Endocrine control of implantation and delayed implantation in rats and mice.

Progesterone and oestrogen are required for implantation in the mouse and rat. In both animals prolactin and LH stimulate ovarian progesterone production oestrogen is stimulated by FSH in the mouse but both LH and FSH appear to be required in the rat. There is no evidence that preimplantation embryonic gonadotrophins, if they exist in these species, are involved in the initiation of implantation. The delay of implantation in suckling mice and rats is due to a lack of oestrogenic stimulus. This is caused by an inhibition of FSH release from the pituitary gland. The inhibition of FSH release appears to be caused by lack of a competent hypothalamic releasing factor. High circulating levels of prolactin and/or LH and/or progesterone do not seem to be responsible for the FSH inhibition during suckling-induced delayed implantation. Increased or decreased prolactin release does not affect FSH release during early pregnancy in the mouse.