First indications that peri-follicular macrophages contribute to the regulation of catagen initiation and progression in the murine hair follicle

The hair follicle (HF) is a complex mini-organ that undergoes a life-long, highly regulated tissue remodelling process (hair cycle) and is comprised of multiple interacting cell populations including including perifollicular macrophages (MAC). These MACs undergo major hair cycle-dependent changes decreasing from anagen to their lowest in telogen. Despite this, the role of MACs in hair growth control remains limited. Recently, a functional role for macrophages was demonstrated in mouse where apoptotic MACs were shown to secrete Wnt7b and 10a which are essential for stem cell and anagen initiation however this has not been studied during the anagen-catagen switch. Therefore to investigate this MAC number was decreased (by 66%) by administering clodronate impregnated liposomes to mouse skin during anagen. This was verified by quantitative immunohistomorphometry using the marker F4/80. Interestingly, MAC number declined both in the injected mouse skin and in surrounding skin sites. Importantly, this was associated with a significant delay in catagen progression in treated mice compared to control mice, who had mostly progressed to telogen or early anagen. To confirm that these differences did not result from differences in hair cycle wave propagation, multiple skin sites were assessed individually and quantified. This confirmed a significant delay in hair cycle progression in all assessed skin areas, with treated mice having a significantly more follicles in either catagen VI or VII. While the underlying mechanisms remain to be elucidated, these results provide the first suggestive evidence that MACs are required during the anagen-catagen switch and/or catagen progression in mice. Moreover, MACS may produce key signalling molecules necessary for hair cycle control and therefore, if similar in human, could be targeted for hair growth manipulation to treat hair pathologies.