Maintenance of pregnancy in the hamster by a single corpus luteum from day 12.

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 the resumption of oestrous cycles within 7 days, while ablation of the corpora lutea on Days 11 to 16 of pregnancy induces abortion or embryonic resorption (McKeown & Zuckerman, 1938). A minimum of two corpora lutea are required to maintain pregnancy in the rat from Day 8 until term (Kelsey & Meyer, 1950). Removal of both ovaries of the hamster on Days 9 to 13 of pregnancy in¬ duces abortion (Klein, 1938). Progesterone alone is unable to maintain preg¬ nancy in the ovariectomized hamster but the concurrent administration of small doses of oestrogen permits pregnancy to continue normally (Klein, 1938; Grady, unpublished observations). On the other hand, it has been reported that exogenous progesterone is sufficient to induce implantation in ovariectomized hamsters (Orsini & Psychoyos, 1965; Harper, 1966) and pregnancy can be maintained at least to Day 14 with progesterone alone (Harper, 1966). Progesterone is, therefore, necessary for the continuation of pregnancy in ovariectomized hamsters, while the necessity for oestrogen is still in question. In the present experiments an electro-cautery needle was used to destroy the corpora lutea of pregnancy in hamsters in an attempt to find the minimal amount of luteal tissue required to maintain pregnancy when follicular tissue was left intact. Only animals which had visible implantation sites at autopsy are included in this report.