Growth and Differentiation of Rabbit Blastocysts in Defined Culture Media

Abstract

The mammalian blastocyst is formed by two fundamental processes which take place in the course of embryonic cleavage. These are first, the development of a certain number of species-specific blastomeres, and second, the formation of the blastocyst’s cavity (blastocoele) following the intercellular accumulation of fluid, the origin of which is the vacuolization of the blastomeric cytoplasm and the extrusion of the content of these vacuoles. The most important step in development of the blastocyst is the differentiation of embryoblast and trophoblast cells. During cleavage, the first eight or ten blastomeres within the zona pellucida are arranged such that one or two blastomeres are inner cells and the others surround them as outer cells, forming a morula. Between the 16-and 32-cell stages the blastocyst cavity is created by coalescence of the enlarged fluid-filled intercellular spaces (Calarco and Brown 1969). When the early blastocyst stage is reached after the relatively slow process of cleavage, at the age of 3–4 days, a characteristic species-dependent growth begins. Usually, the blastocyst enters the uterus at the same time, and cellular proliferation and expansion proceed quickly up to the time of implantation. However, at the beginning of implantation the numbers of cells in blastocysts vary remarkably between species, e.g., the mouse blastocyst is comprised of about 60, that of the rabbit of more than 5000 cells.