LOSS OF THE ZONA PELLUCIDA IN MICE, AND THE EFFECT OF TUBAL LIGATION AND OVARIECTOMY

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Abstract
Loss of the zona pellucida begins late on the 4th day of gestation in the mouse. By 23.00 hours, about half the blastocysts are zona-free. Pontamine Blue sensitivity, indicating the first stage of attachment, begins at about the same time, or a little later. Once the zona is lost, recovery of blastocysts becomes increasingly difficult. However, because attachment in the early stages is very superficial, some zona-free blastocysts can be flushed out of the uterus up to the evening of the 5th day of gestation. The decidual reaction develops during the middle of the 5th day. Loss of the zona pellucida is delayed by about 12 hr when the ova are retained in the oviducts by ligation, and for a longer period when ovariectomy is performed early in pregnancy. In such females, no Pontamine Blue reaction is observed in the uterus. Under these experimental conditions, fracture of the zona eventually occurs, probably owing to active pulsating movements on the part of the blastocyst; the blastocyst may remain for a time within the ruptured zona, or may emerge. In a normal, oestrogen-stimulated uterus, loss of the zona also seems to be by hatching, but in addition some hormonally-determined uterine factor is probably acting on the zona to weaken it, and thus accelerate the process of emergence of the blastocyst.