10.2Fertilization and Cleavage
Besides the important task of combining two sets of genes from the father and mother into one, fertilization plays a number of other significant roles. As an example, in frog embryos, as discussed later, the direction of the body is determined by the site of the oocyte that the fertilizing sperm enters. The entry of the sperm also sets off the process of cleavage.
The cell cycle of cleavage during the early stages of development is very rapid. As an example, the average cell cycle in the cell division of frogs is approximately 16 hours, while early embryonic cells complete their cycle in 30 minutes. This is because the cell cycle lacks phases equivalent to the extent of the G1 and G2 phases. In other words, cell division takes place through repetition of the DNA synthesis period (S phase) and the mitotic period (M phase), making the cell cycle very short (see Chapter 9). In the early stages of development, a certain number of cells are thus created by the repetition of cell division within a short space of time.
Cleavage patterns are diverse, and include a type with unique cleavage directions as well as one with uneven blastomere sizes (Fig. 10-2). This diversity is due to the uneven distribution of nutrients in the embryo, or may reflect the need to unevenly distribute maternal factors localized in the cytoplasm (see 9.4). Among these, the uneven distribution of maternal factors plays a particularly important role in the developmental process. This can be easily understood by taking the destiny of the blastomeres in nematodes as an example (Fig. 10-3). In this case, in each cell division, germ cell granules (maternal factors necessary for the embryo to form germ cells) are unevenly allocated to just one of the two cells. Those that receive the granules remain in the germ cell line and finally become germ cells. All others become somatic cells that form various tissues and organs. Cell lineage describes which cells become the cells of which tissues during the process of a fertilized egg becoming an adult. (see the Column in 10.3).
Fig. 10-2. Cleavage patterns
There are various types of cleavage, which are classified by the direction of division and the size of blastomeres.