Symmetry breaking and Biology

“During the latter half of the twentieth century, spontaneous symmetry breaking became important in biology. All organisms start off as highly symmetric entities, such as a single spherically symmetric cell. As the organism grows, this highly symmetric state becomes unstable, owing either to internal stresses and strains, or to influences from the environment. Enter spontaneous symmetry breaking: the organism will move to one of a set of possible stable, but less symmetric, states. In this way, the dynamics of stability and spontaneous symmetry breaking constrain the possible general forms that an organism may take during its growth. Which of the possible states the organism moves to at each stage can be controlled internally (by a nudge from the DNA, for example) or by the environment (through temperature or a chemical) … the domain of the “biologically possible” is highly constrained by dynamical stability, in which spontaneous symmetry breaking plays a key role.”

From —

Brading, K. (2003). Symmetry and symmetry breaking. In The Oxford Companion to the History of Modern Science. Oxford University Press.

Quick bibliography: Articles–classic and recent–about symmetry breaking in Biology.

*Chen, Q., Shi, J., Tao, Y., & Zernicka-Goetz, M. (2018). Tracing the origin of heterogeneity and symmetry breaking in the early mammalian embryo. Nature Communications, 9(1), 1819. [PDF] [Cited by]

A fundamental question in developmental and stem cell biology concerns the origin and nature of signals that initiate asymmetry leading to pattern formation and self-organization. Instead of having prominent pre-patterning determinants as present in model organisms (worms, sea urchin, frog), we propose that the mammalian embryo takes advantage of more subtle cues such as compartmentalized intracellular reactions that generate micro-scale inhomogeneity, which is gradually amplified over several cellular generations to drive pattern formation while keeping developmental plasticity. It is therefore possible that by making use of compartmentalized information followed by its amplification, mammalian embryos would follow general principle of development found in other organisms in which the spatial cue is more robustly presented.”

*Houston, D. W. (2017). Vertebrate axial patterning: From egg to asymmetry. Advances in Experimental Medicine and Biology, 953, 209-306. [PDF] [Cited by]

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal “organizer.” This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.”

*Shahbazi, M. N., & Zernicka-Goetz, M. (2018). Deconstructing and reconstructing the mouse and human early embryo. Nature Cell Biology, 20(8), 878-887. [PDF] [Cited by]

The emergence of form and function during mammalian embryogenesis is a complex process that involves multiple regulatory levels. The foundations of the body plan are laid throughout the first days of post-implantation development as embryonic stem cells undergo symmetry breaking and initiate lineage specification, in a process that coincides with a global morphological reorganization of the embryo. Here, we review experimental models and how they have shaped our current understanding of the post-implantation mammalian embryo.”

*Srivastava, P., & Kilian, K. A. (2019). Micro-engineered models of development using induced pluripotent stem cells. Frontiers in Bioengineering and Biotechnology, 7, 357. [PDF] [Cited by]

“During fetal development, embryonic cells are coaxed through a series of lineage choices which lead to the formation of the three germ layers and subsequently to all the cell types that are required to form an adult human body. Landmark cell fate decisions leading to symmetry breaking, establishment of the primitive streak and first tri-lineage differentiation happen after implantation, and therefore have been attributed to be a function of the embryo’s spatiotemporal 3D environment. These mechanical and geometric cues induce a cascade of signaling pathways leading to cell differentiation and orientation. Due to the physiological, ethical, and legal limitations of accessing an intact human embryo for functional studies, multiple in-vitro models have been developed to try and recapitulate the key milestones of mammalian embryogenesis using mouse embryos, or mouse and human embryonic stem cells. More recently, the development of induced pluripotent stem cells represents a cell source which is being explored to prepare a developmental model, owing to their genetic and functional similarities to embryonic stem cells. Here we review the use of micro-engineered cell culture materials as platforms to define the physical and geometric contributions during the cell fate defining process and to study the underlying pathways. This information has applications in various biomedical contexts including tissue engineering, stem cell therapy, and organoid cultures for disease modeling.”

*Zhang, H. T., & Hiiragi, T. (2018). Symmetry breaking in the mammalian embryo. Annual Review of Cell and Developmental Biology, 34, 405-426. [Cited by]

“We present an overview of symmetry breaking in early mammalian development as a continuous process from compaction to specification of the body axes. While earlier studies have focused on individual symmetry-breaking events, recent advances enable us to explore progressive symmetry breaking during early mammalian development. Although we primarily discuss embryonic development of the mouse, as it is the best-studied mammalian model system to date, we also highlight the shared and distinct aspects between different mammalian species. Finally, we discuss how insights gained from studying mammalian development can be generalized in light of self-organization principles. With this review, we hope to highlight new perspectives in studying symmetry breaking and self-organization in multicellular systems.”

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