PRL stimulates summer anagen in guard (G& I) hair follicles while activating only a subset of underhair follicles.  In winter, PRL inhibits anagen in both guard and underhair follicles. To explain this apparent paradox, we hypothesize that mink hair follicles exhibit seasonally determined, bi-phasic responses to PRL. These responses may reflect seasonally driven patterns of PRL receptor expression in the skin and/or hair follicles. Currently under investigation, is the possibility that only a subset of underhair follicles express PRL receptors during summer anagen. This might explain why all underhair follicles do not respond to PRL during growth of the summer fur.

    Although pharmacological stimulation of PRL secretion in mink resulted in larger hair follicles, there was no difference in the size of guard or underhair follicles during spontaneous summer or winter anagen.  This finding supports the hypothesis that the primary role of PRL in hair growth cycles, may be to stimulate (summer) or inhibit (winter) onset of anagen. Nevertheless, our findings suggest that PRL may increase the rate and/or duration of hair matrix mitoses during anagen. Alternatively, because the size of the dermal papilla (DP) determines in part, the size of the hair fiber, PRL may also promote the proliferation of DP fibroblasts and/or the recruitment of fibroblasts from the outer root sheath. Studies are currently in progress to evaluate these possibilities