Osaka, Japan – For many endangered species, breeding programs are an important part of efforts to increase population numbers. Now, researchers from Osaka University have successfully induced the precursors of eggs and sperm using cells from threatened white rhinoceros, a critical step toward possibly rebuilding northern white rhino populations.
The northern white rhino, Ceratotherium simum cottoni, is extinct in the wild. Only two females remain, despite a decades-long effort to preserve the species through a breeding program alongside efforts to tackle poaching and habitat loss. The species decline outpaced conservation efforts and the last male died in 2018. Given the status of this species, new ways of approaching breeding and conservation are urgently needed.
Advanced assisted reproductive technologies developed for humans and livestock are a promising avenue for rebuilding populations of endangered species. Techniques such as artificial insemination, induction of ovulation, and in vitro culture of blastocysts have all been used. The challenge with the northern white rhino is the lack of eggs and sperm to produce embryos.
"Cells from the skin or blood can be reprogrammed back into an embryonic-like state," explains Masafumi Hayashi, lead author of the study. "These cells are called induced pluripotent stem cells, or IPSCs. As with embryonic stem cells, they can develop into any tissue in the body, including sperm and eggs."
Previous studies have shown that, in mice, embryonic stem cells can be differentiated into primordial germ cell-like cells (PGCLCs), which are equivalent to the origins of eggs and sperm. PGCLCs have been generated from pluripotent stem cells in mice, humans, monkeys, and rabbits, but never in wild animals.
The researchers established a culture system using pluripotent cells from southern and northern white rhinos and through repeated refinement produced PGCLCs.
"This is the first time that PGCLCs have been induced in a wild animal," says Katsuhiko
Hayashi, senior author. "This a significant breakthrough as it paves the way for generating reproductive cells from endangered species that can then be used for breeding."
Production of mature reproductive cells requires a cell environment that provides the chemical signals needed for sex-dependent differentiation and functional maturity of the gametes. In mice, PGCLCs were fully functional after transfer into the ovary and testis. While some challenges remain, in theory, it may be possible to produce mature northern white rhino eggs suitable for fertilization with banked sperm derived from four deceased males. For a species on the very brink of extinction, these technological advances offer hope.
