Epithelial tissue
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.
The principal functions of epithelial tissues include the following:
- Covering, lining, and protecting surfaces (eg, epidermis)
- Absorption (eg, the intestinal lining)
- Secretion (eg, parenchymal cells of glands)
Classification
1. Covering epithelia — cover body surfaces and line body cavities.
2. Glandular epithelia — form an acinus or gland part, in the body
3. Special epithelia — myoepithelial, sensory epithelia, and germinal epithelium.
Figure 1. glandular and covering epithelium
General features of epithelia
Feature 1: abundance of cells in good order , rare intercellular substance
Feature 2: polarization
Epithelial cells have the free surface and basal surface with different morphology and function.
Feature 3: avascularity
No blood vessels in epithelia. Nutrients diffuse from the underlying connective tissue crossing basement membrane.
Feature 4: innervation Rich in nerve terminals
Feature 5: functions of epithelium
Protection; Secretion ; Absorption; Lubrication ; Sensory reception; Reproduction; Excretion
Covering epithelium
Serve as barrier membranes. Seal and separate the organism from various external or internal environments.
- Simple squamous epithelium
- Simple cuboidal epithelium
- Simple columnar epithelium
- pseudostratified columnar epithelium
- Stratified squamous epithelium
- Transitional epithelium
Table 1. types of covering epithelia
Simple squamous epithelium
Figure 2. Renal corpuscle, Simple squamous epithelium
Simple cuboidal epithelium
Figure 3. Kidney, Simple cuboidal epithelium Form tubules, Transverse section
Simple columnar epithelium
Figure 4. Small intestine, Simple columnar epithelium
pseudostratified columnar epithelium
Figure 5. Trachea, pseudostratified columnar epithelium
Stratified squamous epithelium
Figure 6. Nonkeratinized stratified squamous epithelium. (esophagus)
Figure 7. Keratinized stratified squamous epithelium (skin)
Transitional epithelium
Figure 8. transitional epithelium
Secretory Epithelia and Glands
Epithelial cells that function mainly to produce and secrete various macromolecules may occur in epithelia with other major functions or comprise specialized organs called glands.
Glands develop from covering epithelia in the fetus by cell proliferation and growth into the underlying connective tissue, followed by further differentiation.
Exocrine glands remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver the secreted material where it is used. Endocrine glands lose the connection to their original epithelium and therefore lack ducts. Thin-walled blood vessels (capillaries) adjacent to endocrine cells absorb their secreted hormone products for transport in blood to target cells throughout the body.
- Glands can be simple (ducts not branched) or compound (ducts with two or more branches).
- Secretory portions can be tubular (either short or long and coiled) or acinar (rounded and saclike); either type of secretory unit may be branched, even if the duct is not branched.
Compound glands can have branching ducts and can have multiple tubular, acinar, or tubuloacinar secretory portions.
Figure 9. exocrine and endocrine glands
Table 2. Structural classes of exocrine glands, features of each class, and examples.
- Endocrine glands lack ducts; secreted substances are hormones carried throughout the body by the interstitial fluid and blood, with specificity produced by the hormone receptors of target cells.
- Glands have three basic secretory mechanisms: merocrine, which uses exocytosis; holocrine, in which terminally differentiated cells filled with lipid product are released; and apocrine, in which apical, product-filled areas of cells are extruded.
- Exocrine glands producing mucus, or similar individual cells called goblet cells, are called mucous glands; oligosaccharide compo- nents of mucus stain poorly with routine dyes but stain well with PAS stain.
- Exocrine glands producing largely enzymes (proteins) are called serous glands and stain darkly with H&E due to the cells’ content of RER and secretory granules.
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