At fertilization, mouse sperm bind to the zona pellucida (which consists of glycoproteins ZP1, ZP2, and ZP3) that surrounds eggs. A ZP2 cleavage model of gamete recognition requires intact ZP2, and a glycan release model postulates that zona glycans are ligands for sperm. These two models were tested by replacing endogenous protein with ZP2 that cannot be cleaved (Zp2Mut) or with ZP3 lacking implicated O glycans (Zp3Mut). Sperm bound to two-cell Zp2Mut embryos despite fertilization and cortical granule exocytosis. Contrary to prediction, sperm fertilized Zp3Mut eggs. Sperm at the surface of the zona pellucida remained acrosome-intact for more than 2 hours and were displaced by additional sperm. These data indicate that sperm-egg recognition depends on the cleavage status of ZP2 and that binding at the surface of the zona is not sufficient to induce sperm acrosome exocytosis.

At fertilization, spermatozoa bind to the zona pellucida (ZP1, ZP2, ZP3) surrounding ovulated mouse eggs, undergo acrosome exocytosis and penetrate the zona matrix before gamete fusion. Following fertilization, ZP2 is proteolytically cleaved and sperm no longer bind to embryos. We assessed Acr3-EGFP sperm binding to wild-type and huZP2 rescue eggs in which human ZP2 replaces mouse ZP2 but remains uncleaved after fertilization. The observed de novo binding of Acr3-EGFP sperm to embryos derived from huZP2 rescue mice supports a 'zona scaffold' model of sperm-egg recognition in which intact ZP2 dictates a three-dimensional structure supportive of sperm binding, independent of fertilization and cortical granule exocytosis. Surprisingly, the acrosomes of the bound sperm remain intact for at least 24 hours in the presence of uncleaved human ZP2 regardless of whether sperm are added before or after fertilization. The persistence of intact acrosomes indicates that sperm binding to the zona pellucida is not sufficient to induce acrosome exocytosis. A filter penetration assay suggests an alternative mechanism in which penetration into the zona matrix initiates a mechanosensory signal transduction necessary to trigger the acrosome reaction.

Maintenance of sex-specific germ cells requires balanced activation and repression of genetic hierarchies to ensure gender-appropriate development in mammals. Figla (factor in the germ line, alpha) encodes a germ cell-specific basic helix-loop-helix transcription factor first identified as an activator of oocyte genes. In comparing the ovarian proteome of normal and Figla null newborn mice, 18 testis-specific or -enhanced proteins were identified that were more abundant in Figla null ovaries than in normal ovaries. Transgenic mice, ectopically expressing Figla in male germ cells, downregulated a subset of these genes and demonstrated age-related sterility associated with impaired meiosis and germ cell apoptosis. Testis-associated genes, including Tdrd1, Tdrd6, and Tdrd7, were suppressed in the transgenic males with a corresponding disruption of the sperm chromatoid body and mislocalization of MVH and MILI proteins, previously implicated in posttranscriptional processing of RNA. These data demonstrate that physiological expression of Figla plays a critical dual role in activation of oocyte-associated genes and repression of sperm-associated genes during normal postnatal oogenesis.Mouse gestation takes ϳ20 days, and at embryonic day 10.5 (E10.5) the bipotential mammalian gonad commits to becoming a testis or an ovary, depending on the presence of the male-specifying Y chromosome. Subsequent gender-specific genetic hierarchies modulate the transition of the undifferentiated gonad to an ovary or testis, which produces haploid gametes through the reductive divisions of meiosis (5). However, sexual fate is plastic among metazoans (e.g., flies, worms, and mice), and this reversible commitment indicates an ongoing need for genetic controls to maintain one sex with concomitant nonexpression of genes that support the opposite identity (21, 33). Recently, Foxl2, a forkhead transcription factor expressed in somatic cells, has been implicated in maintaining female gonad sexual identify in adult mice; its ablation in postnatal ovaries leads to sex reversal via upregulation of Sox9 in somatic tissue independent of oocytes (51). It seems likely that genetic hierarchies expressed within oocytes complement these somatic signals to maintain appropriate germ cell identity by activating oocyte-associated genes and repressing sperm-associated genes during postnatal oogenesis.Members of the basic helix-loop-helix (bHLH) family of transcription factors are key regulators of cell growth and differentiation that can either activate or repress transcription of target genes (19). Figla (factor in the germ line, alpha; official name, folliculogenesis-specific basic helix-loop-helix) encodes a germ cell-specific bHLH transcription factor that was initially identified by its ability to coordinate the expression of the oocyte-specific zona pellucida genes (28). Although Figla transcripts are first detected in oocytes at E14.5, few differences are observed between normal and null transcriptomes prior to birth (20), at which time null oocytes do...