Compaction

The zygote and the first blastomeres are totipotent. This means that each cell has the potential for developing into a complete embryo. However, totipotency ceases between the 8- and 16-cell stages and the cells become pluripotent, i.e. each cell no longer has the potential to develop into an embryo, but instead can develop into different cell types in the adult body. There is a gradual transition from totipotency to pluripotency. The fewer cell divisions that have taken place, the greater the capacity for totipotency. At the same time, the blastomeres start to display morphologically distinguishable features. At the 12-cell stage a central blastomere that probably contributes to the formation of the embryoblast can be recognized.
From this point, cell divisions can occur radially (perpendicular to the common surface of the zona pellucida) or tangentially (parallel to this surface). Radial division produces two polar daughter cells, each of which remains on the surface of the zona pellucida. Tangential division generates a polar, superficial polar cell and a nonpolar inner daughter cell, which differentiate in the internal metabolic environment. The outer cells form the trophoblast, and the inner cells the embryoblast.
The cells begin to move towards each other (compaction). Specialized connections become established between the trophoblast cells (tight junctions, zonulae occludentes). As a result, the inner cells of the embryoblast become isolated and so may differentiate in their own environment. The outer cells appear morphologically polarized: they form microvilli at their outer surface, establish specialized contacts at the lateral borders and show an asymmetrical distribution of cell organelles.