Hair Embryology
During the third month of gestation, the epithelial primary hair germs develop in the eyebrow region and scalp. Subsequently, hair development continues in a cephalocaudal direction.
The primary hair germ begins as an epithelial bud that protrudes into the dermis and seems to be directed by a collection of mesenchymal cells that eventually form the dermal (follicular) papilla.
The germ continues to grow deeper in the dermis under the guidance of the dermal papilla. It then acquires multiple epithelial buds that grow transversely and differentiate into the many components of the primary hair follicle (ie, sebaceous gland and duct, apocrine gland and duct, attachment point of the erector pili muscle [bulge region]).[1]
Sebaceous glands are absent in the palms and soles; apocrine glands develop only in certain body regions. The hair grows at a slant angle.
Melanocytes are scattered haphazardly in the primary epithelial bud.
Anatomy of the hair follicle. Classification of Hair
Human hair varies with respect to texture, color, and length.[2, 3] Lanugo hairs cover most of the fetal skin and shed perinatally. These lightly pigmented hairs are fine in texture.
Thicker and darker than lanugo hairs, vellus hairs cover most of the glabrous skin surface except palms, soles, palmar and plantar surfaces of fingers and toes, inner aspect of the prepuce, and the glans penis.
Darkly pigmented terminal hairs are long and thick and are most commonly located on the scalp and the facial areas of men. These hairs can reach a preprogrammed length based on length of the anagen phase. Ultimately, terminal hairs undergo involution and convert to catagen and then to the telogen phase. The mechanism and factors that induce the hair to terminate growth and involute are largely unknown.
Some hair follicles have the ability to produce different types of hair at various times. For example, hair follicles on an adult scalp that normally produce terminal hairs can, in individuals with androgenetic alopecia, undergo miniaturization and produce short, fine, and lightly pigmented hairs. Similarly, hair follicles on the male beard, which until puberty produce only vellus hairs, are capable of producing terminal hair thereafter.
Hair Growth Cycles
Histological features of the human hair follicle vary greatly during the growth cycle.[4] All hair, terminal and vellus, goes through a growth phase (anagen), a regression or transitional phase (catagen), and a resting phase (telogen).[5]
Anagen hair bulbs are located in the subcutaneous fat, catagen bulbs are in the dermis, and telogen bulbs are in the mid-to-upper dermis. The anagen phase for vellus hairs is much shorter than that for terminal hairs.
The hair growth cycle in different regions of the body varies, and the duration of anagen, catagen, and telogen phases differs according to body site. While the entire cycle duration for the eyelashes is 4 months, the scalp cycle requires about 3-4 years to complete.
In the normal human scalp, the anagen phase duration averages 2-3 years (occasionally much longer); the catagen phase, 2-3 weeks; and the telogen phase, approximately 3 months.
Hair located on other body sites (eg, eyebrows, trunk, extremities) is characterized by longer telogen phases (up to 9 months) and shorter anagen periods (4-7 months). The catagen phase remains constant at 3-4 weeks.
Absent a disease state, 85-90% of all scalp hair follicles are in the anagen phase, 10-14% are in the telogen, and only 1-2% are in the catagen. Estimates place daily scalp hair shedding at about 100 hairs per day.
Scalp hair grows at an average rate of 0.4 mm/day (approximately 0.5 in/month). At any particular time, not all human hair follicles in any given anatomical location are in identical growth stages; some are in the anagen, while others are in the telogen or catagen. This phenomenon is referred to as the mosaic pattern and is in contrast to some animals whose follicular units all are synchronized. This explains some animals' ability to shed winter coats when the ambient temperature becomes warm, while humans do not go bald during the follicular rest period.
Kollar EJ. The induction of hair follicles by embryonic dermal papillae. J Invest Dermatol. Dec 1970;55(6):374-8. [Medline].
Hashimoto K. The structure of human hair. Clin Dermatol. Oct-Dec 1988;6(4):7-21. [Medline].
Sperling LC. Hair anatomy for the clinician. J Am Acad Dermatol. Jul 1991;25(1 Pt 1):1-17. [Medline].
Murphy GF. Histology of the skin. In: Lever's Histopathology of the Skin. 1997:5-50.
Kligman AM. The human hair cycle. J Invest Dermatol. Dec 1959;33:307-16. [Medline].
Tiede S, Kloepper JE, Bodò E, Tiwari S, Kruse C, Paus R. Hair follicle stem cells: walking the maze. Eur J Cell Biol. Jul 2007;86(7):355-76. [Medline].
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