Human embryonic sexual differentiation
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Genetic and Chromosomal Basis of Human Embryonic Sexual Differentiation
Human embryonic sexual differentiation begins with genetic sex determination at fertilization, where the presence of XX or XY chromosomes sets the stage for future development. Until about the sixth week of embryonic life, the gonads are bipotential, meaning they can develop into either testes or ovaries, and no visible sexual differences are present in the embryo at this stage 256.
Initiation of Gonadal Differentiation: SRY and Key Signaling Pathways
The process of sex-specific gonadal differentiation is triggered by the expression of the SRY gene on the Y chromosome in male embryos. SRY activates a signaling cascade that leads to testicular development, while its absence in female embryos allows pro-ovarian factors to promote ovarian development. These pathways not only initiate the development of one gonadal type but also actively suppress the alternative pathway. In males, the SOX9/FGF9 pathway drives testis formation, while in females, the WNT4/RSPO1 pathway supports ovarian development 157.
Role of the Mesonephros and Gonadal Cell Lineages
The mesonephros, an embryonic urinary structure, plays a crucial role in gonadal differentiation. In males, proliferation of mesonephric tubules and corpuscles stimulates testis development, with Sertoli and Leydig cells arising from these structures. In females, regression of some mesonephric tubules leads to the formation of ovarian theca cells, which are analogous to Sertoli cells in the testis. This highlights the importance of the mesonephros in both male and female gonadal development .
Hormonal Control and Phenotypic Sexual Differentiation
After gonadal differentiation, hormones produced by the testes, such as testosterone, drive the development of male internal and external genitalia. In the absence of these hormones, the default pathway leads to female phenotypic development. This hormonal influence extends to the brain, where prenatal testosterone is believed to masculinize both the reproductive system and brain structures in humans 2579.
Sex-Biased Gene Expression in Early Development
Sex differences in gene expression are evident even in undifferentiated human embryonic stem cells and become more pronounced during differentiation. Male cells show higher expression of certain X/Y gene pairs, while female cells upregulate autosomal genes important for specific developmental functions. These genetic differences influence the differentiation trajectories of various tissues, including the placenta and nervous system, and may contribute to sex-specific disease risks later in life 3810.
Parallel and Interactive Model of Sexual Differentiation
Recent research suggests that sexual differentiation is not a simple linear process driven solely by gonadal hormones. Instead, it involves multiple, parallel, and interactive molecular pathways, including genetic, hormonal, and environmental factors. These pathways can act synergistically or compensate for each other, and environmental influences can also shape sexual differentiation outcomes .
Conclusion
Human embryonic sexual differentiation is a complex, multi-stage process involving genetic, gonadal, and hormonal factors. It begins with chromosomal sex determination, followed by the activation of sex-specific signaling pathways that direct gonadal development and suppress the alternative pathway. The mesonephros and its derivatives play a key role in forming gonadal cell types. Hormones produced by the differentiated gonads then drive the development of internal and external genitalia and influence brain development. Sex-biased gene expression and environmental factors further contribute to the diversity of sexual differentiation, supporting a model that integrates genetic, hormonal, and environmental influences.
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Deciphering Sex-Specific Differentiation of Human Fetal Gonads: Insight From Experimental Models
Sex-specific differentiation of human fetal gonads involves multiple signaling pathways, with differences between mice and humans, highlighting the need to examine these pathways in human fetal gonads.
Sexual Differentiation of Behavior in the Context of Developmental Psychobiology
Sexual differentiation in behavior is influenced by factors in the environment, resembling the processes by which all specialized cells differentiate from a common precursor, the fertilized egg.
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