The Microanatomy, Cell Replication, and Keratin Gene Expression of Hair Follicles During a Photoperiod-Induced Growth Cycle in Sheep

Exposure to long days after a period of short days induced a synchronized wool follicle growth cycle in New Zealand Wiltshire sheep. Skin biopsies taken between 3 and 30 days post-exposure revealed the follicles' progression through the regressive, resting, and regenerative stages. By day 20, 16% of follicles were in late catagen, and by day 52, all follicles were in various stages of the cycle. The cycle's progression was slower but similar to that seen in rodents and goats. Early catagen was seldom seen, late catagen was marked by specific structural features, and during telogen, mitotic activity and keratinocyte differentiation ceased. Cell proliferation resumed in early proanagen before anatomical changes were visible. This photoperiod-induced cycle offers a model for studying follicle structure and the regulation of seasonal hair growth.