FOLLICULAR DETERMINANTS OF THE LENGTH AND DIAMETER OF WOOL FIBERS .2.COMPARISON OF SHEEP DIFFERING IN THYROID-HORMONE STATUS

The thyroid hormone status of Corriedale sheep was manipulated in orde r to produce a change in the rate of fibre elongation (L) with no conc omitant change in the diameter (D) of the fibres, to allow the follicu lar factors responsible for fibre L to be determined. Thyroidectomy re sulted in a 60% decrease in wool growth per unit area of skin and a 40 % decrease in the rate of fibre volume output, due largely to a declin e in fibre L from 412 mu m/day to 277 mu m/day (P < 0.0001) while fibr e D was unchanged (23.9 mu m v. 21.9 mu m, P > 0.05). Elevation of pla sma thyroxine levels to 250% of Control values had no significant effe ct on fibre L and D, but patch wool growth was increased (P < 0.05). A high proportion (22.9%) of the follicles in the hypothyroid sheep was inactive in comparison to the low levels of inactivity in the control (1.2%) and hyperthyroid animals (0.1%). The rate of division of folli cle bulb cells was depressed by hypothyroidism and increased by hypert hyroidism (P < 0.0001), but there was no effect of thyroid status on t he size of the cortical cells released from the wool fibres. Hypothyro id sheep had slightly smaller follicle bulbs and dermal papillae than the control and hyperthyroid animals. The proportion of dividing cells entering the fibre cortex (estimated from cortical cell volume, rate of fibre volume growth and rate of cell division) was depressed by thy roidectomy suggesting that relatively more dividing cells entered the inner root sheath in the follicles of hypothyroid sheep. This is suppo rted by the lower production ratio (ratio of area of fibre to area of fibre-plus-inner root sheath) of the hypothyroid sheep. It is conclude d that the rate of fibre elongation is depressed in hypothyroid sheep as a result of both a reduction in the rate of division of cells in th e follicle bulb and fewer of these dividing cells entering the fibre c ortex. This mechanism allows rapid changes in the rate of fibre elonga tion to occur with little change in the size of the follicle bulb, and may account for a number of scenarios in which fibre length changes r apidly with no concomitant change in fibre diameter. Fibre diameter, o n the other hand, appears to be closely related to the dimensions of t he follicle bulb, and by association, to the rate of bulb cell product ion. Selection of sheep with small follicle bulbs and at the same time , high production ratios, may be means of increasing fibre length whil st maintaining low fibre diameter.