The Influence of the Thyroid on Wool Follicle Development in the Lamb

Abstract

SummaryThyroidectomy of the newborn lamb prevents the maturation of secondary wool follicles. The administration of L-thyroxine to thyroidectomized lambs allows normal follicle development.Body growth and wool growth are also depressed by thyroidectomy and most thyroidectomized lambs do not survive more than a few weeks without replacement therapy. The effects on wool growth do not appear to be secondary to the effects on follicle development.The thyroxine requirements for normal wool growth and for normal secondary follicle maturation appear to be greater than the requirements for general body growth. I. INTRODUCTIONThe development of wool follicles in the lamb has been described by Carter (1943), Carter and Hardy (1947), Burns (1949), Fraser (1954, Schinckel (1953Schinckel ( , 1955, and Short (1955). Primary follicles are fully developed and are producing fibres at birth. Development of most if not all secondary follicles commences by the time of birth but in the Merino only about 20 per cent. of secondary follicles are producing fibres at this time. In the first month after birth this proportion increases to 70-80 per cent. Further development then occurs more slowly.The thyroid is well known to affect growth and differentiation. Simpson (1924) found that thyroidectomy of lambs retarded growth and reduced fleece weights at 13-14 months of age by 50 per cent. but no observations were reported on follicle development. The hair cycles of rats are greatly retarded by thyroidectomy (Salmon 1938; Scow and Marx 1945;Scow and Simpson 1945; Dicke 1948), and the administration of throxine to normal rats has been reported to accelerate the hair cycle (Butcher 1937). The lamb does not exhibit several cycles of wool development similar to the hair cycles in the rat, but development of individual follicles appears to be a similar process in both species. This paper reports the results of an experiment carried out to determine the effect of the thyroid on the maturation of the secondary follicles of the lamb. The results show that the thyroid does markedly influence this process.

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

“…However, the observations that suggest secondary wool follicle maturation were enhanced by exogenous T4, which agrees with the reported literature (Ferguson et al, 1956). Differences between the studies are likely to be due to differences in the timing of the treatment, where we treated pregnant ewes for a short period of time (D55 to D64), v. Ferguson et al treating lambs post partum (Ferguson et al, 1956). This is significant considering that different populations of wool follicles develop at different times pre-and postnatally (Hardy and Lyne, 1956), with our treatments coinciding with the initial stages of primary wool follicles development compared with postnatal treatment with T4, during secondary follicle development.…”

“…However, the observations that suggest secondary wool follicle maturation were enhanced by exogenous T4, which agrees with the reported literature (Ferguson et al, 1956). Differences between the studies are likely to be due to differences in the timing of the treatment, where we treated pregnant ewes for a short period of time (D55 to D64), v. Ferguson et al treating lambs post partum (Ferguson et al, 1956).…”

“…The decrease in postnatal wool follicle densities in this study was surprising as previous studies have shown that exogenous T4 treatment during gestation did not influence wool follicle initiation (Ferguson et al, 1956). However, the observations that suggest secondary wool follicle maturation were enhanced by exogenous T4, which agrees with the reported literature (Ferguson et al, 1956).…”

“…The influence of long-term manipulation of thyroid hormone status on the development of wool follicles late in gestation has previously been reported (Ferguson et al, 1956;Hopkins and Thorburn, 1972;Chapman et al, 1974), as has the influence of thyroid hormones on the function of postnatal wool and hair follicles (Ferguson et al, 1956;Williams et al, 1990;Villar et al, 2000). However, the long-term effects of transient manipulation of thyroid hormone status coinciding with primary wool follicle initiation on wool follicle development and function are unknown.…”

“…Postnatal wool follicle development was also affected, since lambs that underwent thyroidectomy at birth had perturbed in secondary wool follicle maturation (Ferguson et al, 1956). Conversely, administration of exogenous thyroid hormones restored the maturity of secondary follicles in athyroid lambs (Ferguson et al, 1956). Thyroidectomy surgery performed in adult Merino sheep resulted in the induced shutdown of large numbers of wool follicles (Hynd, 1994).…”