Hair is an essential element of physical appearance, well-being and attractiveness. The hair follicle (HF) is the hair shaft-producing unit that has been described as a complex and complete organ; indeed, it is characterized by continuous cycling with successive phases of growth (anagen), illustrated by the intense activity of hair matrix keratinocytes. The growth stage is followed by a complete regression of the lower segment of follicle and bulb (catagen) and relative quiescence (telogen), resulting in hair shedding (exogen). At the end of telogen, the HF is reactivated by intrafollicular and extrafollicular signals in order to re-initiate a new cycle.
This intense cellular activity is the result of complex interactions taking place in different compartments of the HF. In particular, the concentric organization of the HF implicates regulated interactions between the epithelial and connective tissue sheath. These epithelial-mesenchymal interactions have been shown to play a central role in HF activity and involve components of the HF basement membrane, which is located between the connective tissue sheath that surrounds the HF and the basal layer of the outer root sheath (ORS). This basement membrane also continues at the level of the dermal papilla, which is composed of mesenchymal tissue and considered a conductor of hair regeneration and growth.
In the skin, the basement membrane has been well-described as far as its molecular organization and role in maintaining epidermal-dermal cohesion. However, its role in relation to hair growth remains incompletely understood. The aim of the present study was to characterize the main components of the HF basement membrane by immunohistology to evaluate the modulation of their synthesis by a compound designed to boost cell energy levels.
Integrins are transmembrane adhesion proteins that are involved in cell-cell and cell-matrix interactions and exert signalling functions. In the HF, they play key roles in interactions between the outermost cell layer of the ORS and the basement membrane. In particular, b1 integrin mediated signaling has been implicated in hair growth control. Moreover, b1 and a6 integrin subunits have been identified as epidermal stem cell markers.
With respect to their role in cellmatrix adhesion, a6b4 and a3b1 integrins have been described as main receptors for laminin-332 and laminin-511, respectively, which regulate hair growth. In particular, laminin-511, through interaction with its b1 integrin receptor, has been shown to promote dermal papilla development. Moreover, blocking laminin-511 has been shown to provoke alopecia on human scalp grafts.