The induction by topical minoxidil of increased fenestration in the perifollicular capillary wall

In the 1999 study, researchers investigated the effects of topical minoxidil on the fenestration (small openings) in the capillary walls surrounding hair follicles in male Wistar rats. The study involved four rats, with two in the experimental group receiving 4% minoxidil solution and two in the control group receiving only the vehicle solution. After 10 days of treatment, transmission electron microscopy was used to compare the capillaries and dermal fibers between the two groups. The results showed that while there was no significant difference in the proliferation of follicular capillaries or changes in dermal fibers, the density of fenestrations in the follicular capillary walls around anagen (growing phase) hair bulbs was significantly increased in the minoxidil-treated group compared to the control group (0.32/µm vs. 0.11/µm). This suggests that minoxidil may stimulate the expression of vascular endothelial growth factor (VEGF) from anagen hair bulbar regions, leading to increased fenestrations in follicular capillary walls, which could contribute to its hair growth-promoting effects. However, minoxidil did not induce fenestrations in subepidermal capillary walls or affect the proliferation of dermal collagen or elastic fibers.