SEASONAL FUR GROWTH CYCLES OF MINK The growth of hair in mammals is an intermittent process, occurring in fairly predictable cycles. During hair growth cycles, each hair follicle exhibits periods of growth and fiber production (anagen) followed by a short transition phase during which the lower half of the follicle involutes via highly organized apoptotic processes (catagen) resulting finally in a period of rest or relative inactivity called telogen. After a species and body-site-specific duration of telogen, the dormant follicle is reactivated, by as yet poorly defined mechanisms, resulting in renewed growth, differentiation and subsequent regeneration of the lower portion of the follicle. As the new hair shaft grows upward, the old hair from the previous cycle, is shed or molted.  In the mink ( Mustela vison), as with many mammals, hair growth cycles are photoperiodically regulated. In the the spring, increasing daylength (photoperiod) initiates onset of summer anagen during late April. The activated follicles continue to produce hair shafts until approximately mid-June, at which time cell division slows and finally ceases, the melanocytes stop producing pigment and the lower half of the follicle degenerates (catagen) leaving the hair shafts anchored rather shallowly in the dermis of the skin. The follicles (or what remains of them) are inactive (telogen) until late-August to early-September, when the decreasing photoperiod initiates winter anagen. Each follicle then produces a hair shaft of increasing length, until late November, when, once again, hair growth ceases, and the follicles undergo the transition to telogen. The physiological mechanism through which photoperiodic changes influence hair growth cycles in mink appears to involve the pineal gland hormone melatonin (MEL). Sometimes referred to as the "Hormone of the Night", MEL is secreted during the dark phase of the 24 hour day. Thus, the daily duration of MEL secretion decreases during the spring and summer (as photoperiod increases) and increases during the fall and winter (as photoperiod decreases). Teleologically, this provides the animal with an internal clock, enabling it to determine what time of the year it is, and thus develop the proper fur coat for greater insulation in the winter or reduced insulation in the summer. In a similar manner, the changing secretory pattern of MEL, also determines in part, the time of year when the animals are sexually active. This results in the young being conceived and born at the most advantageous time of year for survival. Although, the target tissue/s and mechanism through which MEL acts are unknown, there is much evidence to suggest that part of MEL's actions are mediated through the pituitary hormone prolactin (PRL). We have shown that exogenous MEL, during late summer, inhibits PRL seceretion, while simultaneously initiating early winter anagen. In contrast, exogenous MEL, during early spring, likewise inhibits PRL secretion, but completely blocks the onset of the summer fur growth cycle. Although this might suggest that PRL plays a stimulatory role during the summer and a inhibitory role in winter, our recent findings lead us to believe that the role of PRL in hair growth cycles of mink is more subtle, and that the hormone does not function as an absolute on-off switch in hair growth cycles, but does influence the number of follicles that become active during each cycle. In other words, there are other signals that determine the time at which summer and winter anagen begins, but the concentration of PRL around the follicles, determines in part, how many follicles will be activated. It is an interesting phenomenon, that mammals appear to have all of the hair follicles they will ever have at birth or shortly thereafter. And yet, in the mink (female as an example) the hair density (number of hairs per follicular bundle) in winter fur ranges from 15.4 ± 1.0 to 17.8±0.5, whereas the summer fur density ranges from 10.4 ± 0.8 to 15.1±0.8. Thus, even though the follicles are present in the skin, a great many of them are not activated during the summer fur growth cycle. The number of large guard hair follicles that are active during the summer and winter hair growth cycles, does not differ. Therefore seasonal differences in hair density are the result of changes in the number of small under hair follicles that are either active or inactive. It remains an enigma, why a subset of these under hair follicles remain dormant during the summer fur growth cycle. Evidence from our lab, and others, suggests that PRL may be involved in regulating the number of follicles that are activated during each hair growth cycle. Recently we have shown that when circulating PRL levels are elevated in mink, the number of underhair follicles that become activated (spontaneous or artificially by plucking) is significantly reduced. Thus, it would appear that increasing PRL levels in the spring reduces hair density by inhibiting the activation of a subset of under-hair follicles. In contrast, decreasing PRL levels in the fall would appear to "permit" a greater number of underhair follicles to be activated resulting in a denser winter pelage. In further support of the hypothesis that PRL is involved in regulating hair growth cycles, we have also demonstrated the presence of PRL receptors (PRL-R) in the skin of mink. Currently, some of our goals are to utilize molecular biology techniques to:
This phase of our work will be important, since we hypothesize that PRL is selectively regulating (inhibiting) a subset of the under hair follicles.
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Last Modified: 08/25/09 at 12:23:37 PM