Early reproductive failure of ewes in a hot environment. II. The uterus

Publisher Website
Google Scholar
Abstract
1. A balanced factorial experiment was carried out with forty-eight Merino ewes. There were two levels each of the four factors: environmental temperature, thyroxine status, vitamin A intake and progesterone status. Each ewe was killed at 25 days' pregnancy; its uterus was weighed and portions were prepared for histological examination. A number of morphological indices were then measured.2. With respect to uterus weight there were significant interactions between thyroxine level, environmental temperature and progesterone level, and between thyroxine level and vitamin A intake. In the absence of thyroxine injections, heat and supplementary vitamin A increased mean uterus weight; in the absence of either of these, thyroxine injections increased it. Heat or vitamin A with thyroxine injections maintained normal mean weight. Progesterone influenced the temperature effect only in the presence of thyroxine.3. Where thyroxine injections were given at normal temperatures uterus weight was significantly correlated with plasma vitamin A concentration. There was a similar correlation where thyroxine injections were given in the absence of dietary vitamin A.4. There were no significant treatment effects or interactions on any of the histological indices examined. The height of the glandular layer of the endometrium, however, increased with increasing uterus weight, while the density of transected glandular tubules declined. There was no correlation between height of the stratum compactum or of the glandular epithelium and uterus weight.5. In ewes with dead embryos the means of the following indices were significantly greater than in ewes with live ones: height of the stratum compactum, height of the glandular layer, height of the gland epithelial nuclei; the density of glandular tubules was less.6. In the hot-room, gland epithelium height was correlated with mean rectal temperature.7. Expansion of the glandular layer is not associated in all subgroups with more embryonic deaths and cannot therefore be their main cause. The observations do not indicate that early embryonic deaths are due to insufficiency of ovarian hormones; nor do they account for the marked improvement in viability associated with thyroxine injections.