In the process of forming a body from an egg, cells need to be arranged in an orderly manner along the front-back, left-right, and top-down axes (body axis). In fish and amphibians, the anteroposterior axis originates from the polarity (heterogeneity) of the egg, a property that sperm do not have. On the other hand, the sex of medaka fish is determined by the presence or absence of the Y chromosome (XX/XY), and the Y becomes male from the fourth day after fertilization. Eggs and sperm are produced from a common progenitor cell (germ cell), but the germ cell can become either an egg or a sperm before sex is determined.

　The research group discovered that the dynamics of molecules called microtubules changes significantly at the timing when germ cells are destined to become eggs and sperm. Staining of microtubules revealed the presence of dome-shaped hollow microtubule structures (microtubule domes) within germ cells. This microtubule dome is stably maintained when germ cells differentiate into eggs, but disappears when germ cells differentiate into sperm. In addition, the microtubule dome provided the basis for intracellular structures called Balbianibodies that are required to create egg polarity.

　The microtubule dome, which is the unique base of eggs, already exists in germ cells before they are determined to be female (egg) or male (sperm). This result suggests an interesting phenomenon in which the initial sex of germ cells is female, and both females and males initially prepare to produce eggs. At the same time, for the first time, they were able to demonstrate the process by which the microtubule dome creates egg polarity, going back to early development.

Paper information:【Development】Sexually dimorphic dynamics of the microtubule network in medaka (Oryzias latipes) germ cells