HORMONAL REQUIREMENTS FOR THE UTERINE ATTACHMENT REACTION AND BLASTOCYST IMPLANTATION IN THE MOUSE, HAMSTER AND GUINEA-PIG

Before implantation occurs, a lumen containing secretion can be demonstrated in the uterus of the rat, mouse, hamster and guinea-pig. This luminal space disappears when implantation occurs, the apposing luminal surfaces of the mucosa coming into close contact. This phenomenon has been named the `attachment reaction ' of the uterine epithelium (Nilsson, 1970). In the rat, the preimplantation appearance of the uterine mucosa can be maintained, by giving progesterone during experimentally delayed implanta-tion (Nilsson, 1966) or by giving progesterone to spayed rats (Ljungkvist, un-published). Implantation and the attachment reaction do not take place until oestrogen is given (Mayer & Nilsson, 1966). In the hamster, implantation occurs without the addition of oestrogen when progesterone is given to animals ovariectomized early in pregnancy (Prasad, Orsini & Meyer, 1960; Harper, Dowd & Elliott, 1969) and in the guinea-pig, even progesterone is not neces-sary (Deanesly, 1960). Although, in the mouse, the blastocysts of an experimen-tally delayed implantation do not implant until oestrogen is given (Humphrey