A comparison of protocols for the photoperiodic induction of synchronised wool follicle growth cycles

A.J. Craven, A.L. Parry, M.G. Ashby, A.J. Pearson

AgResearch, Flock House Agricultural Centre, Private Bag 1900, Bulls, New Zealand

Proceedings of the New Zealand Society of Animal Production. 1995, 55: 35-38

The decline in wool growth in long-woolled sheep breeds during winter is associated with a reduction in follicle mitotic rate that is comparable to early catagen, the regressive stage of the follicle growth cycle. The identification of morphological or biochemical markers which indicate the onset of catagen would be facilitated by an experimental protocol that induces a synchronous entry into catagen. Four different photoperiod manipulations were compared, all involving short-to-long day transitions with New Zealand Wiltshire sheep. The percentage of primary (P) and secondary (S) follicles in the growth and resting phases of the cycle were assessed histologically from skin biopsies, and blood samples were collected to determine circulating prolactin (PRL) by radioimmunoassay.

In control sheep, plasma PRL levels increased over the spring to a maximum in December (254+/-45 ng/ml). In short day (8L:16D) treated sheep, plasma PRL levels were suppressed (<23 ng/ml) but rose rapidly after photoperiod treatment was terminated. In sheep maintained in short-days from 25 July to 22 October and released into either natural photoperiod, or into artificial long days (16L:8D), mean follicle activity declined by 58+/-14% (P) and 49+/-10% (S) during November and December. This reduction was coincident with, and did not differ significantly from, a natural spring decline in follicle activity observed in control sheep (55+/-2% P; 56+/-18% S). Short day treatment beginning on either 25 July or 5 October and ending on 6 January caused a rapid synchronised drop in follicle activity of 65+/-7% (P) and 58+/- 7% (S) 6 weeks after exposure to natural long-day photoperiod. Contemporary P and S follicle activities in all untreated sheep remained at 100%.

Conditioning with short days for 3 months in late spring and early summer followed by sudden exposure to long days increased PRL and induced synchronous entry of wool follicles into catagen.

Keywords: NZSAPAB; Wiltshire sheep; animal model; wool follicle; prolactin; histology; seasonality

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Last Updated 25-01-1997