Developmentally, germ cells do not initially exist in the gonads but migrate through the body to reach them. After completing their migration, germ cells undergo mitosis within the gonads. While normal mitosis involves nuclear division followed by cell division, germ cells do not complete cytokinesis and instead form intercellular bridges to maintain connections between cells. This division continues, resulting in germline cysts where several germ cells exist as a cluster.
This phenomenon is observed in both vertebrates and invertebrates before meiosis and occurs in both males and females.
Regarding female germ cells:
The function of germline cysts is thought to serve quality control of oocytes and to concentrate germline cell resources into a single oocyte. Organelles such as mitochondria and endoplasmic reticulum gather in one oocyte through the intercellular bridges. As a result, a Balbiani body, which is a cluster of organelles, can be observed in the dominant oocyte.
Additionally, during this period, meiosis begins in female germ cells.
As oocytes and surrounding pre-granulosa cells mature, cyst breakdown occurs, leading many immature oocytes to undergo apoptosis, ultimately forming primordial follicles where pre-granulosa cells surround a single oocyte.
While cyst formation and cyst breakdown are opposite phenomena, cadherin adhesion and Kit-KitL binding are thought to play crucial roles in both processes.
During the mitotic and cyst formation phases, cadherin expression is decreased in both germ cells and pre-granulosa cells, resulting in weak cell adhesion. Consequently, germ cells connect via intercellular bridges. During this period, germ cell motility is known to be active, suggesting low cell adhesiveness.
Conversely, during cyst breakdown and primordial follicle formation, cadherin expression increases in both oocytes and pre-granulosa cells, leading to increased cell adhesion. This is thought to reduce germ cell motility, enabling the formation of fixed structures called primordial follicles. Additionally, the receptor-ligand signaling between oocytes and pre-granulosa cells through Kit-KitL is widely known to enhance primordial follicle survival.
In humans, this phenomenon occurs around 20 weeks of gestation. Subsequently, these follicles remain dormant with little change until puberty.
Currently, since there is no germ cell division after this period, the number of primordial follicles formed during this time is considered the maximum number.
Culturing primordial follicles is expected to help elucidate their formation process and may be useful for toxicity studies.
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