Experiments were conducted in pregnant mice in an attempt to clarify the role of the pituitary gland in implantation in this species. Hypophysectomy was performed at various times during Days 2 to 5 of pregnancy and replacement studies with human chorionic gonadotrophin (hcg) or progesterone were carried out.
Results indicated that pituitary involvement in the initiation of implantation was confined to the early part of Day 3. Changes in the photoperiod altered the time of pituitary involvement, thus exposing a possible source of error in between-laboratory comparisons in this type of study.
Pregnancy was maintained in hypophysectomized mice by daily injection with hcg. The minimum daily dose was between 1·1 and 3·3 i.u. Embryos maintained by hcg were smaller than those in hypophysectomized mice injected with progesterone.
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