Envelopment by endothelial cells initiates translocation of avian primordial germ cell into vascular tissue

The journey of a generation: advances and promises in the study of primordial germ cell migration

The germline provides the genetic and non-genetic information that passes from one generation to the next. Given this important role in species propagation, egg and sperm precursors, called primordial germ cells (PGCs), are one of the first cell types specified during embryogenesis. In fact, PGCs form well before the bipotential somatic gonad is specified. This common feature of germline development necessitates that PGCs migrate through many tissues to reach the somatic gonad. During their journey, PGCs must respond to select environmental cues while ignoring others in a dynamically developing embryo. The complex multi-tissue, combinatorial nature of PGC migration is an excellent model for understanding how cells navigate complex environments in vivo. Here, we discuss recent findings on the migratory path, the somatic cells that shepherd PGCs, the guidance cues somatic cells provide, and the PGC response to these cues to reach the gonad and establish the germline pool for future generations. We end by discussing the fate of wayward PGCs that fail to reach the gonad in diverse species. Collectively, this field is poised to yield important insights into emerging reproductive technologies.

Early gonadogenesis in Columba livia (birds: Columbiformes): Migration, colonization, and differentiation of germ cells

In birds, primordial germ cells (PGCs) use the bloodstream to travel to a specific region, where the cells undergo extravasation followed by intrastromal migration to the gonadal crest for further colonization. Currently, DDX4, SSEA1, and Oct4 are used to identify germ cells. Other germline cell-associated molecules are N-cadherin, GnRHR, and 3β hydroxysteroid dehydrogenase (3βHSD), which have been used in mice and birds during gonadal development; however, its role in early gonadogenesis in birds is poorly described. This study aimed to evaluate the differential immunodetection of N-cadherin binding molecule, Oct4 pluripotency protein, GnRHR receptor, and 3βHSD enzyme in Columba livia embryos during migration colonization of PGCs in the gonadal crest and early gonadogenesis. These markers were revealed by immunohistochemistry in histological preparations of C. livia corresponding to stages (S)15 to S40. Immunodetection of N-cadherin, Oct4, GnRHR, and 3βHSD in the germ line of C. livia allowed the identification of PGCs in the yolk sac membrane at the level of the splanchnic mesoderm during migration to the genital crest and its colonization. In the same way, it was possible to characterize and localize PGCs during early gonadogenesis. This study in C. livia demonstrates that Oct4, N-cadherin, GNRHR, and 3βHSD are immunodetected in PGCs and could be used as potential germline cell markers during cell migration out of blood vessels, colonization in the genital crest, and early gonadogenesis. Furthermore, this study could be used as a novel general model to understand the early gonadogenesis in altricial species.

LIN28 is essential for the maintenance of chicken primordial germ cells

Understanding the mechanism of stem cell maintenance underlies the establishment of long-term and mass culture methods for stem cells that are fundamental for clinical and agricultural applications. In this study, we use chicken primordial germ cell (PGC) as a model to elucidate the molecular mechanisms underlying stem cell maintenance. The PGC is a useful experimental model because it is readily gene-manipulatable and easy to test gene function in vivo using transplantation. Previous studies to establish a long-term culture system have shown that secreted factors such as FGF2 are required to maintain the self-renewal capability of PGC. On the other hand, we know little about intracellular regulators responsible for PGC maintenance. Among representative stem cell factors, we focus on RNA-binding factors LIN28A and LIN28B as possible central regulators for the gene regulatory network essential to PGC maintenance. By taking advantage of the CRISPR/Cas9-mediated gene editing and a clonal culture technique, we find that both LIN28A and LIN28B regulate the proliferation of PGC in vitro. We further showed that colonization efficiency of grafted PGC at the genital ridges, rudiments for the gonads, of chicken embryos were significantly decreased by knockout (KO) of LIN28A or LIN28B. Of note, overexpression of human LIN28 in LIN28-KO PGC was sufficient to restore the low colonization rates, suggesting that LIN28 plays a key role in PGC colonization at the gonads. Transcriptomic analyses of LIN28-KO PGC reveal that several genes related to mesenchymal traits are upregulated, including EGR1, a transcription factor that promotes the differentiation of mesodermal tissues. Finally, we show that the forced expression of human EGR1 deteriorates replication activity and colonization efficiency of PGCs. Taken together, this work demonstrates that LIN28 maintains self-renewal of PGC by suppressing the expression of differentiation genes including EGR1.

An inducible germ cell ablation chicken model for high-grade germline chimeras

Chicken embryos are a powerful and widely used animal model in developmental biology studies. Since the development of CRISPR technology, gene-edited chickens have been generated by transferring primordial germ cells (PGCs) into recipients after genetic modifications. However, low inheritance caused by competition between host germ cells and the transferred cells is a common complication and greatly reduces production efficiency. Here, we generated a gene-edited chicken, in which germ cells can be ablated in a drug-dependent manner, as recipients for gene-edited PGC transfer. We used the nitroreductase/metronidazole (NTR/Mtz) system for cell ablation, in which nitroreductase produces cytotoxic alkylating agents from administered metronidazole, causing cell apoptosis. The chicken Vasa homolog (CVH) gene locus was used to drive the expression of the nitroreductase gene in a germ cell-specific manner. In addition, a fluorescent protein gene, mCherry, was also placed in the CVH locus to visualize the PGCs. We named this system 'germ cell-specific autonomous removal induction' (gSAMURAI). gSAMURAI chickens will be an ideal recipient to produce offspring derived from transplanted exogenous germ cells.

Current Approaches to and the Application of Intracytoplasmic Sperm Injection (ICSI) for Avian Genome Editing

Poultry are one of the most valuable resources for human society. They are also recognized as a powerful experimental animal for basic research on embryogenesis. Demands for the supply of low-allergen eggs and bioreactors have increased with the development of programmable genome editing technology. The CRISPR/Cas9 system has recently been used to produce transgenic animals and various animals in the agricultural industry and has also been successfully adopted for the modification of chicken and quail genomes. In this review, we describe the successful establishment of genome-edited lines combined with germline chimera production systems mediated by primordial germ cells and by viral infection in poultry. The avian intracytoplasmic sperm injection (ICSI) system that we previously established and recent advances in ICSI for genome editing are also summarized.

Fate Decisions of Chicken Primordial Germ Cells (PGCs): Development, Integrity, Sex Determination, and Self-Renewal Mechanisms

Primordial germ cells (PGCs) are precursor cells of sperm and eggs. The fate decisions of chicken PGCs in terms of their development, integrity, and sex determination have unique features, thereby providing insights into evolutionary developmental biology. Additionally, fate decisions in the context of a self-renewal mechanism have been applied to establish culture protocols for chicken PGCs, enabling the production of genome-edited chickens and the conservation of genetic resources. Thus, studies on the fate decisions of chicken PGCs have significantly contributed to both academic and industrial development. Furthermore, studies on fate decisions have rapidly advanced owing to the recent development of essential research technologies, such as genome editing and RNA sequencing. Here, we reviewed the status of fate decisions of chicken PGCs and provided insight into other important research issues that require attention.

Prime editing in chicken fibroblasts and primordial germ cells

CRISPR/Cas9-based genome editing technologies are revolutionizing developmental biology. One of the advanced CRISPR-based techniques is prime editing (PE), which enables precise gene modification in multiple model organisms. However, there has been no report of taking advantage of the PE system for gene editing in primordial germ cells (PGCs) thus far. In the current study, we describe a method to apply PE to the genome of chicken fibroblasts and PGCs. By combining PE with a transposon-mediated genomic integration, drug selection, and the single-cell culture method, we successfully generated prime-edited chicken fibroblasts and PGCs. The chicken PGC is widely used as an experimental model to study germ cell formation and as a vector for gene transfer to produce transgenic chickens. Such experimental models are useful in the developmental biology field and as potential bioreactors to produce pharmaceutical and nutritious proteins. Thus, the method presented here will provide not only a powerful tool to investigate gene function in germ cell development but also a basis for generating prime-edited transgenic birds.