Cover Story:
In vitro growth (IVG) culture systems provide a controlled environment for immature and developmentally incompetent oocytes to mature and acquire developmental competence in vitro. However, the reported culture periods have been longer than the follicular development in vivo. To address this, Chelenga et al. developed an 8-day IVG culture system closely resembling the in vivo follicular development period. Notably, this system yielded developmentally competent bovine oocytes when cultured in a gas-permeable culture device. In addition, the combination of the 8-day IVG culture with pre-IVM culture for IVG oocytes significantly enhanced their maturational and developmental competences (Chelenga et al. Pre-maturational culture promotes the developmental competence of bovine oocytes derived from an 8-day in vitro growth culture system, pp. 214–217). The oocyte-cumulus-granulosa complex at Days 0, 4, and 8 of IVG culture (upper left, upper right, and bottom left, respectively) and the matured IVG oocyte after pre-IVM followed by IVM cultures (bottom right) were shown on the cover page.
Cover Story:
The development of germ cells is accompanied by alterations in the cell cycle in response to external signals and intrinsic cellular mechanisms. During fetal development, male germ cells undergo G0/G1 arrest, whereas female germ cells exit the mitotic phase of the cell cycle and enter meiosis. The NANOS2 and CYP26B1 proteins in the fetal testes cause the germ cells to remain in G0/G1 arrest, which prevents them from entering the meiotic cell cycle. External signals such as RA, BMP, and WNT promote the female germ cells in the fetal ovaries to enter the meiotic phase of the cell cycle. MEIOSIN and STRA8 are transiently co-expressed in the pre-leptotene phase in spermatocytes and oocytes. The MEIOSIN-STRA8 complex ensures the establishment of the meiotic phase by activating meiotic genes in such a manner that the entry into meiosis coincides with the S phase of the cell cycle. This review discusses the development of germ cells from the viewpoint of cell cycle regulation and highlights the mechanism by which germ cells enter the meiotic phase of the cell cycle (Shimada and Ishiguro. Cell cycle regulation for meiosis in mammalian germ cells, pp. 139–146).
Cover Story:
In chickens, cryopreservation of primordial germ cells (PGCs), the embryonic precursors of gametes, is the best way to cryobank chicken germplasm. However, as cryoprotectants are yet to be optimized for chicken PGCs, the efficacy of cryomedia can be further improved. Hamai et al. designed dimethyl sulfoxide-based and propylene glycol-based cryomedia in conjunction with trehalose and serum that achieved >50% recovery of viable PGCs after thawing while maintaining germline competency (Hamai et al. Development of cryopreservation media for the slow-freezing of cultured primordial germ cells in chicken, pp. 109–117). The offspring of Kurokashiwa, a rare chicken breed in Japan, was successfully revived from PGCs cryopreserved in this cryomedia.
