Magic embryology: requires exact timing for every tissue (Introduction)

by David Turell @, Wednesday, February 05, 2020, 16:55 (917 days ago) @ David Turell

Making an exact copy fetus requires exact programming:

"In all sorts of animals, from fruit flies to mice to elephants, cells follow fairly similar sets of steps to grow from embryo to adult. But while these steps follow the same order and often involve the same kinds of genes and molecular signals, they proceed at different rates from species to species.


"So what are the timers that keep things trucking along at the right rate for any given organism, ensuring that it grows to the proper size and with all its parts in place?

"Take one lone example among many: motor neurons, the nerves that make muscles contract. These develop from precursor cells over a few days in mice but a week or two in humans — and the same thing happens when the cells are grown in a dish. “We can look at this carefully and demonstrate it’s the same genetic process, the same gene activities, the same mechanisms involved, and it’s just running slower in humans than in mouse embryos,” Briscoe says. “We’re trying to tackle that problem.”


"First of all, for bodies to properly form, events must unfold in the right sequence: A before B, and B before C, and so on, at right times all over the developing body.


"For example, there’s a crisp order in how Drosophila neurons develop from stem cells in a part of the embryo called the ventral nerve cord. The stem cells produce neurons with different identities, one after the other; all of them develop, from the same pool of stem cells, just at different times, and once it’s time for the later types, there’s no going back to making earlier ones. They’re guided through this process by the sequential rise and fall of activity of a set of key genes.

"The second aspect of timing is far more mysterious: the molecular processes setting the tempo such that clocks run faster or slower in different species.

Scientists already have identified types of clocks in sundry tissues. Speaking generally, such molecular timers either “count up by steadily increasing the levels of a critical regulator until it exceeds a threshold, or count down by gradually decreasing the levels of an inhibitor,” Ebisuya and Briscoe write.


"Some clocks run through feedback loops, wherein proteins build up to a certain level and then act to shut down their production — creating cyclical oscillations that cells can harness to drive developmental steps. Vertebrates of all kinds depend on an oscillating clock of this type to create the right number of structures called somites, which later develop into the bones and muscles of the vertebral column.

"And in other cases, cells seem to keep track of how many times they’ve divided, says developmental biologist Mubarak Hussain Syed of the University of New Mexico, who studies the timing of gene activity during Drosophila early brain development. “The cell might be counting, ‘OK, we have done 20 divisions, and now it’s time’ ” for the next step, he says.


"Zooming in on details could yield a vast view. Scientists believe that changes in developmental timing — heterochronies, as they are called — had profound roles to play in the evolution of the diversity in body shapes and proportions we see in modern creatures. Snakes, for example, have many more vertebrae than do mice; they achieve this by running their segmentation clock at a faster clip relative to the development of other body parts. Giraffes come by their long necks another way: They have the same number of cervical vertebrae as their closest relatives, okapi, but those vertebrae are given more time to grow large.

"By revealing the molecular clocks that time growth, biologists may start to understand the influences that gave the world mice, humans and elephants to begin with."

Comment: Pure automaticity following a planned development. New cells respond to total controls. If this happens in embryos, as it must, it is easy to conclude all cells in fully developed organisms are are constantly following those overall instructions. No magical cell mental activity, making lone decisions by themselves when the cells turn from development to the regular activities of simply conducting the duties of living.

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