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Ultrastructural features of the regressing corpus luteum have been studied in the ewe at the end of the estrous cycle. Luteal cells secrete densely-staining granules which have been reported to contain progesterone. This process appears to be maximal at Day 10 of the cycle and begins to taper off noticeably at Day 12. The secretory activity declines gradually till Day 14, but up to this time the fine structure of other cytoplasmic organdIes in the luteal cell shows no remarkable change, save for a gradual increase in the number of autophagocytic bodies which first begin to appear at Day 12. In contrast, on Day 15 there is evidence of marked degeneration; the luteal cells are shrunken, granule secretion has ceased, autophagocytic bodies are numerous and lipid droplets, rarely seen in the fully functional cell, are present in abundance, Lysosomes are observed at all stages of the cycle and there is no dramatic increase in their number towards the end, In some cells at Day 15 the process of degeneration has advanced to the stage where the fine structure of organelles is no longer recognizable and within autophagocytic bodies there are large sites associated with acid phosphatase activity.

Oxytocin (OT), progesterone and prostaglandin F2α (PGF2α) concentrations were measured in the utero-ovarian vein (UOV) of ewes which displayed persistence of the corpus luteum (CL). During the period of expected luteolysis, the frequency of OT and PGF2α pulses in the UOV was significantly (P < 0.005 for both) lower in ewes with persistent CLs, compared with ewes that underwent normal luteal regression. In contrast, the amplitude of both OT and PGF2α pulses was similar in both groups of animals. It is suggested that persistence of the CL resulted from a decreased PGF2α pulse frequency, which may have arisen from a decreased frequency of stimulation by OT. In two persistent CL ewes, however, it appeared that a failure at the level of the uterus may have contributed to the observed decrease in PGF2α release. Although a PGF2α analogue (Lutalyse) infusion into the uterine vein of two ewes with persistent CLs failed to induced luteolysis, it did stimulate a large release of OT into the UOV. This suggests that persistent CLs maybe more resistant to PGF2α and, that at day 22 post-oestrus, these CLs are capable of releasing large quantities of OT into the UOV.

The ultrastructure of the corpus luteum of the sheep has been examined throughout the estrous cycle. Small, densely staining granules 0.2 im in diameter, are observed within the luteal cell at Day 2 of the cycle; they increase in number as the cycle progresses and at Day 6 the first signs of secretion are noticed. Maximum secretion of granules takes place at Days 10 and 11, and this is followed by a progressive decline in secretory activity from Day 12 to Day 15 of the cycle. These clear-cut changes in granule formation and secretion therefore reflect the established pattern of progesterone secretion by the ovary during the estrous cycle in the sheep. Fractions were obtained from corpora lutea of sheep by homogenization and differential centrifugation. A correlation was obtained between the presence of densely staining granules (approx 0.2 um diam) and the progesterone content of each fraction. Many types of nonsteroidogenic endocrine cells are known to have a secretory pathway whereby hormones are packaged in the Colgi region and then discharged from the cells via granules; the present work provides the first indication of a steroid hormone, viz, progesterone, being secreted in a similar way.

Synacthen has been infused at 10 μg/h into 4 hypophysectomized foetal sheep. This caused a rise in adrenocorticotrophin to a mean value of 512 pg/ml which remained fairly constant and did not show the large fluctuations seen on infusion into intact foetuses. The Synacthen half-life in the circulation had a mean value of 0.27 min. Hypophysectomy did not necessarily delay the foetal corticosteroid response to Synacthen infusion. The corticosteroid concentration achieved had a mean value of about 50 ng/ml which is substantially below that for intact foetuses and in one case it was maintained at only 20 ng/ml. Despite this delivery occurred within 5 days. Dexamethasone infusion reduced the plasma adrenocorticotrophin concentration by about 70 % and the plasma cortisol concentration by about 60 %.