|dc.description.abstract||A series of studies on growth of sheep was carried out to investigate the role of sex hormones. The experimental approaches involved alteration of the endocrinological environment by administration of oestradiol or by gonadectomy. In one study (Chapter 3) wether and entire ewe lambs were implanted at 4 weeks of age with various lengths of silicone rubber implants containing oestradiol-17β. There was a non-implanted group for both sexes, 5 dose levels in wethers and 6 dose levels in entire ewes. Half of the animals were killed at 10 weeks and half at 20 weeks after implantation. Oestradiol treatment slightly stimulated live-weight gain and it significantly stimulated linear growth of vertebral columns and limb bones, with the effect being most pronounced in ewe lambs slaughtered after 10 weeks of treatment. The treatment also increased diameter and diameter:length ratio of limb bones and slightly increased weight of muscles at 10 weeks and their length at 20 weeks of treatment. Castrated male lambs had longer bones than entire ewes suggesting a genetic factor or an effect of sex hormones during prenatal or early postnatal life.
A major study was conducted (Chapter 4) using wether, ram, spayed and entire ewe lambs which were implanted for 180 days with various sizes of silicone rubber implants containing oestradiol-17β to provide 3 different dose levels of the hormone. In addition there were non-implanted control groups. Average dose of oestradiol measured as weight loss from implants was 2.6, 6.0 and 13.3 mg. Untreated entire male animals were superior to females in live weight gain and size of bones. In both sexes castration reduced live-weight gain but stimulated linear growth of bones with the effect being most marked in males. Exogenous oestrogen treatment inhibited linear growth of cannon bones, stimulated that of vertebral column and ribs and had little effect on medial and proximal limb bones. Treatment with oestradiol also increased: diameter and diameter:length ratio in limb bones, maximum cortical thickness, weight, volume, ash and ash: organic matter ratio in bone. Oestrogen treatment increased carcass protein, girth, girth:length ratio and weight of muscles, whereas it decreased fat depots. Also it increased live weight, carcass weight, and weight of; head, pelt, liver, adrenal glands and uterus. There were sex differences in response to oestrogen treatment with the effect being more pronounced in spayed ewe lambs. In both studies (Chapters 3 and 4) the effects of oestradiol were generally linearly related to its dose level.
The proteolytic enzyme papain was used in an attempt (Chapter 5) to manipulate linear growth of bones and enable their relationships with growth of attached muscles to be studied. Papain inhibited linear growth of bones and generally negatively affected live weight gain and growth of muscles but the effects were not large.
In these experiments, growth in length of muscles was intimately related to that of bones to which these muscles were attached. There were significant correlations between girth, weight and, in many cases length, of these muscles to length of bones. Longitudinal growth of limb bones may thus be regarded as an important factor in stimulating growth of attached muscles.
These studies have provided an important oversight of the relationships between effects of sex steroids and the pattern of development of the skeleton. It is possible to explain the often contradictory reports of effects of steroids on bones by recourse to the differential maturation rates of regions of the skeleton. In bones close to reaching mature size, sex steroids accelerate cessation of growth (presumably by hastening epiphyseal closure), preventing these bones from reaching their normal size. In the case of relatively immature bones (e.g. vertebral column, ribs, proximal limb bones), sex steroids stimulate growth. Therefore the effects of sex steroids must be considered in light of the stage of maturity of the skeleton when such treatments are applied.||en