Epithelial tissues represent fundamental tissues in animals’ bodies. As examples, many tissues and organs, including exocrine glands (e.g., salivary glands and the pancreas), the skin, the liver, the kidneys, the lungs and the gastrointestinal tract mainly consist of epithelial tissues. In this section, the formation of epithelial tissues - key players in tissue architecture - and the roles cell adhesion plays in epithelial tissues are discussed. The direction of cells (cell polarity), which is one of the characteristics of epithelial cells, is also covered.
Epithelial tissues are sheet or tubular tissues consisting of cells connected by unique cell adhesion structures called junctional complexes (Column Fig. 11-4). Epithelial cells have apical, lateral and basal sides, and are bound on their lateral sides by junctional complexes. The basal side adheres to a basal lamina.
One of the characteristics of epithelial tissues is their role in creating an impermeable wall separating two environments (the body’s external and internal environments). Another characteristic is their active involvement in the transport of materials between the two environments. The roles played by epithelial tissues are closely associated with the junctional complexes formed between epithelial cells as well as with cell polarity.
Epithelial cells have cell polarity; the structure and functions differ between the apical side and the basal side (Fig. 11-11A). The nucleus, endoplasmic reticula, Golgi bodies and the cytoskeleton are arranged in certain directions in accordance with cell polarity. The reason epithelial cells have polarity is that they come into contact with and mediate between two different environments, and one of their important roles as a mediator is material transport between the two. The transport of nutrients from the outside to the inside of the body (through epithelial cells in the small intestine) and the transport of secretory materials from the inside to the outside of the body (through secretory cells in the pancreas and salivary glands) are examples of these functions. To fulfill these roles, epithelial cells have obtained polarity.
There are many other examples of cell polarity besides epithelial cells (Fig. 11-11B), including amphibian eggs (which have animal and vegetal poles), asymmetric cell division, migrating cells (which have anterior and posterior directionality), and neurons (which have directionality in axons and dendrites). This cellular directionality is formed in accordance with the functions of each cell.
Fig. 11-11. Cell polarity of epithelial cells
A) A model of the cell polarity of epithelial cells. Epithelial cells have apical, basal and lateral sides. Organelles and the cytoskeleton are arranged in particular directions in association with the directional intracellular trafficking - one of the roles of epithelial cells.
B) Cell polarity is observed in many cases other than epithelial cells. ↔ indicates the direction of polarity.