Estimation of Cell Birth Rate in the Wool Follicle Bulb using Colchicine Metaphase Arrest or DNA Labelling with Bromodeoxyuridine

Abstract

The birth rate of cells in the wool follicle cells (CBR) was measured in two experiments on sheep differing widely in wool growth rate as a consequence of differences in nutritional regime. In Experiment 1, CBR was determined in one sheep on a high and one on a low plane of nutrition by two methods: the colchicine-metaphase arrest method, or from the product of total cell number and cell cycle time (the latter determined by DNA labelling with 5-bromo-2'-deoxyuridine (BrdU) and analysis of the fraction of mitoses labelled curve). CBR by colchicine was 0.73 and 0.56 of that determined using BrdU for the low- and high-plane sheep respectively. Cell cycle times were 26.5 h (low nutrition) to 18.6 h (high nutrition). In Experiment 2, CBR was again measured by the colchicine method, and from the rate of increase in labelled cells following a pulse dose of BrdU, in sheep on two levels of nutrition (n = 4llevel) and with a 3.7-fold difference in wool growth rate. Again, CBR by colchicine was only 0.41 (high nutrition) to 0.74 (low nutrition) of estimates made using the BrdU method, this difference being statistically significant only for sheep on the high plane of nutrition. Nutrition significantly altered CBR determined by colchicine (Pc0.001) and by BrdU (Pc0.034). The rate of fibre growth per follicle measured on two separate occasions was closely related to CBR measured using colchicine (r= 0.83, P< 0.01 and 0.8 1, P c 0.02) or BrdU (r= 0.73, P c 0.04 and 0.64, P< 0.08). It is concluded that CBR is a major determinant of fibre growth, but that colchicine depresses CBR. Previous estimates of mitotic activity in wool follicle bulbs have been made using colchicine and are thus probably underestimated. The technique using BrdU labelling and the subsequent rate of increase in labelled cells appears to be a useful one for estimating follicle cell kinetics.