Magic embryology: mechanical forces reviewed (Introduction)

by David Turell , Wednesday, January 13, 2021, 19:07 (1410 days ago) @ David Turell

Still poorly understood:

https://www.nature.com/articles/d41586-021-00018-x?utm_source=Nature+Briefing&utm_c...

"At first, an embryo has no front or back, head or tail. It’s a simple sphere of cells. But soon enough, the smooth clump begins to change. Fluid pools in the middle of the sphere. Cells flow like honey to take up their positions in the future body. Sheets of cells fold origami-style, building a heart, a gut, a brain.

"None of this could happen without forces that squeeze, bend and tug the growing animal into shape. Even when it reaches adulthood, its cells will continue to respond to pushing and pulling — by each other and from the environment.

"Yet the manner in which bodies and tissues take form remains “one of the most important, and still poorly understood, questions of our time”, says developmental biologist Amy Shyer, who studies morphogenesis

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"Maître’s group investigated how the initial ball of cells comprising the very early mouse embryo develops a large, fluid-filled cavity called the lumen. As this cavity fills, the cells that will become the fetus are pushed together on one side. This first symmetry-breaking event ensures that the embryo implants into the uterine wall correctly, and also governs which side of the embryo will be the back and which the belly. What wasn’t clear was how the embryo creates and positions the lumen

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"Later in development, embryos break symmetry in another direction, differentiating head from tail.

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"They found that the tip of the growing tail was in a state that physicists call ‘fluid’ — the cells flowed freely, and when pressed, the tissue deformed easily. The farther the scientists got from the tail end, the more rigid the tissue became. “We knew that it was solidifying, but we didn’t know the mechanism,” Campàs recalls.

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"As the cells crowded together, the tissue solidified. Campàs compares the transition to coffee grains being packaged: the grains flow freely into a bag, but become so tightly packed that the filled bag feels like a brick. He plans to investigate whether this mechanism underlies the formation of other embryonic structures, such as limb buds.

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"Once the developing embryo has mapped itself out, individual organs begin to form. “Fundamentally, we have a poor understanding of how any internal organ forms,” says Timothy Saunders, a developmental biologist at the National University of Singapore.

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"There’s a crucial event when two pieces of tissue come together to form a tube that will ultimately become the heart. Each piece contains two kinds of heart-muscle cell. The pieces must zip up correctly, pairing like with like, for a healthy heart to emerge. “We often saw misalignment that was then corrected,” says Saunders. “What’s causing the correction?”

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"Simple cell proliferation can also signal cells to arrange themselves properly, as researchers at the University of Cambridge, UK, discovered in embryos of the clawed frog Xenopus. The team, led by physical biologist Kristian Franze, already knew that as the eye and brain wire up, the eye neurons send out their axons — long projections that neurons use to contact each other — along a pathway defined by the stiffness of the brain tissue. Eye axons follow softer tissues towards a central hub in the developing brain

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"As in the developing zebrafish tails, the stiffer tissue in the frog brains seemed to contain a greater density of cells. When the team blocked cell division in the developing embryos, the stiffness gradient never appeared — and the axons couldn’t find their way. Packing a space with cells seems to be a quick and effective way to guide the wiring-up of the nervous system."

Comment: All fun and games in research. We see how it happens but have no idea what is guiding the developments, except the general knowledge it must come from the genome. Only a designing mind can create this process of embryological reproduction.