The nutrition of epithelial cells is by indirect exchange through the BL and matrix substances with blood in the capillaries of the lamina propria. In glands, the working epithelial cells constitute the parenchyma . The supporting connective tissue and other elements make up the stroma . Cell shapes give the secondary classes. Simple epithelia , in general, are adapted to absorptive and secretory roles, while compound epithelia protect against damaging mechanical and chemical actions.
Compound epithelia frequently, and simple sometimes, have several types of cell present.
Cells lying basally on the BL are mitotically active and migrate upwards, differentiating to replace cells lost from the surface, or cells that have destroyed themselves by apoptosis. Epithelia shed cells continually. Cells above the base become polyhedral and are held together by many desmosomes to resist the abrasive forces on this protective epithelium. Chapter 21.A . No connective tissue papillae indent the epithelium.