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So, why does it take us nine months to develop? What drives our developmental clock?

Mark Johnson
Milwaukee Journal Sentinel

New discoveries lead scientists to new questions, and it has been no different for James Thomson 20 years after his isolation of human embryonic stem cells.

“So the basic question,” he said, “is why do we take nine months (to develop from embryo to birth) and a mouse takes three weeks?

“You never ask that question because it seems so intuitively obvious that a big thing is going to take longer, but the question of how that’s controlled and why is never really addressed. ... It’s been something that’s fascinated biologists for well over 100 years with very little insights into how scale and time is controlled.”

James Thomson, director of regenerative biology at Morgridge Institute for Research in Madison on the UW-campus, works in his office.

In work published in 2017, Thomson and his colleagues examined mouse and human cells and found strong evidence for some kind of mechanism inside the cell that controls development.

Put both into the same conditions in a lab dish and the mouse cells developed on mouse pace; the human cells on human pace.

Thomson said he found that if he put human cells into a lab dish that mimics the mouse environment, “it still follows human time. It doesn’t say, ‘Oh, I’m in a mouse. I’ve got to go in mouse time. It still follows human time.”


Three elephants cross the road in Kruger National Park, South Africa. African elephants have a gestational period of 645 days, one of the longest in nature.

The period from conception to birth varies greatly among mammals — from 19 days for a domestic white mouse to 187 days for a baboon, 270 days for a human, 345 days for an alpaca, 450 days for a black rhinoceros and 645 days for an African elephant. And size doesn’t appear to be the only factor.

“Whales develop far too fast for their size,” Thomson said. “They grow at a phenomenal rate.”

Sperm whales take an average of 535 days to develop.

At present, Thomson is focusing on developmental timing in mice, humans and a variety of non-human primates.

Other fundamental questions loom over the stem cell landscape.

Shinya Yamanaka, the Japanese scientist who pioneered the method of reprogramming mature adult cells back to an embryonic state, wrote by email that, “one of the big questions to puzzle out is the mechanism of reprogramming."

In his Nobel-winning experiments, Yamanaka used a virus to ferry four crucial genes inside adult skin cells, rewinding the developmental clock. But how did the transformation unfold exactly?

“I believe this question will elucidate the mystery of life,” said Yamanaka, who works at Kyoto University, the Gladstone Institutes in San Francisco and the University of California, San Francisco.

Another key question, said Yamanaka: “How can we evaluate the safety and qualities” of cells created through reprogramming in order to turn the cells into safe and effective medicine?