The determinants of wool production per unit area of skin are reviewed, and found to be dominated by follicle volume. Evidence is presented to show that follicle cells destined to become fibre cells differentiate into a continuum of cell types rather than discrete populations of medullary, ortho- and paracortical cells as the literature suggests. Fibres of the same dimensions grown by a sheep at one point in time contain differing proportions of these cell types. It is hypothesised that there is a degree of independence between fibre volume and cell differentiation. Medulla formation is explained in terms of competition between follicles for keratin precursors, so that fibre volume exceeds the available volume of keratin. A similar mechanism is proposed for the cortex, where paracortex and mesocortex become less prevalent as total fibre volume increases. It is concluded that a model for fibre formation can be considered in two parts: one which generates fibre volume and another which determines cell type within that volume.
Proceedings of the New Zealand Society of Animal Production, Volume 52, , 225-228, 1992
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