It is secreted by the adjacent layer of epithelial cells. This is responsible for preventing large molecules like protein from reaching the epithelial cells. The deeper part of the basement membrane, the reticular lamina, contains coarse protein fibres secreted by the connective tissue, and provides the strength of the lamina. Epithelial cells are avascular, meaning that they do not have any contained blood vessels. They obtain nutrients by diffusion across the basal or apical surface of the cell, and dispose of waste matter by the same routes.
Epithelia have an enormous capacity for growth and repair.
Each epithelium includes stem cells that divide to replace cells removed from the apical surface, and to allow for growth of the tissue. Epithelial cells may be exposed to enzymes or to abrasion. Under these circumstances, they do not last long, and must be replaced when they are destroyed or shed. Epithelia are replaced by the division of stem cells, or germinative cells, within the epithelium, located in the deepest layer closest to the basement lamina. Classification of Epithelial Cells.