Differentiation of germ-line cells

Germ-line cells of fishes arise from cells that have taken up the special cytoplasm called “germ cytoplasm” stored in the egg cytoplasm. The germ cytoplasm gathers on both sides of the early cleavage furrow during cleavage. As cell division progresses, only the cells that have taken up this cytoplasm differentiate into germ cells. Early germ cells are called primordial germ cells (PGCs). The germ cytoplasm contains proteins and mRNA. The physical removal of this germ cytoplasm or inhibition of mRNA translation into protein leads to the loss of germ cells, resulting in infertile individuals. Moreover, with the microinjection of an artificial mRNA that imitates germ cytoplasm into a fertilized egg, it is possible to confer fluorescence on the germ cytoplasm (Figure 8) or make the germ cells glow (Figure 9).