Thomas, David Granville; (1997) The hormonal control of hair growth in the red deer (Cervus elaphus). Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Red deer exhibit a marked seasonal cycle of coat growth and replacement which results in a dense insulatory winter coat and a sparse summer coat that allows evaporative heat loss. The aim of this study was to describe fully this annual cycle and investigate the hormonal control of initiation and growth of the two coats in the species. Analysis of skin biopsies revealed that a complete moult of primary and secondary coat hairs occurred in the spring and autumn. Within the summer coat, in addition to fully developed primary coat follicles, a second 'vestigial' population of primary follicles was identified which failed to produce functional hair fibres. Thus, primary follicle density remained constant from coat to coat throughout the year, but this structural adaptation to the summer coat resulted in reduced fibre density and probably acts to aid heat dispersal from the animal. A strong temporal relationship was established between the spring rise in plasma prolactin levels and the initiation of summer coat growth. Further experiments showed that local infusion of ovine prolactin during the winter induced an advance in summer coat growth. The development of the following winter coat was also affected by prolactin treatment, showing that a local phase shift of the annual pelage cycle was induced. This study identified a seasonal 'window' through which prolactin could adjust the inherent activity rhythm within the population of coat follicles. An in vitro model for hair follicle culture was also developed. This technique was used to show that ovine prolactin administration stimulated growth of fully developed summer coat follicles. This effect on growth was augmented by concomitant administration of triiodothyronine. The fact that summer follicles could be stimulated to further growth suggests that prolactin maybe involved in the maintenance of the summer coat growth. Autoradiographic studies localised high numbers of prolactin receptors in the bulb region of both growing summer and winter coat follicles. The demonstration of target tissues for prolactin in red deer coat follicles together with the results from in vivo and in vitro experiments strongly supports a role for prolactin in seasonal coat growth regulation in this species.

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