Apoptosis in mouse follicles during catagen regression

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The mechanism of catagen hair follicle involution has been identified as apoptosis although the precise mechanism has never been satisfactorily explained. However, studies done before are based on morphological or electron microscopical observation. We examine whether or not the expected location of apoptotic cells wit hfragmented DNA is observed in murine hair follicles on the first postnatal hair cycle using terminal deoxy-nucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Furthermore, the electrophoresis of DNA isolated from the hair follicles was carried out from the same animal to define apoptosis.Day 10, all follicles were growing, there was no positive staining with TUNEL in the bulb. The electrophoresis did not show characteristic fragmentation, i.e. 'DNA ladder'. Day 15, a few positive cells were observed in the bulb. Thereafter, several positive cells were seen in the bulb day 16. Day 17, many positive cells were present in the lower portion of the follicle. The electrophoresis showed 'DNA ladder' day 15, day 16 and day 17 although 'DNA ladder' in day 15 was more slight than in day 17.We stained with TUNEL cultured cells from the murine hair tissues adding Transforming growth factor beta (TGF beta). The percntage of positive cells with TUNEL among cultured cells was more than in the control.Our studies confirmed that apoptosis identified with DNA fragmentation occurs in the lower follicles, towards the end of catagen, and TGF beta might be at least partly responsilbe for regulating the onset of the catagen phase of the hair growth.