At the time of birth, mouse ovaries are essentially a bag full of primordial oocytes, around which primordial follicles are forming. This key time in ovarian development establishes the population of oocytes and follicles and instigates evelopment of a cohort of follicles. If follicles fail to form, the result is oocyte demise, yet oocytes are also essential for follicular development. Several factors key to follicular formation and oocyte survival have been discovered recently, such as FIGLA and LHX8. Now, Choi et al. (p. 1176) report that SOHLH2 spermatogenesis- and oogenesis-specific basic helix-loop-helix transcription factor 2) is crucial for follicular development and oocyte survival. Although some primordial follicles appear to form in Sohlh2-null ovaries, the oocytes are rapidly lost and follicles do not develop to the primary stage. Importantly, only low levels of key oogenic transcripts such as Figla, Gdf9, Lhx8, and Pou5f1 are expressed by the null oocytes at birth, suggesting that SOHLH2 production requires full expression of these important factors. Therefore, SOHLH2 is of fundamental importance for both oogenic and folliculogenic processes.
Youngsok Choi, Daniel Yuan, and Aleksandar Rajkovic. Germ Cell-Specific Transcriptional Regulator Sohlh2 Is Essential for Early Mouse Folliculogenesis and Oocyte-Specific Gene Expression. Biol Reprod 2008; 79:1176�. Published in BOR-Papers in Press 27 August 2008;
Background noise, or a symphony?
Genetic background effects can plague researchers studying developmental processes in mammalian organisms. However, because such effects are due to differences in many allelic combinations, they are also key indicators of regulatory complexity. In a paper on p. 1038, Park et al. report genetic background effects on the phenotype produced by Dax1 (Nrob1) deficiency in mice. Intriguing work from several laboratories has demonstrated that the severity of the Dax1-deficiency phenotype is greatest on a C57BL/6J background and milder on a 129 genetic background. Park et al. studied individuals comprised of mixed, 129 and C57BL/6J backgrounds and defined a quantitative range of intermediate phenotypes and gene expression patterns (such as the augmentation of Stra8 expression by C57BL/6J background noted by others). Appreciating the significance of these results will require defined mapping crosses to determine the C57BL/6J alleles modifying gonadal sex determination and germ cell fate. Nonetheless, the collective studies to date highlight the importance of genetic context in the analysis of sex differentiation pathways.
Susan Y. Park, Eun-Jig Lee, Donna Emge, Carolyn L. Jahn, and J. Larry Jameson. A Phenotypic Spectrum of Sexual Development in Dax1(Nr0b1)-Deficient Mice: Consequence of the C57BL/6J Strain on Sex Determination. Biol Reprod 2008; 79:1038�. Published in BOR-Papers in Press 16 July 2008;
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Society for the Study of Reproduction