Human embryology

Segmentation of the human zygote

Because once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions that increase the number of cells. These, which decrease in size with each division, are called blastomeres (Figure 1-1). Until the eight-cell stage, they form a loose clump (Fig. 1-2A). After the third division, however, the blastomeres maximize contact with each other and form a compact ball of cells that are held together with tight junctions (Fig. 1-2B). This compaction process separates the inner cells, which are fully communicated by gap junctions, from the outer cells. Approximately 3 days after fertilization, the cells of the compacted embryo divide again, forming a 16-cell morula (mulberry). The inner cells of the morula make up the inner cell mass and the cells surrounding them make up the outer cell mass. The inner cell mass originates the embryo's own tissues, and the outer cell mass forms the trophoblast, which will later contribute to the formation of the placenta. The trophoblast cells in the embryonic pole begin their differentiation first into cytotrophoblast, from which the syncytiotrophoblast differentiates, which initiates the invasion of the maternal endometrium around day 6 of development and ends on day 12.