Branched germline cysts and female-specific cyst fragmentation facilitate oocyte determination in mice

During mouse gametogenesis, germ cells derived from the same progenitor are connected via intercellular bridges forming germline cysts, within which asymmetrical or symmetrical cell fate occurs in female and male germ cells respectively. Here, we have identified branched cyst structures in mice, and investigated their formation and function in oocyte determination. In fetal female cysts, 16.8% of the germ cells are connected by three or four bridges, namely branching germ cells. These germ cells are preferentially protected from cell death and cyst fragmentation, and to accumulate organelles and cytoplasm from sister germ cells to become primary oocytes. Changes in cyst structure and single-cell mRNA profiles suggested that cytoplasmic transport in germline cysts is conducted in a directional manner, in which cellular content is first transported locally between peripheral germ cells and further enriched in branching germ cells, a process causing selective germ cell loss in cysts. Cyst fragmentation occurs extensively in female cysts, but not in male cysts. Male cysts in fetal and adult testes have branched cyst structures, without differential cell fates between germ cells. During cyst formation, E-cadherin junctions between germ cells position intercellular bridges to form branched cysts. Disrupted junction formation in E-cadherin-depleted cysts led to an altered ratio in branched cysts. Germ cell-specific E-cadherin knockout resulted in reductions in primary oocyte number and oocyte size. These findings shed light on the mechanism of how the size of the ovarian reserve, the number of primary oocytes available to sustain adult ovarian function, is determined during ovary formation. ### Competing Interest Statement The authors have declared no competing interest.