Apoptosis in murine hair follicles during catagen regression

Catagen hair follicle involution has been reported to involve apoptosis, although the precise mechanism has not been satisfactorily resolved. Previous studies have involved solely morphological or electron microscopical methods. We report here studies on murine hair follicles during the first postnatal hair cycle conducted using the terminal deoxy-nucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method. Electrophoresis of DNA isolated from the hair follicles of the same animals was carried out in order to confirm the systematic fragmentation of DNA that typifies apoptosis. On day 10, when all the follicles were growing, there was no evidence of staining with TUNEL in the hair bulbs. Electrophoresis similarly did not show characteristic DNA ladders. By day 15, a few positive cells were observed in the hair bulbs and the numbers had increased by day 17 when many positive cells were seen, especially in the lower portions of the follicles. Electrophoresis demonstrated DNA ladders on days 15, 16 and 17, although the DNA ladder on day 15 was less prominent than that on day 17. These studies confirmed that apoptosis, as identified by techniques that measure DNA fragmentation, occurs in the lower regions of hair follicles towards the end of catagen.