Primordial germ cells in mice arise in the proximal extraembryonic mesoderm of the mouse embryo, and migrate to the embryonic gonad primordia [1, 2]. In female mice, primordial germ cells that enter the embryonic ovaries divide mitotically until approximately embryonic day (E) 13.5. These mitotic oocyte progenitors are termed oogonia. Oogonia enter meiosis after E13.5, and are then termed oocytes. Oocytes arrest in the diplotene stage of the first meiotic division.
Initially, oocytes develop in clusters termed germ-cell nests (also called germ-cell clusters, cysts, or syncytia). These nests arise through the processes of incomplete cytokinesis and cellular aggregation [2–6]. During the first few days after birth, the germ-cell nests break apart, and the oocytes individually become surrounded by somatic cells to form primordial follicles. Temporally, the process of germ-cell nest breakdown and primordial-follicle formation is accompanied by the apoptotic cell death of approximately two-thirds of the oocytes. The surviving oocytes become surrounded by a single layer of somatic pre-granulosa cells, forming the primordial follicles [2–5]. Primary follicles are formed from the primordial follicles as the oocytes start to grow and the surrounding somatic pre-granulosa cells become cuboidal and proliferative. The prevailing view in the field has been that the oocytes which are present in the primordial follicles of the ovaries represent the entire reservoir of gametes available to a female mouse throughout its reproductive life. However, a vigorous debate has developed over the existence of female germ-line stem cells in ovaries of mice and humans [7, 8].
Breakdown of germ-cell nests and formation of primordial follicles are key early events in mammalian folliculogenesis. Breakdown of germ-cell nests occurs during the same time window as the apoptotic death of approximately two-thirds of the oocytes within those nests; however, the mechanistic connection between these two events is not clear. It has long been known that exposure of neonatal mice to various estrogenic compounds results in formation of multi-oocyte follicles, and it is believed that defects in the process of germ-cell nest breakdown leads to the formation of these multi-oocyte follicles [4, 5].
The Notch signaling pathway is an evolutionarily conserved, intercellular signaling mechanism [9, 10]. Notch signaling frequently plays a crucial role in precursor cells, making binary cell-fate decisions. However, Notch signaling also regulates additional developmental decisions, such as boundary formation between cell populations, cell proliferation, and cell death. Notch family receptors are large single-pass Type I transmembrane proteins. In mammals, four Notch family receptors have been described, encoded by the Notch1, 2, 3 and 4 genes.
A Notch family receptor exists at the cell surface as a proteolytically cleaved, non-covalently associated heterodimer, consisting of a large ectodomain and a membrane-tethered intracellular domain. During canonical Notch signaling, Notch receptors interact with ligands that are also single-pass Type I transmembrane proteins. This restricts the Notch pathway to regulating juxtacrine intercellular interactions. In mammals, the canonical Notch ligands are encoded by the Jagged (Jag1, Jag2) and Delta-like (Dll1, Dll3, Dll4) gene families.
The signal induced by ligand binding is transmitted intracellularly by a process involving proteolytic cleavage of the receptor and nuclear translocation of the intracellular domain of the Notch family protein. The receptor/ligand interaction induces two additional proteolytic cleavages in the membrane-tethered fragment of the Notch heterodimer. The final cleavage, catalyzed by the gamma-secretase complex, frees the intracellular domain of the Notch receptor from the cell membrane. The cleaved fragment translocates to the nucleus owing to the presence of nuclear localization signals located in the Notch intracellular domain. Once in the nucleus, the Notch intracellular domain forms a complex with a sequence-specific DNA binding protein, the RBPJ protein, (also known in mammals as CSL or CBF1), and activates transcription of Notch target genes.
Notch signaling plays an essential role during oogenesis in Drosophila, and is required at several different stages of oocyte development [11–13]. Recent work has suggested that Notch signaling probably also plays an essential role during oogenesis and ovary development in mammals. The Notch2 receptor is expressed at high levels in pre-granulosa [14, 15] and granulosa  cells of the neonatal and adult mouse ovary, and ex vivo culture of neonatal mouse ovaries in gamma-secretase inhibitors (which abrogate Notch signaling) resulted in defects in granulosa-cell proliferation and primordial-follicle formation [14, 15]. However, no in vivo loss-of-function studies have been performed to establish whether Notch family receptors have an essential physiological role during normal ovary development. Mice homozygous for a Notch2 null allele die early during embryogenesis [17, 18], thus necessitating a conditional gene-deletion strategy to examine the requirement for Notch2 gene function during oogenesis.
In this paper, we report that Notch2 gene function in the somatic-cell lineage of the mouse ovary is essential for breakdown of germ-cell nests and formation of primordial follicles, and that Notch2 function in granulosa cells non-cell-autonomously regulates apoptosis of oocytes in the early postnatal period.