exocrine Glands
- are scattered within the epithelial lining of the intestines and respiratory tubes, between columnar cells with other functions
- produce mucin, a glycoprotein that dissolves in water when secreted
- mucin+water=mucus
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Three factors act to bind epithelial cells to one another:
- (1) Adhesion proteins in the PM of the adjacent cells link together in the narrow extracellular space
- (2) the wavy contours of the membranes of adjacent cells join in a tongue-and-groove fashion
- (3) there are special cell junctions
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Tight Junction or zonula occludens ("small zone that shuts off")
- in the apical region of most epithelial tissue, a beltlike junction extends around the periphery of each cell
- the adjacent cells are so close that some proteins in their PM are fused
- this fusion forms a seal that closes off the extracellular space; thus preventing molecules from passing between the cells of epithelial tissue
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Adhesive Belt Junction (zonula adherens)
- located just below the tight junctions in epithelial tissues
- a type of anchoring junction
- this junction reinforces the tight junctions, particularly when the tissues are stretched
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Gap Junction or nexus
- is a tunnel-like junction that can occur anywhere along the lateral membranes of adjacent cells
- connected by hollow cylinders of protein=connexons
- function in intercellular communication by allowing small molecules to move directly between neighboring cells
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Basal Lamina
- at the border between the epithelium and the connective tissue
- functionally, acts as a selective filter; it determines which molecules from capillaries in the underlying connective tissue are allowed to enter the epithelium
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