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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