Early reproductive failure of ewes in a hot environment V. The conceptus

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Abstract
1. A balanced factorial experiment was carried out with forty-eight Merino ewes. There were two levels of each of the four factors: environmental temperature, thyroxine status, vitamin A intake and progesterone status. Each ewe was killed at 25 days' pregnancy. The embryos and their membranes were weighed and examined histologically.2. There appeared to be no treatment effects on the embryonic length/weight ratio. The ratio between embryonic weight and ‘conceptus minus fluid’ weight may have been affected. There was no evidence of treatment effects on the degree of differentiation of either the embryo or the chorioallantois.3. Surviving embryos tended to be large where thyroxine was supplied, i.e. where viability was good, and vice versa. Dead embryos fell into two groups: those less than 3 mm. long and those 6 mm. or more which had probably died shortly before the ewes were killed.4. There were no significant correlations between the weights of live embryos and mean rectal temperature, plasma vitamin A concentration or any of the histological indices measured in the endometrium. In the hot-room live embryo weight was correlated with the height of the ewe's bladder epithelium (P < 0·01).5. It is suggested that in the absence of adequate maternal thyroid hormone (i) the trophoblast may enlarge too slowly to stimulate adequately some response in the endometrium essential for maintenance of the embryo, or (ii) disproportionate growth in different parts of the conceptus may result directly in embryonic failure.