Development of the follicle population in N-type sheep.

Several types of follicles occur in the skin of sheep. These have been shown to differ in the types of fibres they form, when these are classified on shape of tip curl, occurrence of medulla, size of crimps, rate of growth, and frequency of shedding as kemps. Fibre type frequency distributions have been analysed. In non-N-type, halo-hairs and fibres with a sickle-shaped tip, forming the pre-curly-tip group, can only be formed in primary central follicles, which ordinarily all grow pre-curly-tip fibres, although occasionally these follicles produce curly-tip fibres. The primary lateral follicles all grow curly-tip fibres which grow also in the early secondary follicles. The late secondary follicles grow fibres with a tip of indefinite shape, histerotrichs. In N-type sheep primary centrals always form pre-curly-tip fibres, with the reservation that in Plateau arrays with few or no fibres classed as supersickle- fibres it is believed that some fibres called hairy-tip–curly-tip fibres grow in primary central follicles; primary laterals form hairy-tip–curly-tip fibres save in some N/+ lambs; and the secondaries form curly-tip and histerotrich fibres. The primary follicles are strongly affected by the N and nr genes, which have much less effect, if any, on the secondaries. Fibre growth rates and consequently size of crimps, and frequency of occurrence of brown fibres and of shedding as kemps, are all higher in primary than in secondary fibres in N-type sheep, and in N-type than in non-N-type, showing that the N and nr genes affect the primary more than the secondary follicles in these characteristics.