Female Germ Cell Development in Chickens and Humans: The Chicken Oocyte Enriched Genes Convergent and Divergent with the Human Oocyte

Abstract

The development of germ cells and other physiological events in the differentiated ovary of humans are highly conserved with several mammalian species, except for the differences in timing. However, comparative knowledge on this topic is very scarce with respect to humans and lower vertebrates, such as chickens. In chickens, female germ cells enter into meiosis around embryonic day (E) 15.5 and are arrested in meiotic prophase I as primary oocytes. The oocytes arrested in meiosis I are accumulated in germ-cell cysts; shortly after hatching, they are enclosed by flattened granulosa cells in order to form primordial follicles. In humans, the process of meiotic recombination in female germ cells begins in the 10-11th week of gestation, and primordial follicles are formed at around week 20. In this review, we comprehensively elucidate both the conservation and the species-specific differences between chickens and humans with respect to germ cell, oocyte, and follicle development. Importantly, we provide functional insights into a set of chicken oocyte enriched genes (from E16 to 1 week post-hatch) that show convergent and divergent expression patterns with respect to the human oocyte (from week 11 to 26).

Keywords: chicken; gene conservation; gene expression; germ cell; human; oocyte.

Figure 1

Schematic comparison of developing germ cells, oocytes, and ovarian follicles in chickens and humans. The primordial germ cells (PGCs) are specified by the inherited mode in chickens and by the epigenesis mode in humans. After specification, PGCs migrate to the gonads; this occurs via the germinal crescent and blood circulation in chickens and via the hindgut in humans. Further physiological events in the differentiated ovary, including oogonia proliferation and entry into meiosis, oocyte arrest in meiotic prophase I, the accumulation of oocytes in germ-cell cysts, and the formation of early follicles, are all essentially the same in both chickens and humans, except the differences in timing. In contrast, late folliculogenesis is meaningfully different between chickens and humans. SWF: small white follicles; LWF: large white follicles; SYF: small yellow follicles; LYF: large yellow follicles.

Figure 2

Biological processes of the genes reviewed in this article. (A) Biological processes of the chicken oocyte enriched genes that are convergent with the human oocyte. (B) Biological processes of the chicken oocyte enriched genes that are divergent from the human oocyte. The involvement of convergent and divergent genes in the biological processes is emphasized in red and blue, respectively.

Figure 3

Interacting networks of the genes reviewed in this article. (A) Interacting networks of the chicken oocyte enriched genes that are convergent with the human oocyte. (B) Interacting networks of the chicken oocyte enriched genes that are divergent from the human oocyte. Interacting genes found in clusters 1, 2, and 3 are emphasized with red, green, and blue, respectively.