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The negative correlation between the number of foetuses and the length of gestation characteristic of most multiparous species is not found in the pig. This observation, reported in previous studies, was confirmed using 1450 records from the Duroc and Hampshire breeds. The gestation period was also unrelated to either the number of late foetal deaths or to the weight and fatness of the sow but prolonged gestation at first parturition reduced the probability of a second litter.

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Percentages of cell types in the adenohypophysis were compared with the content of sth and prolactin at different reproductive stages (e.g. immature, oestrous cycle, pregnancy, lactation and after hysterectomy) in the pig. Acidophils represent sth and prolactin activities. Chromophobes are undifferentiated, inactive or depleted cells.

Acidophils represented half (53·5%) the cell population. Percentages of sth cells were higher in 17-day-old pigs than in mature animals. This is a period of rapid growth in the young pig. During the cycle, during pregnancy and after hysterectomy, the percentages of prolactin cells were higher than those found in immature or lactating pigs.

The chromophobes represented 13·1% of the cell population. Chromophobes were the predominant cell type during lactation, but these seemingly inactive cells may have been active acidophils that were synthesizing and secreting prolactin. By rapid turnover of hormone, the cytoplasmic granules in prolactin cells lack differentiation.

The sth activity was similar in pigs during pregnancy and lactation and after hysterectomy. There were no significant correlations in the percentages of sth cells and the concentration or content of sth during pregnancy and lactation. There was a trend of increased percentages of prolactin cells in later stages of pregnancy and after hysterectomy. Exogenous oestrogen caused a marked increase in the pituitary content of sth and prolactin in hysterectomized pigs as compared with uninjected hysterectomized animals. Prolactin levels remained relatively constant during pregnancy and after hysterectomy, but declined during lactation. Exogenous oestrogen caused a consistently higher trend in prolactin activity in hysterectomized animals.

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J. G. Thompson, R. J. Partridge, F. D. Houghton, C. I. Cox and H. J. Leese

The consumption of oxygen, uptake of pyruvate and glucose and production of lactate were determined for groups of bovine embryos produced in vitro from the one-cell to the blastocyst stage (day 0–6 of culture). Measurements were made in Hepes-buffered synthetic oviduct fluid medium supplemented with 1.0 mmol pyruvate l−1, 10 mmol d,l-lactate l−1 and 1.5 mmol glucose l−1 and also 3 mg BSA ml−1 and, from day 5 of development, 10% (v/v) fetal calf serum. The amount of ATP production was determined from oxygen consumption and the proportion of glucose taken up that could be accounted for by lactate production. The data revealed that oxygen consumption was relatively constant from days 0–4 of culture (0.24–0.27 nl per embryo h−1), but increased with the initiation of compaction (0.39 nl per embryo h−1) and continued to increase with the formation and expansion of the blastocoel (0.9 nl per embryo h−1). Both pyruvate and glucose uptake followed similar patterns. Furthermore, when plotted against oxygen consumption, both pyruvate and glucose uptake increased significantly (P < 0.001) in a linear relationship (R 2 = 0.61 and 0.49, respectively). Lactate production also increased with development and accounted for 40% of glucose uptake at day 0 of culture (putative zygotes), increasing to 70% by day 2 (eight-cell stage) and 100% of glucose uptake from day 4 of culture onwards. ATP production followed a similar pattern to that of oxygen consumption (60–85 pmol per embryo h−1 from day 0 to day 4) increasing with compaction (124 pmol per embryo h−1) and blastulation (221 pmol per embryo h−1). For precompaction stages, 93–96% of ATP production was derived from oxidative phosphorylation, decreasing to 82% with compaction. ATP produced by oxidative phosphorylation could be accounted for by the uptake of pyruvate, suggesting that bovine embryos produced in vitro utilize little endogenous substrates when appropriate exogenous substrates are present in the culture medium. The data revealed that bovine embryos were dependent on oxidative phosphorylation for energy (ATP) production at all stages of pre-elongation development, with perhaps a shift in dependence towards glycolysis in conjunction with compaction. It follows that oxidizable substrates, such as pyruvate and certain amino acids, are preferred in embryo culture medium during development in vitro.