research highlights

stem cells from stem cells In vitro reconstitution of mouse development makes it possible to convert mouse pluripotent stem cells into primor- dial germ cells, which go on to generate functional sperm in vivo. Gametes— and sperm—are argu- ably one of the more interesting cell types in the body. To pass on the genome to the next generation, these very specialized cells must successfully participate in the intricate dance of fertilization and then give rise to pluripotent cells that go on to generate all by ESC-derived primordial germ cells. Hematoxylin and eosin–stained tissue sections body tissues, including more germ cells. of germ cell–deficient mouse testis without (left) and with (right) transplant of in vitro–differentiated The ability to make germ cells from plu- PGCs. Images courtesy of K. Hayashi and M. Saitou. ripotent stem cells would aid basic studies of development, provide a source of germ cells to study and could in principle established culture conditions (notably, from mouse induced pluripotent stem cells also be useful for assisted reproduction. In this must be done serum-free) to convert (iPSCs). What is more, they screened their a recently published paper, Mitinori Saitou these ESCs into -like cells. Finally, cultures for cell-surface markers that can and colleagues at Kyoto University describe the researchers applied the floating three- be used to enrich the in vitro–generated methods for the directed differentiation of dimensional culture conditions they had PGC-like cells by FACS, without the need pluripotent stem cells to primordial germ previously developed for converting isolated for genetically encoded reporters. cells (PGCs) in the mouse. epiblast into PGCs and monitored germ cell This work has immediate value for gener- PGCs go on to give rise to both male fate specification from the ESC-derived cells ating large numbers of mouse PGC-like cells and female gametes. In the mouse, they are using the fluorescent reporter transgenes. for basic research (several orders of magni- derived from the embryonic epiblast early in Under the right conditions, the researchers tude more cells can be obtained this way development. In previous work, researchers observed rapid (a few days) and efficient than from mouse embryos). But in addition, in Saitou’s group had derived PGCs ex vivo (about 40%) conversion of the epiblast-like one of the exciting distant vistas opened up from the isolated embryonic epiblast 5–6 cells to PGC-like cells in vitro. As they had by robust and functional in vitro germ cell days after fertilization. They reasoned that seen previously, BMP4 signaling is key in differentiation is that such methods could lessons learned from these previous stud- this process. be used to generate gametes from stem ies could be harnessed to define a method An analysis of whole-genome expres- cells of other species. Indeed, in this issue that let them go all the way from pluripotent sion profiles indicated that the epiblast-like of Nature Methods, Jeanne Loring and col- stem cells to PGC-like cells in vitro. stem cells are closest to in vivo embryonic leagues report the derivation of iPSCs from Although the so-called epiblast stem cells day (E)5.75 epiblast and that PGC-like cells two endangered species, the silver-maned are in principle good candidates for generat- are closest to in vivo E9.5 PGCs. From this drill and the white rhinoceros. Although ing PGC-like cells, based on their biologi- and other analyses, Saitou and colleagues translating knowledge about mouse germ- cal origin in the embryonic epiblast, this is conclude that they essentially recapitulated line specification to deriving germ cells known to occur at a very low efficiency. germ-cell development in vitro. Perhaps from different, nonrodent species will be a Could this efficiency be increased, Saitou most importantly, the PGC-like cells they huge challenge, it is tempting to speculate and colleagues asked, by starting with epi- generate undergo functional spermato- that the findings of Saitou and colleagues blast-like cells that have not been cultured genesis when transferred to the gonads of could eventually have some bearing on in vitro over time? The researchers set out germ cell–deficient mice. Sperm derived efforts in other species as well. to generate epiblast-like cells afresh from from these transplanted PGCs yield viable Natalie de Souza embryonic stem cells (ESCs). and grossly normal offspring when used to RESEARCH PAPERS They derived ESCs from mice bearing fertilize oocytes in vitro. Hayashi, K. et al. Reconstitution of the mouse germ transgenes for expression of fluorescently The researchers generated PGC-like cells cell specification pathway in culture by pluripotent tagged markers of PGC fate. Then, they from several ESC lines, but notably, also stem cells. Cell 146, 519–532 (2011).

nature methods | VOL.8 NO.10 | OCTOBER 2011 | 789