Wool growth rate in vitro is independent of host animal nutrition, season, and the potential mediators of photoperiod, melatonin and prolactin.

In most sheep breeds, wool growth varies seasonally and is correlated with changes in photoperiod, temperature, and nutrition. Wool growth regulation by these factors may result from systemic changes, or to a response localized at the wool follicle. Possible roles of systemic and local factors have been examined here by comparison of wool growth responses in vitro and in vitro. Wool follicles were isolated and maintained under conditions facilitating fibre growth for at least 4 days. The wool growth rate by follicles isolated regularly over a 10 month period was invariate (P > 0.10), in contrast to a seasonally varying wool growth rate by follicles from the same animals in vivo (P < 0.001). Although a high energy, high protein diet increased wool growth in vivo (P < 0.05), cultured follicles isolated from these animals showed no change in growth rate (P > 0.10). These observations suggest that the regulation of wool growth by environmental factors is extrafollicular and that there is no carryover of the in vivo growth rate when follicles are isolated and systemic signals are therefore removed. Additional work discussed here examined the regulation of pelage by prolactin and melatonin, potential endocrine mediators of photoperiod. The fibre production rate by isolated follicles maintained in control media was not different to that of prolactin or melatonin supplemented follicles (P > 0.10). It is concluded that selective breeding of domesticated sheep has suppressed the response of follicles to regulation by prolactin and melatonin. Studies currently underway using in situ RT-PCR will provide further characterization of this conclusion.

Adrenalectomy of sheep: a novel technique.

A rapid technique for adrenalectomy of sheep was developed. Bilateral adrenalectomy was performed in a single stage operation by inducing ischaemia with latex rings of the type used commonly for the castration of lambs and calves. The success of the technique was demonstrated by failure of exogenous ACTH to increase circulating cortisol concentrations. Adrenal steroid replacement was used to maintain the sheep during the post-operative period (cortisol 0.25 mg.kg-1, deoxycorticosterone acetate 0.05 mg.kg-1). Adrenalectomised sheep were maintained for up to 20 days without glucocorticoid, provided a lowered dose of mineralocorticoid was administered (0.04 mg.kg-1 x day-1). Adrenalectomised sheep had significantly (P < 0.007) lower packed cell volume (23.75%) than normal sheep (31.24%), across a broad range of cortisol concentrations (0 to 734 ng/mL), indicating that plasma cortisol may not reflect true blood concentrations when drawing comparisons between adrenalectomised and normal sheep.

Mitotic activity in cells of the wool follicle bulb.

Mitotic activity in the cells of the germinative region of wool follicle bulbs was quantified by using small (0.1-0.5 ml) intradermal doses of colchicine and selective staining of the metaphase-blocked nuclei using either crystal violet, iodine and eosin or haematoxylin and eosin. The number of metaphase nuclei present 3 h after colchicine administration increased with colchicine dose from 0 to 1 microgram and thereafter remained relatively constant up to 200 micrograms colchicine. The accumulation of metaphase nuclei was linear for up to 6 h after intradermal colchicine. The metaphase-blocking effect of intradermal colchicine was confined to a radius of less than 5 cm from the injection site, allowing a number of estimates of mitotic rates to be made over a small area of skin. Such estimates revealed little variation in mitotic activity over the midside region of the sheep, although there were substantial differences in follicle activity at different sites over the body. The technique is simple, allows serial or concurrent estimates of mitotic activity to be made in the same animal, and eliminates problems associated with intravenous colchicine administration. It was used to derive the relationship between follicle activity and fibre production after nutritional changes, and to define the time course of mitotic events after administration of the antimitotic defleecing agent cyclophosphamide.