2.4Formation of Multicellular Organisms by Embryogenesis
In several multicellular organisms such as animals or flowering plants, fertilization occurs through the fusion of a sperm and an egg. The zygote repeatedly undergoes cytokinesis (cleavage) to form many cells, which then differentiate and form individual bodies (see Fig. 2-3). In these cases, the appropriate production and arrangement of these different cells are required for the formation of various tissues and organs that constitute the multicellular organism. Such morphogenesis not only requires the formation of cell clusters through cell division but also relies on the interactions between various cells formed by ovum heterogeneity or cleavage in the original oocyte, and depending on the circumstances, cell migration. The process by which a single zygote develops into an embryo (multicellular organism) is called embryogenesis. In embryogenesis, differentiation refers to the development of cells with various functions. Despite the fact that the genetic information shared by all differentiated cells is fundamentally identical, expression of various genes that are not required by the differentiated cells is repressed and expression of specialized genes becomes easy to occur. This is considered to be due to epigenetic changes; therefore, modifications such as DNA methylation have been brought to light (see Chapters 10 and 24). The process of development of multicellular organisms also involves interactions between cells and delivery of substances such as hormones to cells (Chapters 19 and 22).