As documented in two recent articles, an effective 'luteo-placental shift' in the support of the maintenance of pregnancy is not restricted to the human species. Under appropriate experimental conditions, the rat (Csapo, 1967) and even the rabbit (Csapo & Csapo, 1968) are capable of performing this shift and thus may sustain pregnancy after bilateral ovariectomy. The success of this regulatory change depends on the compensatory changes in the placenta. In both species, only those animals which responded to ovariectomy with distinct placental hypertrophy sustained pregnancy, while those which did not, showed premature delivery (Csapo, 1967; Csapo & Csapo, 1968). Placental hypertrophy in ovariectomized, gestagen-treated rats has also been recently observed by Van der Vies & Feenstra (1967).
These observations focused attention on the regulatory significance of placental hypertrophy. It was of interest to determine (in addition to the histological evidence indicating genuine hypertrophy without hyperplasia) to what extent this placental weight increase is due to hydration. This was examined by measuring placental dry weight, in addition to wet weight.
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