Magic embryology: more on mechanical forces (Introduction)

by David Turell , Wednesday, January 10, 2024, 21:02 (318 days ago) @ David Turell

A study in sea squirts:

https://www.sciencedaily.com/releases/2024/01/240109121139.htm

"sea squirt oocytes (immature egg cells) harness friction within various compartments in their interior to undergo developmental changes after conception.

***

"Sea squirts or ascidians in particular are very unusual: after a free-moving larvae stage, the larva settles down, attaches to solid surfaces like rocks or corals, and develops tubes (siphons), their defining feature.

"Although they look like rubbery blobs as adults, they are the most closely related invertebrate relatives to humans.

"Especially at the larval stages, sea squirts are surprisingly similar to us.

"Therefore, ascidians are often used as model organisms to study the early embryonic development of vertebrates to which humans belong.

***

"The findings suggest that upon fertilization of ascidian oocytes, friction forces play a crucial role in reshaping and reorganizing their insides, heralding the next steps in their developmental cascade.

"Oocytes are female germ cells involved in reproduction. After successful fertilization with male sperm, animal oocytes typically undergo cytoplasmic reorganization, altering their cellular contents and components.

"This process establishes the blueprint for the embryo's subsequent development.

"In ascidians, for instance, this reshuffling leads to the formation of a bell-like protrusion -- a little bump or nose shape -- known as the contraction pole (CP), where essential materials gather that facilitate the embryo's maturation.

***

"The scientists microscopically analyzed fertilized ascidian oocytes and realized that they were following very reproducible changes in cell shape leading up to the formation of the contraction pole.

The researchers' first investigation focused on the actomyosin (cell) cortex -- a dynamic structure found beneath the cell membrane in animal cells.

"Composed of actin filaments and motor proteins, it generally acts as a driver for shape changes in cells.

"'We uncovered that when cells are fertilized, increased tension in the actomyosin cortex causes it to contract, leading to its movement (flow), resulting in the initial changes of the cell's shape," Caballero-Mancebo continues.

"The actomyosin flows, however, stopped during the expansion of the contraction pole, suggesting that there are additional players responsible for the bump.

***

"...they came across the myoplasm, a layer composed of intracellular organelles and molecules (related forms of which are found in many vertebrate and invertebrate eggs), positioned in the lower region of the ascidian egg cell.

"'This specific layer behaves like a stretchy solid -- it changes its shape along with the oocyte during fertilization," Caballero-Mancebo explains.

During the actomyosin cortex flow, the myoplasm folds and forms many buckles due to the friction forces established between the two components.

"As actomyosin movement stops, the friction forces also disappear.

"'This cessation eventually leads to the expansion of the contraction pole as the multiple myoplasm buckles resolve into the well-defined bell-like-shaped bump," Caballero-Mancebo adds.

"The study provides novel insight into how mechanical forces determine cell and organismal shape. It shows that friction forces are pivotal for shaping and forming an evolving organism."

Comment: the push-pull of forces is just one mechanism. There are electrical influences, changes in molecular shapes, the actions of enzymes driving reactions, etc. We see the driving forces but do not know how DNA really controls the actions.