Oestrogens are essential for implantation in progesterone-treated, ovariectomized rats and mice (Psychoyos, 1961; Humphrey, 1967). It has been suggested that oestrogens induce blastocyst nidation by releasing histamine and perhaps other mast cell factors (Shelesnyak, 1963). If this is so, then it would be expected that mast cell factors and substances that cause their release (histamine liberators, Paton, 1957) should be as effective as oestrogens in inducing implantation, and that oestrogen-induced implantations should be affected by antihistamines and by changes in the tissue levels of histamine. The dependence of delayed blastocysts on mast cell or adrenal factors was investigated in the following experiments.
Randomly bred mice of the QS strain received gonadotrophins to induce ovulation and mating; the day of finding the vaginal plug was termed Day 1 of pregnancy. The mated mice were ovariectomized between