Men, especially, know that hair thins with age and that there are many products on the market to assist with hair growth—but what exactly is meant by a hair growth solution? Does it deliver more hair, thicker hair, hair that grows longer before it breaks or hair that grows faster? Biologically, hair growth is a well-described cycle involving four phases: growth (anagen), regression (catagen), resting (telogen) and shedding (exogen). This cycle occurs in virtually all hair-bearing species and the dynamics of the hair cycle have been the subject of recent studies.
The hair follicle is a multicellular tissue that retains an element of developmental dynamics recapitulated in the adult hair cycle. This is reflected in the continual interplay between the mesenchymal and epithelial elements. This review offers a brief summary of some of the biology of hair growth and comments on some of the differences between hairs that are critical to describing what kind of hair is produced by hair growth technologies.
Hair growth is not completely understood, despite a rapid increase in research in recent years. Although factors regulating the transition between the key stages in adult hair cycling are well-documented, it is perhaps not surprising that the two US Food and Drug Administration (FDA)-approved treatments for hair loss were existing drugs for which hair growth is a side effect of their primary use. Originally, minoxidil was developed for hypertension, and finasteride was designed for prostate disease. Disappointingly, few clues about hair follicle changes in hair loss have been gleaned from these serendipitous findings.