The effects of short-term manipulation of thyroid hormone status coinciding with primary wool follicle development on fleece characteristics in Merino sheep

Abstract

Thyroidectomy surgery performed late in gestation results in perturbations in wool follicle development in foetal sheep, showing the importance of thyroid hormones for wool follicle development. The aim of this study was to determine the influence of transient manipulation of thyroid hormone status at a time corresponding with foetal primary wool follicle initiation. Pregnant Merino ewes (n 5 12 per treatment) were treated daily between gestational days 55 and 64 with control (vehicle), exogenous thyroxine (T4) or propylthiouracil (PTU), an inhibitor of T4 synthesis, and conversion to the active form of the thyroid hormone (triiodothyronine). There were no significant differences in birth weight, gestational lengths and birth coat scores of the resultant lambs. The total primary and secondary follicle densities were significantly lower in lambs exposed to exogenous T4 compared with other treatments (P , 0.05). However, the T4 group displayed a higher proportion of mature secondary follicles (reflected by increased mature secondary follicle densities and mature secondary/primary follicle ratios) than the other treatment groups (P , 0.05). The skin morphology of the lambs differed 12 months later, with the T4 group having significantly higher total follicle densities compared with the PTU group, largely attributed to increased mature and total secondary follicle densities. However, this increase in wool follicle densities did not translate to differences in the fleece yields and weight, fibre diameter, staple lengths or any other fibre parameters. This study showed that transient manipulation of thyroid hormone status during foetal primary follicle initiation does have long-term consequences on the morphology of wool follicles, in particular the maturity of secondary wool follicles.

Genome-wide DNA methylation and transcriptome analyses reveal the key gene for wool type variation in sheep

“…In the Merino breed, the development of secondary follicles continues until three months after birth [ 35 ]. Larger primary wool follicles generate medullated coarse wool, and smaller primary and secondary wool follicles generate non-medullated fine wool [ 7 ]. In general, fine wool sheep are coated with non-medullated wool from birth [ 36 ].…”

“…The ALC wool lambs found in this study in a population of fine wool sheep with the same genetic background, but the ALC wool lambs retained significant medullated wool after birth. It was observed that its medullated wool gradually faded away with age, just as its late embryonic primary wool follicles gradually decreased [ 7 ]. Our study compared the embryos of modern coarse wool sheep and ALC wool lambs, which have distinct similarities in gene expression.…”

“…The medullated wool is developed from primary wool follicle, while the non-medullated wool is developed from primary and secondary wool follicle [ 6 ]. Thus, the wool type variation should be due to the diversity of wool follicle types, however, the main determining period of wool follicle morphogenesis is at the embryonic stage, where, primary wool follicles concentrate at embryonic d 50–90 and secondary wool follicles at embryonic d 80–100 [ 7 ]. Moreover, the epigenome comprising different mechanisms, e.g., DNA methylation, remodeling, histone tail modifications, chromatin microRNAs and long non-coding RNAs, interact with environ-mental factors like nutrition, pathogens, climate to influence the expression profile of genes and the emergence of specific phenotypes [ 8 ].…”

On-farm factors affecting physical quality of Merino wool. 1. Nutrition, reproduction, health and management

“…Kenyon et al (2005) investigated this hypothesis through use of thyroidectomies and exogenous administration of triiodothyronine and thyroxine, but found that any effects on fibre attributes could not be solely attributed to elevated maternal thyroid hormones. Another study showed thyroxine definitely involved in the development of follicle morphology, but that any alterations which occurred from supplementation with thyroxine or inhibition of thyroxine (with propylthiouracil) did not alter subsequent fleece attributes (McDowall et al, 2011).…”