Magic embryology: embryo timekeeper (Introduction)

by David Turell @, Wednesday, April 28, 2021, 18:18 (427 days ago) @ David Turell

A metabolic clock to time repeated events is found and studied:

"Animal cells bustle with activity, and the pace varies between species. In all observed instances, mouse cells run faster than human cells, which tick faster than whale cells. These differences affect how big an animal gets, how its parts are arranged and perhaps even how long it will live. But biologists have long wondered what cellular timekeepers control these speeds, and why they vary.

"A wave of research is starting to yield answers for one of the many clocks that control the workings of cells. There is a clock in early embryos that beats out a regular rhythm by activating and deactivating genes. This ‘segmentation clock’ creates repeating body segments such as the vertebrae in our spines. This is the timepiece that Ebisuya has made in her lab.


"Biologists have been studying the segmentation clock since the 1990s, and they know that it runs about twice as fast in mouse embryos as it does in human embryos. The speed at which an embryo develops, or at which different parts of it develop, has an important influence on the adult body. Ebisuya and others want to understand how differences in developmental pace give rise to organisms with such different bodies and behaviours.


"The findings are already overturning some long-held assumptions about how different animals develop. So far, there is no sign of a master gene controlling the speed of the segmentation clock. Instead, its speed seems to be controlled by the differing rates at which proteins are broken down. Scientists had assumed the speed was mostly constant for each protein across animals, so the discovery might require them to revise some molecular-biology textbooks.


"Speed matters when it comes to building species. Evolution didn’t give giraffes long necks by adding extra bones; they have the same number of vertebrae as their stubby-necked okapi relatives. Rather, neck vertebrae in giraffes grow over longer periods of time, which allows them to reach bigger sizes.


"Pourquié and other biologists have been trying to take the segmentation clock apart and understand how it works, building a long list of genes and proteins that help the clock to keep time. One key gene is Hes7, the mammalian equivalent of the bird gene c-hairy1. Hes7 can repeatedly turn itself on and off, as can several other genes involved in the clock. That makes it “a key pacemaker for the segmentation clock”, says Ryoichiro Kageyama, a developmental biologist at Kyoto University in Japan who has studied the gene for almost two decades.

"But it is still unclear why Hes7 turns on and off at different speeds in different species, and thus how the speed of the segmentation clock is ultimately controlled. A series of studies over the past three years point to an answer.


"These studies revealed many similarities between the segmentation clock of humans and those of other animals. Analogues of the same genes and proteins are involved in mice and humans, for instance.

"But there was one striking difference. The human segmentation clock is slow. Each oscillation takes 5–6 hours, twice as long as the 2–3 hours it takes in mouse embryos: a clear example of heterochrony (see ‘Unlocking the segmentation clock’). But why is the human segmentation clock so slow, and what is controlling it?


"The in vitro segmentation clock studies could well resolve this question, but also suggest a broader mystery: do human cells run slower than those of other species, not just during specific periods of development, but throughout our lives? If so, that could help to explain why our lifespan is extended compared with that of other species."

Comment: We are all based on the same basic body plans so size of parts plays a big role in what types of species develop. This is highly complex design plan when timing is part of the process. Not by chance.

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