Reduction in wool follicles prior to birth in Merino sheep

This study was undertaken to identify whether all secondary follicles that are initiated are present at birth in the Merino fetus, and if not, when does net initiation of secondary follicles cease. Skin was sampled from fetal lambs at 36, 26 and 16 days before the estimated date of parturition and from lambs at birth and 2, 4, 6, 8 and 13 weeks after birth. The ratio of secondary to primary follicles (S/P) reached a maximum 16 days before birth and was significantly lower at birth (P<0.002) and at all postnatal ages (P<0.05). There was no difference between S/P at birth and S/P at later ages. Postnatal primary follicle density, secondary follicle density and the percentage of fibre-producing follicles followed similar patterns to those reported by others. This is the first conclusive demonstration that secondary follicle initiation is completed several weeks prior to birth and that there are less secondary follicles at birth than at 134 days of gestation.