Isolation and in vitro expansion of porcine spermatogonial stem cells

Recent Progress of Induced Spermatogenesis In Vitro

Sperm, a crucial gamete for reproduction in sexual reproduction, is generated through the proliferation, differentiation, and morphological transformations of spermatogonial stem cells within the specialized microenvironment of the testes. Replicating this environment artificially presents challenges. However, interdisciplinary advancements in physics, materials science, and cell engineering have facilitated the utilization of innovative materials, technologies, and structures for inducing in vitro sperm production. This article offers a comprehensive overview of research progress on inducing in vitro sperm production by categorizing techniques into two major systems based on matrix-based and non-matrix-based approaches, respectively. Detailed discussions are provided for both types of technology systems through comparisons of their similarities and differences, as well as research advancements. The aim is to provide researchers in this field with a comprehensive panoramic view while presenting our own perspectives and prospects.

The cellular expression patterns of gdnfa and gdnfb in the gonads of Nile tilapia and their differential response to retinoic acid

In mammals, glial cell derived neurotrophic factor (GDNF) plays a critical role in the self-renewal and maintenance of spermatogonial stem cells (SSCs) in testis and oogenesis in ovary, whilst retinoic acid (RA), the key factor of meiosis initiation, can downregulate its expression. Unlike mammals, two Gdnf replication genes are widely present in teleost fishes, however, our understanding of them is still poor. In the present study, two paralogous gdnf from Nile tilapia (Oreochromis niloticus), namely as Ongdnfa and Ongdnfb, were characterized, and then their cellular expression profiles in testis and ovary and responsiveness to RA treatment at the tissue and cellular levels were investigated. In phylogenetic tree, the Gdnfa and Gdnfb from teleost fishes were clustered into two different subclasses, respectively, and then clustered with the homologs from cartilaginous fish and tetrapods, suggesting that OnGdnfa and OnGdnfb are orthologous to GDNF and paralogous to each other. Ongdnfa is expressed in Sertoli cells and Leydig cells in testis and oocytes in ovary. The expression pattern of Ongdnfb is similar to Ongdnfa. In the ex vivo testicular organ culture, RA down-regulated the expression of Ongdnfa, whereas up-regulated the expression of Ongdnfb (P < 0.05), suggesting that they have differential responsiveness to RA signaling. RA treatment of the cultured cells derived from adult Nile tilapia testis which have the expression of RA receptors (RAR), Ongdnfa and Ongdnfb further confirmed the above result. Collectively, our study suggests that Ongdnfa and Ongdnfb have non-germline expression patterns in testis and germline expression patterns in ovary; furthermore, they have differential responsiveness to RA signaling, implying that they might have differential biological functions. This study broadens and enriches our understanding of fish GDNF homologs and lays foundation for the study of their biological functions in the future.

Use of assisted reproductive technologies (ARTs) to shorten the generational interval in ruminants: current status and perspectives

The challenges posed by climate change and increasing world population are stimulating renewed efforts for improving the sustainability of animal production. To meet such challenges, the contribution of genomic selection approaches, in combination with assisted reproductive technologies (ARTs), to spreading and preserving animal genetics is essential. The largest increase in genetic gain can be achieved by shortening the generation interval. This review provides an overview of the current status and progress of advanced ARTs that could be applied to reduce the generation time in both female and male of domestic ruminants. In females, the use of juvenile in vitro embryo transfer (JIVET) enables to generate offspring after the transfer of in vitro produced embryos derived from oocytes of prepubertal genetically superior donors reducing the generational interval and acceleration genetic gain. The current challenge is increasing in vitro embryo production (IVEP) from prepubertal derived oocytes which is still low and variable. The two main factors limiting IVEP success are the intrinsic quality of prepubertal oocytes and the culture systems for in vitro maturation (IVM). In males, advancements in ARTs are providing new strategies to in vitro propagate spermatogonia and differentiate them into mature sperm or even to recapitulate the whole process of spermatogenesis from embryonic stem cells. Moreover, the successful use of immature cells, such as round spermatids, for intracytoplasmic injection (ROSI) and IVEP could allow to complete the entire process in few months. However, these approaches have been successfully applied to human and mouse whereas only a few studies have been published in ruminants and results are still controversial. This is also dependent on the efficiency of ROSI that is limited by the current isolation and selection protocols of round spermatids. In conclusion, the current efforts for improving these reproductive methodologies could lead toward a significant reduction of the generational interval in livestock animals that could have a considerable impact on agriculture sustainability.

Additional glial cell line-derived neurotrophic factor in vitro promotes the proliferation of undifferentiated spermatogonia from sterile cattleyak

Cattleyak is a typically male sterile species. The meiosis process is blocked and the scarcity of spermatogenic stems cells are both contributing factors to the inability of male cattleyak to produce sperm. While Glial cell line-derived neurotrophic factor (GDNF) is the first discovered growth factor known to promote the proliferation and self-renewal of spermatogenic stem cells, its relationship to the spermatogenesis arrest of cattleyak remains unclear. In this report, we studied the differential expression of GDNF in the testis of yak and cattleyak, and discussed the optimal concentration of GDNF in the culture medium of undifferentiated spermatogonia (UDSPG) of cattleyak in vitro and the effect of GDNF on the proliferation of cattleyak UDSPG. The results indicated that GDNF expression in the testicular tissue of cattleyak was inferior to that of yak. Moreover, the optimum value for the UDSPG in vitro culture was determined to be 20-30 ng/mL for cattleyak. In vitro, the proliferation activity of UDSPG was observed to increase with additional GDNF due to the up-regulation of proliferation-related genes and the down-regulation of differentiation-related genes. We hereby report that the scarcity of cattleyak UDSPG is due to insufficient expression of GDNF, and that the addition of GDNF in vitro can promote the proliferation of cattleyak UDSPG by regulating the expression of genes related to proliferation and differentiation. This work provides a new insight to solve the issue of spermatogenic arrest in cattleyak.

Optimized Recovery of Immature Germ Cells after Prepubertal Testicular Tissue Digestion and Multi-Step Differential Plating: A Step towards Fertility Restoration with Cancer-Cell-Contaminated Tissue

Undifferentiated germ cells, including the spermatogonial stem cell subpopulation required for fertility restoration using human immature testicular tissue (ITT), are difficult to recover as they do not easily adhere to plastics. Due to the scarcity of human ITT for research, we used neonatal porcine ITT. Strategies for maximizing germ cell recovery, including a comparison of two enzymatic digestion protocols (P1 and P2) of ITT fragment sizes (4 mm3 and 8 mm3) and multi-step differential plating were explored. Cellular viability and yield, as well as numbers and proportions of DDX4+ germ cells, were assessed before incubating the cell suspensions overnight on uncoated plastics. Adherent cells were processed for immunocytochemistry (ICC) and floating cells were further incubated for three days on Poly-D-Lysine-coated plastics. Germ cell yield and cell types using ICC for SOX9, DDX4, ACTA2 and CYP19A1 were assessed at each step of the multi-step differential plating. Directly after digestion, cell suspensions contained >92% viable cells and 4.51% DDX4+ germ cells. Pooled results for fragment sizes revealed that the majority of DDX4+ cells adhere to uncoated plastics (P1; 82.36% vs. P2; 58.24%). Further incubation on Poly-D-Lysine-coated plastics increased germ cell recovery (4.80 ± 11.32 vs. 1.90 ± 2.07 DDX4+ germ cells/mm2, respectively for P1 and P2). The total proportion of DDX4+ germ cells after the complete multi-step differential plating was 3.12%. These results highlight a reduced proportion and number of germ cells lost when compared to data reported with other methods, suggesting that multi-step differential plating should be considered for optimization of immature germ cell recovery. While Poly-D-Lysine-coating increased the proportions of recovered germ cells by 16.18% (P1) and 28.98% (P2), future studies should now focus on less cell stress-inducing enzymatic digestion protocols to maximize the chances of fertility restoration with low amounts of cryo-banked human ITT.

Perspectives: Approaches for Studying Livestock Spermatogonia

At present, the knowledge base on characteristics and biology of spermatogonia in livestock is limited in comparison to rodents, yet the importance of studying these cells for comparative species analysis and enhancing reproductive capacity in food animals is high. Previous studies have established that although many core attributes of organ physiology and mechanisms governing essential cellular functions are conserved across eutherians, significant differences exist between mice and higher order mammals. In this chapter, we briefly discuss distinguishing aspects of testicular anatomy and the spermatogenic lineage in livestock and critical considerations for studying spermatogonial stem cell biology in these species.

A Role for Exchange of Extracellular Vesicles in Porcine Spermatogonial Co-Culture

Spermatogonial stem cells (SSCs) provide the basis for lifelong male fertility through self-renewal and differentiation. Prepubertal male cancer patients may be rendered infertile by gonadotoxic chemotherapy and, unlike sexually mature men, cannot store sperm. Alternatively, testicular biopsies taken prior to treatment may be used to restore fertility in adulthood. Testicular SSC populations are limited, and in vitro culture systems are required to increase numbers of SSCs for treatment, demanding culture systems for SSC propagation. Using the pig as a non-rodent model, we developed culture systems to expand spermatogonia from immature testis tissue, comparing different feeders (Sertoli cells, peritubular myoid cells (PMCs) and pig fetal fibroblasts (PFFs)). Spermatogonia co-cultured with Sertoli cells, PMCs and PFFs had comparable rates of proliferation and apoptosis. To elucidate the mechanism behind the beneficial nature of feeder layers, we investigated the role of extracellular vesicles in crosstalk between spermatogonia and feeder cells. Sertoli cell-released exosomes are incorporated by spermatogonia, and inhibition of exosomal release reduces spermatogonial proliferation. Together, these results show that PMCs, PFFs and Sertoli cells promote spermatogonial proliferation in co-culture, with exosomal exchange representing one possible mechanism. Further characterization of exosomal cargo may ultimately allow the development of feeder-free culture systems for clinical use.