By and large, androgenetic alopecia is easily recognizable and histological diagnosis is rarely necessary for male androgenetic alopecia. However, at times different skin conditions can look similar to the naked eye, and the additional information obtained by looking at the structure of the skin under the microscope can give valuable information towards a conclusive diagnosis. This is done by a scalp biopsy, a simple procedure in which a small area of the scalp is removed after administration of a numbing medication. Therefore, in patients where the diagnosis is ambivalent, small disk-shaped samples of tissues are removed from the vertex (top) of the scalp using a sharp, hollow device, and are examined under the microscope. 4 mm punch biopsies provide considerable data on the number and type of hair follicles seen in the scalp.
As a rule, two biopsies are taken for histological analysis, one horizontal section and the other vertical. Horizontal sections are likely to generate more information on the number and types of follicles seen, thereby facilitating a definitive diagnosis.
The earliest, but relatively nonspecific, histological sign of male pattern hair loss may be a spotty peri-vascular degeneration of the lower third of the connective tissue sheath of affected follicles which are in the anagen or growth phase. Normal connective tissue sheath would have pink and fibrilly collagen fibers, whereas the affected follicles in evolving pattern alopecia are recognized by a characteristic basophilic smudging as well as by retention of elastic tissue stains in these foci.
Amongst the three types of hair seen on human skin, terminal hairs are the thick pigmented hairs found on the scalp, beard, armpits, and pubic area, the growth of which is influenced by hormones; and vellus hairs are tiny colorless hairs, the growth of which are not influenced by hormones. The major feature seen on routine vertical sectioned scalp biopsies taken from androgenetic alopecia patients is the reduction in terminal anagen hairs, which normally penetrate through the dermis into the subcutis. These hairs are replaced by secondary psuedo-vellus hairs with angiofibrotic tract remnants called follicular streamers or stellae. There is an apparent reduction in the number of follicles, but the miniaturized follicles can be identified on horizontal sections of scalp biopsies.
Horizontal sections are useful in the diagnosis of androgenetic alopecia as they allow a greater number of follicles to be studied. Such samples of scalp biopsies point to identification of many pseudovellus hair follicles in the papillary dermis. This leads to the theory that affected hair follicles in androgenetic alopecia are miniaturized (transformation of large terminal hairs into fine vellus-like hairs) rather than destroyed.
When the hair miniaturizes, it ascends upward from the reticular dermis to the papillary dermis with a long streamer behind it and cycles up and down through anagen and telogen in the papillary dermis as a small vellus-like hair. If this hair is re-stimulated by treatment to transform again into a terminal hair, it travels back down that streamer or tract to the reticular dermis and generates a terminal hair again. In fact it is the presence of arrector pili muscle and angiofibrotic streamers which helps to differentiate miniaturized hairs of androgenetic alopecia from true vellus hairs. In the vast majority of cases, there is no genuine reduction in the number of follicles, and follicular fibrosis is only seen in about 10% of cases. . However, fibrosis is seen in a small number of normal scalp biopsies as well.
Histological analysis also reveals a change in the ratio of terminal to vellus hairs from greater than 6:1 to less than 4:1. Additionally, the anagen to telogen hair ratio reduces from 12:1 to 5:1. Studies and research show that moderate inflammation is more common in balding scalp and inflammation with or without fibrosis is more common in men with androgenetic alopecia vs. normal controls. This inflammation in androgenetic alopecia is seen as a mild to moderate peri-infundibular lymphohistiocytic inflammatory infiltrate. It is present in up to two thirds of biopsies but this again, is a non-specific feature that is also found in up to one-third of normal scalp biopsies. That this inflammation or fibrosis may have prognostic value for re-growth in pattern hair loss is shown by the results of a case study which documented that only 55 percent of those with vs. 77 percent of those without either fibrosis or inflammation demonstrated a response to topical minoxidil therapy.
At a later stage of androgenetic alopecia, the connective tissue streamer, or stele, which is seen in all telogen follicles, may become broader and more cellular in male pattern hair loss than the collapsed sheath seen with normal follicles. With advanced stages of the condition, the stele may become a fibrotic column. A mild lymphohistocytic infiltrate restricted to the upper follicle is commonly seen in male pattern hair loss but this is not a definitive indicator, as this may be seen in those without hair loss as well.
Furthermore, Hori and colleagues have pointed out a decrease in the thickness of the epidermis, dermis, and subcutaneous tissue in advanced male pattern hair loss compared with normal controls. This decreased skin thickness is probably attributed to the loss of substance of the normal hair follicles as well as loss of the connective tissue itself.
Conclusion
The miniaturization of hairs seen clinically in affected scalp areas of male pattern hair loss is reflected in the characteristic histological findings of representative scalp biopsies. These include hairs of vastly different diameter, decreased number of terminal hairs, increased number of vellus hairs, and increased telogen count. The overall total scalp hair density appears to be preserved until late in the course of pattern hair loss. There is also evidence that an increased number of mast cells (cells play an important role in the body’s allergic response) may also be a feature of pattern hair loss. The histopathologic findings in female pattern hair loss are indistinguishable from those of male pattern hair loss, and differ only in matters of degree.