A Morphological Study on the Early Fertilized Ova in Rat—With Special Reference to the Temporal Changes of Zona Pellucida and Implantation Process of Blastocyst

Abstract

It has already been reported that the zona pellucida in the rodentia plays an important role in the mechanism of preventing polyspermatozoan fertilization. The implantation of the blastocyst is known to begin after the zona pellucida is taken off from the surface. The authors carried out a scanning electron microscopical observation on temporal changes in the zona pellucida after mating. After fertilization, a clear spongy structure having a double or triple construction was formed on the zonal surface. The structure was distinguishable until 2.5 days after mating, when it developed into the 2-cell stage. After that, it became indistinct and thinned increasingly as the ovum grew further. When the blastocyst was formed completely 4.5 days after mating, the zonal surface was hatched. It is assumed that the existence of active zonal reaction may physiologically convert almost all the zona pellucida as a fine spongy structure. In the present research observation was made on the implantation process and the implantation chamber with a scanning electron microscope. On the early morning of 5.5 days after mating, the blastocyst tended to adhere to the implantation chamber which had been formed previously in the endometrium by the secretion of mucoidal substances. After that, implantation proceeded. On the evening of 5.5 days after mating, however the implantation of blastocyst onto the endometrium progressed with interdigitation by microvilli.