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GILBERT S. GREENWALD

Summary.

Hamsters injected with 60 i.u. pregnant mares' serum (pms) on the morning of metoestrus (Day 1 of oestrous cycle) will ovulate sixty to seventy ova if they are isolated from males. However, if similarly treated females are caged with males on Day 2 or 3, coitus takes place and the inactive corpora lutea of the oestrous cycle are transformed into functional corpora lutea of pseudopregnancy. Consequently, the animals do not ovulate. The corpora lutea block ovulation by producing progesterone, which inhibits the release of pituitary luteinizing hormone (lh). The corpus luteum of ovulation thus can be brought to full secretory activity as late as 54 hr after ovulation. Pregnant mares' serum-treated hamsters mating late on Day 3 do ovulate presumably because of regression of the corpora lutea. Hamsters placed with males on Day 4 mate at the normal time and ovulate as many ova as isolated controls. Animals injected concurrently with 60 i.u. pms and 5 mg of progesterone on Day 1 and subsequently isolated from males fail to ovulate, thus duplicating the effects of coitus. This experiment also indicates that the ovulation of follicles matured by pms is due to endogenous gonadotrophins rather than the inherent lh activity of the pms preparation.

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GILBERT S. GREENWALD

Summary.

This study is an analysis of the changes occurring in the population of follicles during the oestrous cycle of the hamster. Findings in the intact animal were compared with those in animals semispayed at various times. The results indicate that the number of large follicles (267μ or greater) is largely determined for each ovary by Day 1 (9 a.m. of the morning of ovulation) and is maintained until the 3rd day of the cycle. Follicular atresia between Days 3 and 4 reduces the number of large follicles per ovary to nearly the number that will eventually ovulate.

In general, both ovaries of any one animal contain approximately equal numbers of follicles during the cycle. During the first 3 days of the cycle of the intact hamster, each ovary of a pair contains nearly as many large follicles as will finally ovulate from both ovaries. After the removal of one ovary on any one of the first 3 days of the cycle, numerous follicles persist in the Day-4 ovary that ordinarily would degenerate. This suggests that a decrease in follicular atresia rather than an increase in the proliferation of medium-sized follicles is responsible for compensatory ovulation after semispaying. After unilateral ovariectomy at Day 3, the remaining ovary on the 1st day of the next cycle contains significantly more follicles than the Day-1 ovary of the intact animal or of hamsters that were semispayed on Days 1 or 4.

Removal of one ovary at 9 a.m. during the first 3 days of the oestrous cycle was followed by a doubling in the number of ovulations from the remaining ovary. These results agree with the findings on follicular development in indicating that the follicles destined to ovulate at the next oestrus are determined on the 3rd day of the cycle.

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GILBERT S. GREENWALD

The oestrous cycle of the hamster differs from the mouse and rat in two unique features: the extreme regularity of the 4-day cycle and the fact that the corpora lutea always begin to show signs of histological regression by Day 3 and have almost completely vanished by the next ovulation.

The rapidity of luteolysis raises the question whether luteal regression in the cyclic hamster is attributable to withdrawal of pituitary luteotrophic hormone(s) or to a surge in release of a hormone which directly destroys the corpus luteum. In the following experiment, these alternative possibilities were explored by hypophysectomizing hamsters on Day 1 of the cycle (metoestrus) and determining the subsequent fate of the corpora lutea in comparison with those in intact cyclic or pregnant animals. Serial sections of ovaries from three different hamsters were examined for each day; the ovaries were sectioned at 10 μ and stained with haematoxylin

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GILBERT S. GREENWALD

Summary.

A silk suture inserted into the uterine lumen prevented implantation in the rat. Normal embryos were present in the oviduct until Day 4 of pregnancy but were missing from the tract by Day 5. If the threaded uterus was ligated at the cervix at Day 4, degenerating ova were recovered the next day. This indicates that shortly after the ova entered the uterus they were usually expelled per vaginam.

A leucocytic infiltration of the endometrium was consistently observed in cornua from which embryos were missing. The inflammatory reaction was accentuated by anti-inflammatory compounds which stimulated myelopoiesis. Two months after thread insertion, the uterus showed the histologic modifications associated with a chronic inflammation, i.e. a proliferative rather than a marked exudative response.

The inflammatory reaction appeared to be essential for interruption of pregnancy since the absence of leucocytes was always correlated with implantation in the sutured cornu. The location of the thread determined whether a leucocytic reaction occurred. Implantation took place in the treated cornu if the thread had not penetrated into the lumen but instead was lodged in the stroma. This accounted for the presence of embryos in some of the threaded uteri despite the in situ location of the suture. Pregnancy continued when the thread was deliberately placed in the stroma, at some distance from the lumen.

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JASTI B. CHOUDARY and GILBERT S. GREENWALD

Summary.

A single subcutaneous injection of 10 or 50 μg of oestradiol cyclopentylpropionate (oecp) on Day 1 of the oestrous cycle induced luteolysis in cyclic guinea-pigs by Day 3. Regression of the corpora lutea was completed by Day 10. In addition to inducing luteolysis, a dose of 10 μg of oecp shortened the length of the cycle and also induced precocious ovulation between Days 9 and 12 of the cycle. A dose of 50 μg of oecp induced follicular atresia by Day 8. oecp was effective in causing luteolysis when injected on Days 1 or 4, but not on Day 6.

Neither 5 mg of progesterone injected daily on Days 1 to 9 nor 1 mg of testosterone propionate injected on Day 1 induced luteolysis.

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HARRY M. WEITLAUF and GILBERT S. GREENWALD

Summary.

By means of radioautography, 35S methionine was used as an index of protein synthesis in cleaving mouse ova. Limited amounts of 35S methionine were incorporated in early cleavage stages. However, the normal blastocyst showed intense reactivity shortly before implantation (Day 5 of pregnancy). When a suckling stimulus was used to delay nidation, the delayed implanting blastocysts did not incorporate 35S methionine for at least as late as Day 9 of concurrent pregnancy and lactation. However, when the young were removed on Day 8 of lactation, the blastocysts incorporated 35S methionine 24 hr later to the same extent as normal Day 5 blastocysts. This indicates that the prolonged uterine sojourn of the delayed implanting blastocyst is accompanied by modifications in metabolic activity so that the egg more closely resembles the pattern of development during the first 4 days of pregnancy than the normal blastocyst.

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HARRY M. WEITLAUF and GILBERT S. GREENWALD

The amount of luteal tissue essential for the maintenance of pregnancy has been determined for various laboratory species. From early experiments and those cited below, it seems clear that different amounts of luteal tissue are necessary at different stages of pregnancy if gestation is to continue. In the rabbit, blastocysts fail to expand in the absence of luteal tissue (Corner, 1928; Adams, 1965), but one to two corpora lutea are sufficient for endometrial proliferation (Brouha, 1934; Adams, 1965); whereas two to four corpora lutea are necessary to depress myometrial activity enough to prevent expulsion of the unattached blastocysts (Adams, 1965). Bilateral ovariectomy after implantation terminates pregnancy in the rabbit (Csapo, 1956).

Destruction of the corpora lutea of the rat on Days 3 to 7 of pregnancy or pseudopregnancy leads to