In many animals other than mammals, the materials necessary for continued development to a certain stage need to be pre-stored in the egg. These materials, which are derived from the mother and therefore known as maternal factors, include many substances that play important roles in the early stages of development. The main constituents of maternal factors are mRNA and special proteins. These factors are involved in many of the important events that occur during the early stages of development, such as determining which way the embryo faces (e.g., head-tail direction and dorsal-ventral-side direction), determining the fate of embryonic cells (i.e., deciding which tissues or organs they will become), regulating cell growth, etc.
Many maternal factors are stored in the oocyte during Oogenesis, and many are stored unevenly in the cytoplasm of the oocyte (Fig. 10-1). The uneven distribution of these factors is closely associated with the determination of the direction of the embryo and the fate of embryonic cells. As an example, maternal factors localized in the egg will become unevenly distributed over embryonic cells (blastomeres) through the division of the egg (cleavage). As a result, they become deeply involved in the determination of these variables.

Fig. 10-1. Oogenesis in fruit flies

Oogenesis in fruit flies occurs with the help of cells known as nurse cells. A germ cell divides four times to form sixteen cells, of which only one becomes an oocyte (an immature egg in the process of Oogenesis). The rest become nurse cells and facilitate the creation of the oocyte. The cytoplasm of the oocyte and nurse cells is connected, and many proteins and mRNA (i.e., the mRNA of maternal factors such as nanos and bicoids) that are synthesized in nurse cells are transported through this pathway to the oocyte. Many of these materials are unevenly stored in the oocyte. → indicates the direction of transport of materials synthesized in nurse cells.

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