Magic embryology: guiding body orientation (Introduction)

by David Turell @, Sunday, May 19, 2019, 18:55 (1179 days ago) @ David Turell

The zygote, as a ball of symmetrical cells, must establish head, tail, left and right to develop any organism in the correct shape. Controls found:

"Even before the fertilised egg or zygote can start dividing into daughter cells that form the future tissues and organs during the development of a multicellular organism, the symmetrical zygote needs to become asymmetrical or polarised in shape and molecular organisation. The master switch that triggers the symmetry breaking process in the zygotes of the nematode worm, Caenorhabditis elegans (C. elegans), was identified in a recent study,


"Structurally, cells acquire top and bottom or front and rear surfaces, while at the molecular level, special protein groups called polarity regulators move to distinct regions in the cell cortex (a layer beneath the cell membrane). As a result, different cell regions acquire specific architectures and functions.


"... his team previously revealed how forces generated on the cortex by the contractions of the actin cytoskeleton (a filamentous framework made of actin and myosin proteins) could direct the movement of polarity regulators to their destined locations. Shortly after fertilisation, actomyosin contractions in the cortex puts the surface of the zygote under tension. Upon symmetry breaking, spatially-controlled inhibition of actomyosin contractions lead to an imbalance in the surface tension, causing the cortical cytoskeletal networks to flow and transport the polarity regulators.


"After using a technique called RNA interference to block the synthesis of specific proteins involved in the polarisation process, and observing the effects in live zygotes, they singled out a protein called Aurora-A (AIR-1 in C. elegans) as the master switch for symmetry breaking.

"Aurora-A is a kinase (a type of protein that regulates the activity of other proteins by adding a phosphate molecule to them) that has well-known roles in controlling cell division by assembling centrosomes -- a cellular organelle that organises microtubule filaments and facilitates cell-cycle progression.

"The researchers identified a two-stage process by which Aurora-A influences actomyosin contraction to initiate symmetry breaking and establish cell polarity. In the first stage, Aurora-A accumulates around centrosomes and suppresses actomyosin contractions locally in the surrounding cortex. This creates force differences along different regions of the cortex, and the resulting cortical flow transports myosins and other polarity proteins to the front of the zygote, thereby creating front-rear asymmetry. In the second stage, Aurora-A diffuses into the cytoplasm and suppresses actomyosin contractions across the entire cortex. This prevents further cortex flows or movement of polarity regulators, effectively locking them in place.


"The single master switch, Aurora-A, creates 'polar lights' that cover one of the cell poles for symmetry breaking."

"Intriguingly, the research team discovered that the role of Aurora-A in cell polarisation was independent of its role in centrosome assembly and cell-cycle progression. Through fruitful collaborations with colleagues at NUS, including Associate Professor Yusuke Toyama (MBI) and Professor Thorsten Wohland (NUS Department of Biological Sciences), who brought their expertise in laser ablation and fluorescence correlation spectroscopy respectively, they demonstrated that the local accumulation of Aurora-A was sufficient to induce symmetry breaking via its kinase activity, regardless of the involvement of centrosomes."

Comment: This is a complex process, which had to have been designed, or the embryos would not proceed properly. No way it could have been developed by chance trial. The Aurora-A is an enzyme which has two roles, which is unusual in itself. Further it should be realized any enzyme is a giant molecule, and one must ask how does an unguided chance evolutionary hunt and find such an exact protein molecule? Only a designer fits.

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