Androgenetic Alopecia (AGA): AGA Under the Microscope

AGA Under the Microscope

There is often some degree of confusion as to what constitutes a typical biopsy from a patient with androgenetic alopecia (AGA). Here, I will review some of they key features of both horizontal and vertical sections (i.e. the two main ways a biopsy can be done). AGA is a non-scarring alopecia. In both horizontal and vertical sections sebaceous glands typically appear quite prominent. A reduction in sebaceous gland density should prompt one to consider that another diagnosis, such as scarring alopecia, might be present. 
Miniaturized hairs are a feature of AGA. In vertical sections, the miniaturized and vellus hairs simply appear as hairs which root themselves in the mid dermis rather than deep down in the subcutaneous fat. In horizontal sections, miniaturization can be appreciated by a shift in the terminal to vellus hair ratio (“T:V” ratio) from above 6:1 to well below 4:1 and possibly well below this.

Inflammation is very much a part of AGA. The legendary dermatologist Dr David Whiting showed that 40% of biopsies from patients with AGA showed perifollicular inflammation. This inflammation sits in the lower infundibulum and isthmus. The cells that comprise the inflammation are largely T cells. Perifollicular inflammation occurs early in the course of AGA and likely drives the development of perifollicular fibrosis and drives the progressive miniaturization of hairs.

Perifolliuclar fibrosis is also part of AGA and the concentric fibrosis can best be appreciated in horizontal sections. Perifollicular fibrosis can often be mistaken and misdiagnosed as scarring leading to a misdiagnosis of lichen planopilaris (LPP) and other scarring alopecias. However, the retention of sebaceous glands and absence of lichenoid change in the outer root sheath favours a diagnosis of AGA over LPP.

Cell death of keratinocytes in the outer root sheath is not a typical feature of AGA. Apoptosis is present in a proportion of dermal papilla cells and lymphocytes in the perifollicular inflammatory infiltrate - but not in the actual outer root sheath. Lichenoid change in the outer root sheath might cause one to consider a diagnosis of LPP rather than AGA.