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

Abstract

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.

Effects of prolactin administered to a perfused area of the skin of Angora goats1,2

“…In the present experiment, the sudden reverse in the melatonin:prolactin ratio due to prolactin infusion during the short photoperiod simulated an increasing photoperiod, and this may have contributed to decreased mohair fiber growth. This observation is in contradiction to other experiments investigating prolactin effects on fiber growth using either pinealectomized animals or in vitro measurements (McCloghry et al, 1992;Ibraheem et al, 1993;Winder et al, 1995). Lincoln (1990) suggested that in highly selected Romney and Merino sheep breeds, wool continuously grows throughout the year, is not affected by changes in prolactin concentrations, and there is no clear seasonal shedding.…”

“…Furthermore, melatonin and prolactin have had contrasting effects on fiber growth in several experiments (Parry and Craven, 1992;Pearson et al, 1993;1996;Litherland, 1996). Conversely, with in vitro maintenance of follicles from Drysdale, English Leicester, or Wiltshire sheep, supraphysiological concentrations of prolactin and melatonin did not affect growth rate or viability of wool follicles (Winder et al 1995). It is also possible that the media used and the accumulation of metabolites in the media increased fiber growth in the study by Ibraheem et al (1993) and prevented a response in the Winder et al (1995) study.…”

“…Conversely, with in vitro maintenance of follicles from Drysdale, English Leicester, or Wiltshire sheep, supraphysiological concentrations of prolactin and melatonin did not affect growth rate or viability of wool follicles (Winder et al 1995). It is also possible that the media used and the accumulation of metabolites in the media increased fiber growth in the study by Ibraheem et al (1993) and prevented a response in the Winder et al (1995) study. Differences between results of the present experiment and these reports could indicate that the duration of in vitro incubations used by Ibraheem et al (1993; might have been inadequate for full expression of prolactin or melatonin effects.…”

The effects of exogenous melatonin on wool quality and thyroid function in Rasa Aragonesa ewes

“…Winder et al (1995) found that the regulation of wool growth by environmental factors appears to be extrafollicular and there is no carryover of the in vivo growth rate when follicles are isolated in vitro (when systemic signals are absent) and treated with melatonin and prolactin. They concluded that the selective breeding of domesticated sheep has suppressed the response of follicles to regulation by melatonin.…”

“…In most breeds of sheep, wool growth varies seasonally and is correlated with changes in photoperiod, temperature, and nutrition. The regulation of wool growth through those factors can result from systemic changes or to localized responses at the level of the wool follicle (Winder et al, 1995). Sumner et al (1994) suggested that most of the variation in wool growth rate is due to differences among sheep breeds.…”