INTRODUCTION

 
    The fur of mink ( Mustela vison) consists of an outer layer of large guard hairs and a dense inner layer of short, thin under hair fibers. During development, hair follicles form as bundles, in a trio-type arrangement (Fig 1), with all hairs of a bundle emerging through a common opening in the skin. Blomstedt, (1989) described three types of bundles for mink: (1): L-type (G-type here), containing a large guard hair follicle plus a variable number of under hair follicles, (2): I-type, composed of an intermediate size guard hair follicle plus a variable number of under hair follicles, and (3): U-type, composed of under hair follicles only.

 
Figure 1: (Left) Trio arrangement of mink hair follicle bundles, showing U and G-type bundles,  (Center) Hair follicle bundle, sectioned longitudinally, and (Right) mink hair follicle bundles, showing  I and G-type bundles.

    Mink exhibit two hair growth cycles annually that are photoperiodically regulated (Martinet et al., 1984; Rose et al., 1984).  In the spring, increasing photoperiod induces summer anagen and molting of winter fur whereas decreasing photoperiod in the fall initiates winter anagen and molting of the summer fur.  While the two fur types are similar in color and guard hair density, the summer fur has significantly less under hairs, resulting in more efficient cooling during warm weather.


Figure 2: Relationship between seasonal changes in photoperiod and the growing (anagen) and resting (telogen) stages of the hair growth cycles of mink. The Inset diagram illustrates the various stages of the hair growth cycle, for comparison. (Used with permission of Dr. George Cotsarelis).
       
    Summer anagen in mink appears to be mediated in part, through prolactin (PRL).  Serum PRL levels increase prior to anagen (Rose et al., 1985), suggesting that the hormone stimulates hair growth. As evidence, exogenous haloperidol (HAL) stimulates PRL secretion advancing onset of summer anagen, whereas exogenous MEL or exposure to an artificially short photoperiod (8L: 16D; Rose et al., 1998), reduces PRL secretion, delaying or inhibiting onset of summer anagen.  In contrast, PRL can inhibit hair growth in some species. Exposure of sheep to a  8L: 16D photoperiod beginning in the winter, caused follicles to enter and remain in anagen as long as the short photoperiod was maintained (Pearson et al., 1993, 1996).  When the animals were released into a natural long-day photoperiod, PRL levels increased and 50% of follicles entered telogen.  Only after CB-154 treatment was discontinued did PRL levels increase and follicles enter telogen.  Recently, (Krause et al., 2001) demonstrated that PRL terminated hair growth in rodent and human hair follicles, in vitro, supporting the hypothesis that the hormone might be involved in the anagen to telogen transition.
       
    Adding to  PRL’s  enigmatic role in hair growth biology, are the results of studies where PRL was without effect on hair growth.  Curlewis  et al., (1991) treated ewes with CB-154 to inhibit PRL secretion and still observed growth of the summer coat at the same time as controls.  Pearson et al., (1996) observed that Wiltshire sheep entered anagen before the late spring increase in serum PRL concentrations. In mink, we showed that adrenalectomy (ADX) or exogenous HAL induced early winter anagen even though serum PRL levels were not different from controls at the time anagen occurred  (Johnston and Rose, 1999). Recently, Craven et al., (2001) demonstrated early onset of anagen in PRL knockout mice.  Thus, the actions of PRL related to hair growth will probably have to be considered from the standpoint of individual species, their physiological condition and hormone concentration.
      
     PRL receptors (PRL-R) have been detected in the skin of mink (Rose et al., 1995) as well as skin and hair follicles of sheep (Nixon et al., 2002), and humans (Krause et al., 2001), suggesting that the hormone may act directly on the hair follicle.  In mink, even if PRL does not play a role in anagen onset (as determined by emergence of guard hairs), it may still have important effects on the development of pelage through actions on the under hair follicles.  As evidence, the under hair fibers in mink emerge two weeks after guard hairs during winter anagen (Worthy et al., 1987).  Therefore, our objectives were to determine:
    
         1.     Changes in serum PRL levels in mink exhibiting natural and artificially                                 induced  summer anagen, and      
         
         2.     If the effects of PRL on under hair follicle development are mediated equally                     on all three follicular bundle types.

 
 

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