Epithelial cells are found in many organs, including skin, intestines, liver, and pancreas, where they perform a wide variety of activities. Learning to identify and classify examples of this diverse tissue can be an interesting challenge because it requires that you consider a set of interrelated features that include the locations, organization, and functions of epithelial cells.
It is useful to begin by recognizing that continuous sheets of epithelial cells cover many of the body's free surfaces (e.g., skin and intestines). Tight connections between epithelial cells at these surfaces are necessary to keep the epithelial sheets physically intact. The continuity is an important feature of the epithelia lining surfaces connected to the external environment because it helps prevent pathogens from entering the body. However, the epithelium lining the intestines must also mediate the transport of dietary nutrients into the body. These dual functions of protection and transport, along with the structural features that provide tight connections between cells, are shared by many epithelia.
Epithelia are also the key elements of many glands, such as the liver and pancreas, which produce and export numerous products onto free surfaces or into the blood. The free surfaces in some of these secretory organs may be insignificant, but the characteristic close connections among the epithelial cells are maintained.
A specialized set of epithelial cells provides additional functions in the body. Epithelial cells serve as sensory receptors for the nervous system in the eye, ear, and mouth. Other epithelial cells serve as small muscles, facilitating the expulsion of glandular secretions or dilating the iris of the eye.
CHARACTERISTICS OF EPITHELIA
The human body contains hundreds of different kinds of epithelial cells, making it difficult to provide a concise and complete characterization of epithelial tissue. It is helpful to note that many epithelia cover a surface. This location requires these cells to exhibit a set of structural features to maintain cohesion and include cell–cell junctions, cell–substrate junctions, and intracellular filament systems. Many of these features are common to all epithelial cells. This chapter begins by considering the common features of epithelial cells, briefly discusses replacement and embryology of these cells, and then outlines the scheme used to name different types of epithelium and provides examples of each type. Finally, some of the major epithelial functions are considered.
Epithelia are found in two primary locations (Figure 1-1A–C):
Covering the external or internal surface of an open space (e.g., the outer surface of the skin or the inside lining of the digestive system).
As the parenchyma (functional tissue cells) of internal organs (e.g., the kidney or liver).
Characteristic locations of epithelia within the body. A. Section of the surface of the skin, which consists of layers of epithelial cells that provide covering and protection ...