Terminal hair - Terminal hair length

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What is terminal hair?

Terminal hairs are the thick pigmented hairs found on the scalp and on certain other parts of the body.

Their length is genetically predetermined. In the newborn, the scalp follicles produce short, silky and poorly pigmented vellus strands. As the infant grows the follicle grows in size and the vellus strand is replaced with an intermediate type of strand, having a myelin sheath. By two years of age, the scalp, eyelids and eyebrows have thick pigmented terminal hair. The percentage of vellus strands gets reduced progressively and their presence becomes unnoticeable.

As the child grows, with increased androgenic hormone levels, especially in puberty, other parts of the body, hitherto having vellus growth, start replacing the vellus with terminal type. Each of these follicles develops subcutaneous glands. Boys and girls during puberty develop axillary and pubic hair. The follicle sensitivity to androgens varies in boys and girls.

In young women, normally the androgenic hormones circulating at low levels do not have any effect on the vellus strands on the other areas of the body including face.

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However in young men, the androgens stimulate the vellus follicles on the face, chest, arms and legs to transform into terminal follicles and grow thick pigmented strands.

Terminal hair length

The length of these strands is relative to the length of the anagen phase of growth. The anagen phase length varies and is unique to a particular area of the body.

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The length of anagen phase of the underarm and genital hair is short and may last between three to six months. In the scalp the length of the anagen phase is relatively longer than the other body regions. Depending upon the genetic origins, the length of the anagen phase of scalp hair may vary between two to seven years.

Types of terminal hair

Scalp hair growth stabilizes by the end of two years. In the adulthood, it is transformed into vellus strands in the case of male pattern baldness. The length of the life of strands varies between 2-7 years. Its average growth rate is about 0.35mm per day.

Eyebrows grow over the upper margin of eye sockets. Their average length on the higher side is about 10 mm. They have the slow growth rate of 0.15 mm per day. Eyebrows follicles are sensitive to injury.

Eyelashes grow on the margin of eyelids. Their average length is around 7.5 mm and their growth rate is about 0.15 mm per day.

Beard grows in response to androgenic hormones in men. It is the fastest growing hair and its average growth rate is about 0.38 mm per day.

Body hair is initially vellus and in boys, during puberty, it turns thick and pigmented in certain regions of the body. The axillary and pubic hair grows in boys and girls during puberty.

Complications and anomalies in terminal hair growth

The hormonal imbalances occurring during the length of pregnancy may cause vellus follicles to transform and produce thick, pigmented strands in certain parts of the body. However, the condition returns to normalcy after the childbirth.

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In male pattern baldness, the thick hairs on the scalp are progressively replaced with vellus strands.
In women affected by hirsutism, vellus follicles, especially on the upper lip, transform and produce terminal hair.

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Reference: 1.Vogt, A., Hadam, S., Heiderhoff, M., Audring, H., Lademann, J., Sterry, W. and Blume-Peytavi, U. (2007), Morphometry of human terminal and vellus hair follicles. Experimental Dermatology, 16: 946–950. 2.Navarini AA, Ziegler M, Kolm I, Weibel L, Huber C, Trüeb RM. Minoxidil-induced trichostasis spinulosa of terminal hair. Arch Dermatol. 2010 Dec;146(12):1434-5.

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