Mouse ovarian germ cell cysts undergo programmed breakdown to form primordial follicles

In many organisms, early germline development takes place within cysts of i nterconnected cells that form by incomplete cytokinesis and later undergo p rogrammed breakdown. We recently identified similar cell clusters within th e fetal mouse ovary, but the fate and functional significance of these germ cell cysts remained unclear. Here, we show that mouse cysts undergo progra mmed breakdown between 20.5-22.5 dpc, during which approximately 33% of the oocytes survive to form primordial follicles. This process accounts for mo st of the perinatal reduction in germ cell numbers and germ cell apoptosis reported by previous authors, and suggests that perinatal germ cell loss is a developmentally regulated process that is distinct from the follicular a tresia that occurs during adult life. Our observations also suggest a novel function for a transient cyst stage of germ cell development. Prior to bre akdown, mitochondria and ER reorganize into perinuclear aggregates, and can be seen within the ring canals joining adjacent germ cells. Cysts may ensu re that oocytes destined to form primordial follicles acquire populations o f functional mitochondria, through an active process that has been evolutio narily conserved. (C) 2001 Academic Press.