THE MICROANATOMY, CELL REPLICATION, AND KERATIN GENE-EXPRESSION OF HAIR-FOLLICLES DURING A PHOTOPERIOD-INDUCED GROWTH-CYCLE IN SHEEP

Exposure of New Zealand Wiltshire sheep to long days, following 24 wee ks of short days, caused a synchronised out-of-season wool follicle gr owth cycle. Skin biopsies were collected at intervals between 3 and 30 days and follicles were examined by light microscopy in both transver se and longitudinal section to describe the regressive (catagen), rest ing (telogen) and regenerative (proanagen) stages of the induced growt h cycle. Follicles were generally in the growing phase (anagen) during short day treatment but by day 20 after exposure to long day photoper iod, 16% of follicles were in late catagen. By day 52, all follicles w ere in various stages of catagen, telogen and proanagen. The progressi on through the cycle occurred more slowly, but was morphologically sim ilar to follicle growth cycles reported in rodents and goats, induced by plucking or melatonin, respectively. Follicles in early catagen wer e rarely observed, possibly reflecting the brevity of this phase of th e cycle. Late catagen follicles were distinguished by the presence of a brush end and an inner root sheath, the latter disappearing as folli cles entered telogen. Immunocytochemistry of proliferating cell nuclea r antigen provided evidence that mitotic activity in the follicle bulb ceased completely during the brief telogen phase. The simultaneous ab sence of type I intermediate filament keratin mRNA indicated that kera tinocyte differentiation had also been interrupted. Cell proliferation was re-established in early proanagen prior to observable changes in the follicle microanatomy. The relatively synchronised follicle growth cycle induced by photoperiod manipulation represents a potentially us eful model for the study of changes in follicle ultrastructure and the endocrine and biochemical regulation of seasonal hair growth patterns .