MoBio Cleavage Chapter 8

Cleavage is the rapid mitotic cell divisions that immediately follow fertilization. As the number of cells increases to about 16 - 64, the embryo is called a morula. Cleavage does not increase the size of the embryo. Thus, the cells become smaller as their number increases.

All cells in a morula look similar. However, they may contain different cytoplasmic materials due to cytoplasm rearrangement. These subtle differences are important for subsequent cell differentiation.


Figure 3.1. The early embryo: (a) Two-cell stage. (b) Four-cell stage. (c) Eight-cell stage. (d, e) Morula stage. z.p. zona pellucida. Polar bodies. [Source: Wikipedia]