Surface changes of the rat embryo before implantation.

Study of the rat embryo surface under the scanning electron microscope shows the superficial structure of the pellucid membrane as a perforated network cover which does not change throughout the preimplantation period (1 to 41/2 days). Dissolution of the pellucid membrane by brief ATP treatment reveals a zygotic surface which changes from day to day. The number and length of microvilli increase with development. At the early blastula stage there is a great heterogeneity of microvilli of different size and shape as well as large membranous ruffles mainly located at one pole of the embryo. The significance of these structures may relate to the changing metabolic requirements of the developing embryo and to the invasive properties of the trophoblast.