Final answer:
A follicular cast is a cylindrical mass of keratin that surrounds a hair follicle, associated with the biological structures of hair and skin.
Step-by-step explanation:
A follicular cast is defined as a cylindrical mass of keratin surrounding a hair follicle. This is a biological concept related to the structure and health of the hair and skin. The function of hair is mainly to act as a physical barrier for the skin and to help regulate body temperature. Each hair on the human body grows out of a hair follicle, an anatomical structure found within the dermis of the skin. The structures comprising the hair follicle include the hair bulb, which is the structure at the base of the hair root that surrounds the dermal papilla, the hair matrix, which is a layer of basal cells from which a strand of hair grows, and the hair papilla, consisting of connective tissue, blood vessels, and nerve endings that nourish the cells in the hair bulb.