The effect of steroids on the maintenance of pregnancy in the spayed rat*

Abstract

Marshall & Jolly (1905 a, b) were the first to report that removal of the ovaries from the rat early in pregnancy was followed by foetal loss, although the duration of pregnancy when the rats were spayed is unknown. Subsequently, a number of workers have carried out similar experiments at different stages of pregnancy with or without removal of some of the foetuses (Selye, Collip & Thomson, 1935;Haterius, 1936;McKeown & Zuckerman, 1938;Zeiner, 1943).In all this work, the number offoetuses present in the rats were veryvariable, and it was thought desirable that a standard number of young should be left in the mother at the time of spaying. The present work was simplified by removing both ovaries simultaneously. In order to clarify the effects of the operation, it appeared necessary to spay one group of mother rats early in pregnancy and compare the subsequent findings with those after spaying at a relatively late stage. The number of foetuses in each pregnant rat was accordingly reduced to six on the 9th day as a standard procedure to obtain what is hereafter called the 'standardized' pregnant rat (Alexander & Frazer, 1954). Some rats were also spayed at this time, i.e. before the formation of the chorio-allantoic placenta, and these were compared with a similar series in which the operation of spaying was performed only 4 days before term.In rats spayed on the 9th day, it was expected that resorption of the foetuses might follow. If this should be the case, such a spayed rat would be a useful test animal to ascertain whether progesterone alone was sufficient to maintain pregnancy; and, if successful, whether other hormones and substances would also keep the foetuses alive. The ability to maintain pregnancy in an animal which would lose its foetuses in the absence of progesterone would certainly reveal any agent which had the essential properties of the latter.

Progesterone Receptor–Mediated Actions Regulate Remodeling of the Cervix in Preparation for Preterm Parturition

“…Based on previous reports, dosing of progestogen treatments was based upon the efficacy to sustain pregnancy. 39,[44][45][46][47][48] At about 8 and 24 hours after treatment (days 16.5 and 17 postbreeding), 3 to 5 mice were killed, a segment of reproductive tract including the cervix was excised and processed as described previously. Reproductive tracts from 4 untreated pregnant mice on the morning of day 15 postbreeding served as additional pretreatment or sham-operated controls near the peak in systemic P and softening during pregnancy.…”

Resorption and loss of foetuses in rats living at 35° C

“…Hoet (1954) described human and rabbit foetal death, associated with excess adrenal steroids which reduce placental glycogen and penetrate to the amniotic fluid. In rats, Alexander, Frazer & Lee (1955) found that deoxycortone acetate protected foetuses from the results of oophorectomy. Our finding that acclimatization was the most effective protective measure against resorption (groups Y and Z) lends support to this theory, since adrenal steroid mobilization almost certainly takes place during heat acclimatization (Conn, 1949).…”

The mechanism of foetal loss after pregnant rats are spayed

“…Various workers (Marshall & Jolly, 1905;Selye, Collip & Thompson, 1935;Haterius, 1935Haterius, , 1936Zeiner, 1943;Alexander & Frazer, 1954;Alexander, Frazer & Lee, 1955) have shown that removal of both ovaries from the pregnant rat is usually followed by loss of the majority of the foetuses, unless the operation takes place very late in pregnancy. Haterius (1936), Zeiner (1943) and Alexander & Frazer (1954) all noted distortion of the foetuses.…”

“…The rats had their abdomens opened under ether anaesthesia at the 15th day of pregnancy, and the ovaries removed by a technique already described (Alexander et al 1955). At the same time, a longitudinal incision was made in the uterine muscle over certain conceptuses, care being taken to avoid puncturing the amniotic sac.…”