Fig. 2:  (Top): Computer generated drawings of G-U-U and G-I-I trio-arrangement of hair     follicle bundles. (Bottom): Light-micrographs of same in anagen. The small arrow     indicates an old follicle while the large arrow indicates a follicle in anagen.
 
In the spring, increasing photoperiod and serum prolactin (PRL) levels are correlated with onset of the growing (anagen) phase of the summer hair growth cycle and molting of winter fur.  Hair growth continues until early-summer when all follicles undergo a short, apoptotic-driven regression (catagen) resulting in the resting (telogen) phase of the hair cycle. Follicles remain inactive until early-fall when decreasing photoperiod and serum PRL levels result in onset of the winter hair growth cycle and molting of summer fur. Hair growth continues until mid-Dec when all follicles progress through catagen, to telogen and remain inactive until the following spring.

During the development of the winter pelage, there is a 40% or greater increase in the number of derived under hair follicles that become active, compared to summer anagen. This is of interest because mammals appear to develop all of their hair follicles by birth, or shortly thereafter. Thus, a major challenge will be: To determine why all of the "available" under hair follicles are not activated during summer anagen. Conversely, we need to determine the nature of the endocrine signals that results in activation of those hair follicles during winter anagen, that were inactive during summer anagen.
   
Prolactin (PRL) has long been suspected to play a role in hair growth cycles of mammals, especially those that exhibit seasonal changes in pelage such as sheep, Djungarian hamsters and mink. Recently, PRL has received increased attention as a potential modulator of rodent and human hair growth cycles (Foitzik et al., 2003; Craven et al., 2001). It is intriguing however, that PRL has both stimulatory and inhibitory effects on hair growth, that are dependent on species and perhaps even season of the year.  For example, in mice and humans PRL reduces hair growth rate, blocks anagen and promotes a catagen-like transformation of follicles in vitro.  In contrast, PRL appears to stimulate the onset of summer fur growth, and perhaps inhibit the development of winter fur in Djungarian hamsters and mink.

Thus, our objectives were to determine the effects of PRL on:

(1):    initiation of spontaneous & depilation-induced anagen, and

(2):    hair fiber density, diameter, cross-sectional area, & ellipticity