Follicular cells are tube-shaped cells responsible for the production of certain hormones and substances. These cells usually squeeze together to form a visible tube that holds a hair-like structure. There are three types of follicular cell in the human body: thyroid epithelial cells, ovarian follicles, and hair follicles. Each kind of follicular cell is located in a different part of the body and responsible for producing different substances. Thyroid epithelial cells and hair follicles occur in men and women, but ovarian follicles are only found in women.
Thyroid epithelial cells, sometimes called principle cells, are located in the thyroid gland. The human thyroid gland is located at the base of the throat and is covered by a small pocket of tough cartilage. The small, hair-like follicular cell groups are seated at the core of the gland and harvest amino acids and iodine from the bodily fluids that pass over them. These cells then transform the amino acids into thyroglobulin proteins that help the thyroid gland function. The follicular cell groups can also transform iodine and amino acids into thyroid hormones that travel through the blood to help the body regulate the metabolism and excrete waste.
Ovarian follicular cells are similar to thyroid epithelial cells in shape and function. Located around the interior of the ovary, these tube-shaped structures secrete and contain ovarian eggs. Each immature egg, or oocyte, is contained within a group of follicular cells for about 28 days while it matures. After that, the cells release the egg into the center of the ovary, where it slips down the fallopian tube and into the uterus. Once there, the egg may or may not be fertilized. Either way, the follicular cells in the ovaries are busy readying another egg for departure.
Hair follicles are possibly the most numerous type of follicular cell. Every mammal on Earth has hair follicles somewhere on their bodies. In ocean-dwelling mammals, these follicles may only appear around the nose or lips, while the hair follicles on land-dwelling mammals are usually present over the entire creature. Mammals like cats, dogs, otters, and horses have hair follicles all over their bodies. Elephants and rhinos have follicular cells that produce thicker, sparser hair. How hair is produced in animals depends largely on environment and what kind of hair the animals need.
Humans have several different kinds of hair follicular cells meant to produce several different kinds of hair. Vellus hair follicles produce the soft, downy, colorless hair that often appears on women’s faces and torsos. Terminal hair follicles produce the hard, wiry hairs that develop under the arms and over the genitals during puberty. Pubescent males will also develop terminal hairs on their faces and across their chests. The hair follicles on the head produce hairs of a texture between vellus and terminal hairs.