Wool fibers are outgrowths of the skin; the follicle is an indentation of the skin's outer layer. Basically, the follicle consists of a bulb down deep and a shaft leading from the bulb up to the skin surface. The fiber originates in the bulb and is pushed up the shaft, where it undergoes elongation and hardening or keratinization. From two types of follicles, the primary and secondary, three types of fibers are produced: true wool, heterotype/medullated, and kemp.
The larger primary follicles produce wool fibers, as well as being the normal source of heterotype, often medullated, fibers and kemp. The primary follicle is unique as it has a sweat gland, a sebaceous or grease gland, and an erector muscle. The sebaceous gland coats each fiber with a greasy secretion before it reaches the skin surface. This greasy secretion also keeps the skin soft and protects both the skin and the fiber from drying out. The sweat gland secretes a soapy perspiration called suint, which combines with the grease to produce the yellow colored yolk. When the erector muscle contracts, the outer coat hairs stand up to trap the air and cause pressure on the sebaceous gland releasing the suint. One primary follicle is capable of producing only one fiber.