How does a three-dimensional shape of an organism emerge from a nearly spherical shaped zygote? This is one of the fundamental questions that has puzzled development biologists for a long time. A recent study titled ‘Self-organized shape dynamics of active surfaces’ by Mietke et al. published in the journal PNAS, gives us new insights on the topic.
Morphogenesis is among the fundamental aspects of developmental biology. It is a process through which an organism develops its shape from a single celled zygote (fertilized egg). Renowned mathematician and computer scientist Alan Turing proposed his famous reaction-diffusion equation model to describe the mechanism for such processes. The model assumes interactions between the chemical molecules to be the main driving force in the developmental process. But it neglects the role played by mechanical forces. However, evidence from various studies has strongly indicated the involvement of mechanical forces to be essential, in a way similar to the forces that deform balloons into different shapes. In essence, scientists have identified the process of morphogenesis to involve the interplay between chemical molecules, mechanical forces and cell shape.
Scientists from the Max Planck Institute for the Physics of Complex Systems and Max Planck Institute of Molecular Cell Biology and Genetics, have recently developed a framework to systematically integrate the connections between changes in surface shape, chemical concentration and the mechanical forces generated by these chemicals. This article summarizes the findings of the study.
Cytoskeleton gives a cell its shape and structure. It comprises of a gel-like network made of actin filaments behaving like a fluid situated beneath the cell membrane, thereby driving force-generating molecules called myosin proteins. Myosin proteins are molecular motors that convert chemical energy into mechanical work.
The intensity of force that is exerted by myosin proteins on the cytoskeleton network to change the cell’s shape, depends on its concentration These forces induce changes in the flow of cytoskeleton fluid and cell shape, which in turn influences the distribution of myosin. This entire process functions as a loop leading to the formation of different shapes.
In this study, Alexander Mietke and his colleagues, have proposed a set of equations called reaction-diffusion advection equations that encode the coupling effect between the flow of cytoskeleton fluid, myosin concentration and the cell shape. Using computer simulations, they have demonstrated the emergence of various non trivial shapes- from spherical or tubular shaped cells, throwing light on the fundamental insights into the process of morphogenesis.
Picture 1: Schematic representation of deformed embryo due to mechanochemical process (Gross et al. 2017).
The famous physicist John Wheeler once remarked, “matter tells space-time how to curve and spacetime tells matter how to move”. In some sense this new discovery echoes the same tune; myosin controls they way cells change their shape and the shape of the cell dictates the distribution pattern of myosin. Such kind of mutual communications gives an organism its definite three-dimensional shape.
This new knowledge paves way to for development biologists to further explore how the three-dimensional shape of organisms arises through a mechanochemical process. However, extending the above described framework to study deformations of non-axisymmetric surfaces such as ellipsoidal shapes, still remains to be a challenge in the field.
References
Mietke, Alexander, Frank Jülicher, and Ivo F. Sbalzarini. "Self-organized shape dynamics of active surfaces." Proceedings of the National Academy of Sciences 116.1 (2019): 29-34.
Gross, Peter, K. Vijay Kumar, and Stephan W. Grill. "How active mechanics and regulatory biochemistry combine to form patterns in development." Annual review of biophysics 46 (2017): 337-356.
