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After reprogramming to naïve pluripotency, human pluripotent stem cells (PSCs) still exhibit very low ability to make interspecies chimeras in mice and pigs. Whether this is because they are inherently devoid of the attributes of chimeric competency or because naïve PSCs in general cannot colonize embryos from distant species remains to be elucidated. Here, we have used different types of mouse, human and rhesus monkey naïve PSCs and we have analyzed their ability to colonize both distant (rabbit) and close (cynomolgus monkey) embryos. Mouse embryonic stem cells (ESCs) remained mitotically active after injection into rabbit and cynomolgus embryos and contributed to the formation of the neural tube in post-implantation rabbit embryos. In contrast, primate naive PSCs colonized rabbit and cynomolgus embryos with much lower efficiency, regardless of the reprogramming protocols. Unlike mouse ESCs, they slowed DNA replication following colony dissociation and, after injection into host embryos, they stalled in the G1 phase of the cell cycle and differentiated prematurely, regardless of host species, distant or close. These results show that, unlike mouse ESCs, primate naive PSCs do not make chimeras because they are inherently unfit to stay mitotically active during embryo colonization.

Basic fibroblast growth factor (bFGF) exerts a mitogenic effect on cortical neuroblasts, whereas neurotrophin 3 (NT3) promotes differentiation in these cells. Here we provide evidence that both the mitogenic effect of bFGF and the differentiation-promoting effect of NT3 are linked with modifications of cell cycle kinetics in mouse cortical precursor cells. We adapted an in vitro assay, which makes it possible to evaluate (1) the speed of progression of the cortical precursors through the cell cycle, (2) the duration of individual phases of the cell cycle, (3) the proportion of proliferative versus differentiative divisions, and (4) the influence on neuroglial differentiation. Contrary to what has been claimed previously, bFGF promotes proliferation via a change in cell cycle kinetics by simultaneously decreasing G1 duration and increasing the proportion of proliferative divisions. In contrast, NT3 lengthens G1 and promotes differentiative divisions. We investigated the molecular foundations of these effects and show that bFGF downregulates p27(kip1) and upregulates cyclin D2 expression. This contrasts with NT3, which upregulates p27(kip1) and downregulates cyclin D2 expression. Neither bFGF nor NT3 influences the proportion of glia or neurons in short to medium term cultures. The data point to links between the length of the G1 phase and the type of division of cortical precursors: differentiative divisions are correlated with long G1 durations, whereas proliferative divisions correlate with short G1 durations. The present results suggest that concerted mechanisms control the progressive increase in the cell cycle duration and proportion of differentiative divisions that is observed as corticogenesis proceeds.