Cinemicrographic observations on the trophoblast and zona pellucida of the mouse blastocyst.

The mammalian ovum is surrounded by a secondary membrane, the zona pellucida, which is produced by the follicle cells round the ovarian oocyte. The major constituents of this membrane are neutral or weakly acidic mucoproteins, and it is sensitive to dissolution by a variety of proteases, depending on species (Gwatkin, 1964), especially the bacterial protease 'Pronase' (Mintz, 1962). In the rabbit and rat the zona is impermeable to compounds with a molecular weight above 1200 (Austin & Lovelock, 1958). Since in all mammals, an intact zona surrounds the cleavage stages and early blastocyst, this membrane must be removed, or penetrated by the trophoblast cells before implantation can take place. This could result from the interaction of chemical or mechanical factors produced either by the conceptus or by the uterus, or from a combination of some or all of these. Where marked expansion of the blastocyst occurs before implantation, as in the rabbit, the zona becomes extremely attenuated and almost invisible in the living conceptus. In the mouse, rat, guinea-pig and hamster, however, the diameter of the zona at the time of implantation is not greatly larger than that of the tubal ova, and the membrane is only slightly thinned, therefore facilitating examination and photography. In the case of the mouse, especially, the development of in vitro culture methods for ova and blastocysts has made direct observations of the method of escape from the zona possible. These studies indicate that in this species at least uterine factors play a minimal role. Whitten (1957) observed lobes extruded from the trophoblast and penetrating the zona of mouse blastocysts in vitro and this observation was later confirmed and extended by Mintz (1962), who showed that these lobes were produced even after enzymic dissolution of the zona, and that the mouse blastocyst could escape from the zona in vitro. Similar escape of the mouse blastocyst from its zona in other media has been reported (Brinster, 1963; Cole & Paul, 1965). The