Learning Objectives
Describe the structural characteristics of the various epithelial tissues and how these characteristics enable their functions.
By the end of this section, you will be able to:
- Explain the general structure and function of epithelial tissue
- Distinguish between tight junctions, anchoring junctions, and gap junctions
- Distinguish between simple epithelia and stratified epithelia, as well as between squamous, cuboidal, and columnar epithelia
- Describe the structure and function of endocrine and exocrine glands
Epithelial tissue primarily appears as large sheets of cells covering all surfaces of the body exposed to the external environment and lining internal body cavities. In addition, epithelial tissue is responsible for forming a majority of glandular tissue found in the human body.
Epithelial tissue is derived from all three major embryonic layers. The epithelial tissue composing cutaneous membranes develops from the ectoderm. Epithelial tissue composing a majority of the mucous membranes originate in the endoderm. Epithelial tissue that lines vessels and open spaces within the body are derived from mesoderm. Of particular note, epithelial tissue that lines vessels in the lymphatic and cardiovascular systems is called endothelium whereas epithelial tissue that forms the serous membranes lining the true cavities is called mesothelium.
Regardless of its location and function, all epithelial tissue shares important structural features. First, epithelial tissue is highly cellular, with little or no extracellular material present between cells. Second, adjoining cells form specialized intercellular connections called cell junctions. Third, epithelial cells exhibit polarity with differences in structure and function between the exposed, or apical, facing cell surface and the basal surface closest to the underlying tissue. Fourth, epithelial tissues are avascular; nutrients must enter the tissue by diffusion or absorption from underlying tissues or the surface. Last, epithelial tissue is capable of rapidly replacing damaged and dead cells, necessary with respect to the harsh environment this tissue encounters.
Epithelial Tissue Function:
Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological damage. The cells of an epithelium act as gatekeepers of the body, controlling permeability by allowing selective transfer of materials across its surface. All substances that enter the body must cross an epithelium.
Many epithelial cells are capable of secreting mucous and other specific chemical compounds onto their apical surfaces. For example, the epithelium of the small intestine releases digestive enzymes and cells lining the respiratory tract secrete mucous that traps incoming microorganisms and particles.
The Epithelial Cell
Epithelial cells are typically characterized by unequal distribution of organelles and membrane-bound proteins between their apical and basal surfaces. Structures