Magic embryology: What guides cell development, placement? (Introduction)

by David Turell , Monday, August 12, 2019, 17:42 (1931 days ago) @ David Turell

More mechanical and spatial factors are recognized:

https://www.quantamagazine.org/for-embryos-cells-size-can-determine-fate-20190812/

"The developing embryo is a finely tuned machine. Its cells know what to do, and when to do it. They know to grow or shrink, to divide or lie dormant, to come together into a beating heart or hurtle through the bloodstream in search of a distant invader. And they know to do all that without a central command station or an objective map of their surroundings to guide them.

"Instead, cells are left to devise their own strategies for calculating precisely where to go and what to become. Those calculations depend on a veritable cocktail of signals, some of which have long been established as obviously important — chemical and electrical gradients, the activity of gene networks, patterns of overlap between spreading fields of molecules.

"But recently experts have also started to pay attention to another, often overlooked set of factors: physical constraints such as size. In new work published today in Nature Physics, a team of researchers reported that during the early development of the roundworm Caenorhabditis elegans, a mechanism based on the size of embryonic cells helps to determine the type of mature tissues they will eventually produce.

"While examining the biochemical process that triggers cells to divide either asymmetrically or symmetrically, the scientists discovered that size was the ruling element — namely, that the size of the cells dictated the pattern that led to one kind of division or another, and ultimately to one kind of lineage or another. “The biology is actually exploiting this fact … to generate a set of outcomes that are required for the development of the organism,” said Martin Howard, ... In this case, cells used innate constraints on their size to specify the lineage that would later give rise to the worm’s sex cells. But more broadly, the findings also point to the possibility of a role for physical cues in the behavior of stem cells and the operation of other developmental systems.

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"For instance, when the worm embryo is still a single cell, proteins on its outer membrane create two uneven, yin-and-yang-like domains that tell the cell where to split. That system for designating asymmetric cell division is called polarity. One lineage of embryonic cells in C. elegans (known as the P lineage) uses polarity to divide asymmetrically four times; the fifth division is then symmetric, permanently establishing the germline responsible for egg and sperm cells.

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"By controlling the sizes of the initial embryos, the team was then able to show that there was a minimum size threshold for the P lineage cells, below which they could not set up the polarization pattern. Those smaller cells lost the ability to polarize after just three cell divisions, not four. “Just by manipulating the size of the embryo, we’ve taken a cell that normally would be able to polarize and divide asymmetrically and turned it into a cell that doesn’t polarize and divides symmetrically,” Goehring said.

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"Cells have seemingly evolved to take advantage of the intrinsic limitations of their patterning process — using it as a ruler of sorts — to determine whether to become germ cells. “The specification [of the germ cells] is a kind of self-organized property of the patterning system,” Howard said.

"And that’s a “genuinely interesting” way to think about the system, said Timothy Saunders, a biophysicist at the Mechanobiology Institute of the National University of Singapore who was not involved in the study. “This idea, that by just simply making things smaller you can naturally switch the type of division, is very neat.”

"These findings come at a time when scientists are widening their view of what controls biological systems to encompass more than genetics alone. “The gene does not exist in a vacuum,” Saunders said. “And we’re realizing more and more that the mechanical environment in which those genes are operating matters” — including for decisions about cell fate."

Comment: It is obvious embryonic cells are controlled by chemical and physical influences. but an overall body plan has to exist. Darwin does not explain the development of embryology in any of his theories. It is magical and strongly suggests that only design fits.