The prenatal development of wool follicles has been described separately for the Merino and Romney but a comparative study of both breeds and their cross has not been undertaken. This study compared the rate and timing of follicle development in midside skin samples from Romney, Merino and Merino-Romney cross (MxR) foetuses slaughtered at weekly intervals from day 76 to 144 of gestation. The development pattern of total number of follicles per unit area of skin differed between genotypes as indicated by a significant (P<0.001) genotype x gestational age interaction. The total follicle density of each genotype increased from day 76 to day 105 at which time the follicle density of the Romney peaked at 229 follicles/mm2. Total follicle density of the MxR continued to increase to between 420 and 510 follicles/mm2 between day 104 and 122 of gestation and the Merino reached a peak of 1170 follicles/mm2 at day 125 of gestation. The density of immature (i) and mature fibre-bearing (f) primary follicles (P(f+i)) did not differ (P>0.10) between genotypes over time, with each genotype reaching a maximum density of P(f+i) between days 83 and 90 of gestation. Primary follicles (P) began to reach maturity by gestational day 90 in the MxR and by day 97 in the Merino and the Romney (P>0.10). In all genotypes, secondary follicle (S) initiation began at around day 90 with a small proportion starting to form fibres by day 105 of gestation. There was a significant interaction between genotype and gestational age for both the density of S(f+i) (P<0.0001) and the density of S(f) (P<0.01). Specifically, S(f+i) increased to 175 follicles/mm2 in the Romney, 440 follicles/mm2 in the MxR and 900 follicles/mm2 in the Merino at 105, 105 and 125 days gestation respectively, after which S(f+i) density stayed relatively constant up to birth. These results indicate that follicle initiation and fibre formation in different genotypes occurs at similar ages during fetal growth. The high total follicle density in the Merino is due to a high S density which, in turn, is due to S initiation continuing for longer than in either the Romney or MxR. These data provide a model for the identification of certain physiological factors which affect secondary follicle initiation and development.
Proceedings of the New Zealand Society of Animal Production, Volume 54, , 131-134, 1994
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