In androgenetic alopecia, follicular miniaturization is the main event that characterizes pattern hair loss. Miniaturization of hair can be described as the transformation of large terminal hairs into fine vellus-like hairs, accounting for the balding process.
There are three types of hairs seen on human skin:
- Vellus hairs, which are tiny colorless hairs, the growth of which is not influenced by hormones
- Terminal hairs, which are the thick-pigmented hairs found on the scalp, beard, armpits, and pubic area, the growth of which is influenced by hormones.
- Intermediate hairs, which are thin hairs between vellus and terminal.
In early fetal life, the forehead of the fetus is covered with long lanugo hair like that on the scalp. This hair begins to miniaturize and involute after the fifth month in utero. Vellus hairs are commonly found on the scalp in children, but are rare in late puberty (probably due to conversion to terminal hairs) and then reappear in the twenties. As a normal consequence of aging, hair of both men and women undergo progressive miniaturization and there is reduced terminal scalp hair density.
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How does miniaturization in androgenetic alopecia occur
The hair follicle is one of human biology’s most fascinating structures, and familiarity with the anatomy of the normal hair cycle is useful in understanding the evolution of miniaturizing hairs. The average number of hairs in the adult scalp is 100,000, of which 90% are in the anagen (growth) phase and 10% in telogen (resting state). The average duration of anagen is 1000 days, of telogen, 100 days, and the average daily loss of telogen hairs is 100. This ratio is usually uniformly distributed over the entire scalp.
In the presence of androgens, genes that shorten the anagen phase are activated, and hair follicles shrink or become miniaturized. With successive anagen cycles, the follicles become smaller (leading to shorter, finer hair), and non-pigmented vellus hairs replace pigmented terminal hairs.
With time, the papillae and matrices of hairs as well as resulting hair shafts in androgenetic alopecia become progressively transformed by miniaturization. These result in a reduction in the terminal-to-vellus hair ratio, normally at least 2:1. Earlier it was perceived that this transformation of terminal to vellus hairs progressed gradually over a number of follicular cycles. However, some researchers and hair biologists believe that it is unlikely that miniaturization can be explained only by a series of progressively shorter anagen cycles.
Calculations and documentation show that the process would take too long for significant miniaturization to occur secondary to shorter anagen cycles alone, especially in view of the latent lag period seen in pattern hair loss that occurs between the loss of a telogen hair and the appearance of an anagen hair. Many hair biologists are of the opinion that miniaturization is an abrupt, large-step process that also can be reversed in 1 hair cycle – this theory is supported by confirmatory histological evidence, in patients with pattern hair loss responding to finasteride (a drug used to reduce the amount of male hormone, testosterone, produced by the body) treatment. It is hypothesized that the miniaturization seen with pattern hair loss may be the direct result of reduction in the cell number and, hence, size of the dermal papilla.
Research shows that the initiation of miniaturization may occur at some stage in early catagen or early anagen, when the dermal papilla is moving up or down the temporary lower follicle and is vulnerable to external influences. It is believed that miniaturization does not occur during established anagen, since anagen, hairs maintain the same diameter during each hair cycle, nor in telogen where there is no metabolic activity.
Where does follicular miniaturization take place
Miniaturization probably occurs in the dermal papilla (the connective tissue element which is enclosed by the bulb of the follicle during anagen, and which forms a compact ball of dermal cells underneath the “hair germ” during telogen) and dermal sheath (perifollicular sheath) of the hair follicle, when initiated by stem cells in the follicular bulge. This terminal-to-vellus switch is influenced by androgen receptors, 5a-reductase, stem cell factors, drug factors, and cytokines, say researchers and hair biologists. Following miniaturization of the follicles, fibrous tracts remain.
Miniaturization of hair in male androgenetic alopecia
In men with androgenetic alopecia, all the hairs in an affected region may eventually become involved in the miniaturization process and may with time cover the area with fine vellus hairs. Since pigment production also ceases with progressive miniaturization, the area may appear bald long before the vellus hair covering is finally lost. As the condition progresses, involved areas in men, however, can become totally devoid of hair.
Miniaturization of hair in female pattern hair loss
The degree of miniaturization of hair is not uniform nor is it as potentially extreme in a given area of the scalp in women. Instead, women with pattern hair loss have a mosaic of variable-diameter hairs in the affected region of the top of the scalp. This explains the difference in the clinical signs between men and women – in women with female pattern hair loss there is a visual decrease in hair density in the affected area, as opposed to baldness in men. In extreme cases of women with pattern hair loss that do become bald, evaluation for an underlying pathologic hyperandrogen state is important.
The fact that the affected hairs, although small, are not usually lost results in the hope that there may be possibility of developing suitable treatment that can re-grow vellus hair to a size large enough to reverse hair thinning. In other words, a clearer understanding of the miniaturization process may provide clues to identifying new approaches to treatment. Reversal of miniaturization in pattern hair loss may depend on different factors in males and females. Although it is not possible to distinguish by histopathology the pattern hair loss between the sexes, there is some evidence that more hairs are miniaturized in males than in premenopausal or postmenopausal females. A similar family history may apply in both sexes, but the pattern of balding and response to treatment differs.