When building an embryo, timing is everything

They researchers found mouse cells divide faster than human cells, which will effectively reduce protein levels even further than the lower stability.The researchers acknowledge that they haven't checked whether any proteins specifically involved in motorneuron development are more or less stable, or whether this difference holds in other tissues or at other times. They form in a head-to tail direction, and their formation is run like a clockwork: a set number of hours after the previous somite forms, a new one will condense out of the loose blanket of cells on the side of the developing spinal cord.Given the topic here, it shouldn't surprise you to hear that the clock runs at a different timing in different species: about 30 minutes in zebrafish, 90 in chickens, two to three hours in mice, and four to six hours in humans. Once again, this raises the question of why the timing of the clock can differ so much if all these species have a very similar collection of genes.Like the other researchers, this group used stem cells from mice and humans and induced them to form somites. Again, the stem cells behaved much like the intact embryonic tissues: it took 120 minutes for mouse stem cells to start producing somite-specific genes, and 320 minutes in human stem cells.If the cells were signaling to each other to control the timing, then it would only work if the cells were in close physical proximity. Despite this, the onset of somite-specific gene activity retained the distinctive timing of the two different species.As the other group did, this research team took the human version of a key somite gene and put it into mouse stem cells. When the stem cells were used to make actual mice, the clock was slow again, but the human gene was good enough to produce a healthy adult mouse.So, the researchers started checking how the gene was used to produce a protein.