Cleavage

Once the zygote has reached the two-cell stage, it undergoes a series of mitotic divisions, increasing the numbers of cells. These cells, which become smaller with each cleavage division, are known as blastomeres (Fig. 2.8). Until the eight-cell stage, they form a loosely arranged clump (Fig. 2.9A). However, after the third cleavage, blastomeres maximize their contact with each other, forming a compact ball of cells held together by tight junctions (Fig. 2.9B). This process, compaction, segregates inner cells, which communicate extensively by gap junctions, from outer cells. Approximately 3 days after fertilization, cells of the compacted embryo divide again to form a 16-cell morula (mulberry). Inner cells of the morula constitute the inner cell mass, and surrounding cells compose the outer cell mass. The inner cell mass gives rise to tissues of the

Trto-call stage Fnur-^Hi magn

Figure 2.8 Development of the zygote from the two-cell stage to the late morula stage. The two-cell stage is reached approximately 30 hours after fertilization; the four-cell stage, at approximately 40 hours; the 12- to 16-cell stage, at approximately 3 days; and the late morula stage, at approximately 4 days. During this period, blas-tomeres are surrounded by the zona pellucida, which disappears at the end of the fourth day.

Trto-call stage Fnur-^Hi magn

Figure 2.8 Development of the zygote from the two-cell stage to the late morula stage. The two-cell stage is reached approximately 30 hours after fertilization; the four-cell stage, at approximately 40 hours; the 12- to 16-cell stage, at approximately 3 days; and the late morula stage, at approximately 4 days. During this period, blas-tomeres are surrounded by the zona pellucida, which disappears at the end of the fourth day.

Compaction Morula
Figure 2.9 Scanning electron micrographs of uncompacted (A) and compacted (B) eight-cell mouse embryos. In the uncompacted state, outlines of each blastomere are distinct, whereas after compaction cell-cell contacts are maximized and cellular outlines are indistinct.

embryo proper, and the outer cell mass forms the trophoblast, which later contributes to the placenta.

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