Types of Hair on the Human Body

There are three types of hair in humans: lanugo, terminal and vellus.

• Lanugo Hair: Lanugo hair (foetal hair) develops at about the third month post conception. These hairs grow at the same rate all over the baby’s body are very fine, and lack a medulla. They shed in favour of vellus hair at 36 to 40 weeks of gestation.
• Vellus Hair: Vellus hair is short, fine hair that may be hard to see with the naked eye. These hairs develop in the womb about 4 weeks before a baby is born. They appear on most every part of the body except for the lips, back of ears, palms, soles, the naval, scar tissue, and some parts of the external genitalia (are they on eyelids?). The hairs are usually no longer than 2 mm long, do not have a medulla, contain little or no pigment, and are not attached to a sebaceous gland (glands in the dermis that secrete an oily/waxy substance called sebum to lubricate the hair and skin). Vellus hair acts as thermal insulation for the body.

• Terminal Hair: Terminal hairs are found on the head and other parts of the body and grow from follicles with sebaceous glands. During puberty, an increase in androgen hormones causes many vellus hairs to be replaced by longer, darker terminal hairs in both men and women, although men usually develop them in more places. These include the face, pubic area, armpits, abdomens, legs and arms. Terminal hairs often contain a medulla.

Terminal hairs are affected by both sweat and sebaceous glands.

A. Sweat Glands: Two types of sweat glands exist in humans, eccrine and apocrine glands.

1. The eccrine glands cool the body by producing sweat, a watery liquid comprised of the chemicals or odorants 2-methylphenol (o-cresol) and 4-methylphenol (p-cresol), as well as a small amount of urea, that exits the pores on the epidermis and evaporates. These glands also supply the skin with a constant supply of water and nutrients. It is believed that humans have between 2 and 5 million sweat glands in the body, with the highest concentration in the palms of hands, soles of feet, forehead, and underarms. The scalp and forehead also sweat through the eccrine glands. Eccrine glands do not require a hair to exit the body, instead relying on individual pores.

2. The apocrine glands are the larger of the sweat glands. They may extend as deep as the hypodermis and produce an odor that allows mammals to recognize one another. They may also serve sexual stimuli function. These glands are considered “primitive” and become fully functional at puberty. Apocrine glands are found in the arm pits, the areola, and the anal region, with their ducts opening into the canals of the hair follicles. The sweat is secreted into the upper parts of the hair follicle and exits to the skin via the hair shaft.¹

B. Sebaceous Glands: In humans, sebaceous glands exist in conjunction with hair and are most prevalent on the scalp and face (approximately 400 to 900 per square centimeter). They secrete an oily substance called sebum which lubricates, waterproofs and protects the hair and skin. Androgens (male hormones) are important factors in determining the amount of sebum secreted from sebaceous glands all over the body.

The word Sebum is Latin for “fat” or “tallow.” It is, indeed, made up of fat in the form of lipids, as well as wax and dead cells. Specifically, sebum is 25% wax monoesters (usually derived from an acid), 41% triglycerides, 16% free fatty acids, and 12% squalene (a biochemical precursor to a family of steroids).

Sebum itself is odourless, but odour can occur once it’s broken down by bacteria. In the sebaceous glands, sebum is produced in cells that eventually burst and release the sebum through a small duct leading from the gland to the hair shaft. This bursting action classifies sebaceous glands as holocrine glands (along with the meibomian glands of the eyelids)

A build-up of sebum can give the hair and scalp a greasy appearance and is usually the result of poor hygiene or the onset of puberty. In this case, higher level of male hormones (androgens) can cause sebum production to kick into overdrive.

1 Dermaxine.com, Sweat Glands, http://www.dermaxime.com/skin-sweat-glands.htm