Wool fibre diameter and follicle density are not specified simultaneously during wool follicle initiation

Although the highly negative correlation between wool fibre diameter and wool follicle density has been known for some time, the source of this relationship in terms of the developmental mechanisms involved has been unclear. By comparing follicle rudiment dimensions (measured in the fetus) with fibre diameter values (measured in the same animal as an adult), we present data indicating that this inverse relationship arises largely after follicle initiation. Our data indicate that this is because fibre diameter is largely specified post-initiation, during the period of wool follicle growth and morphogenesis. This result was unexpected, and has permitted us to speculate upon the hierarchy and relationship of genes that control follicle initiation and fibre diameter.