Alikhani Ma,b, Sangsuwon Ca, Teixeira CCc
Figure 1. Cells adapt to their surrounding environment. Cells respond to environmental changes by continuously receiving signals through various cell surface receptors. These signals activate specific proteins inside the cells, called transcription factors. Activated transcription factors move to the nucleus, attach to the DNA, and unravel only a tiny segment of the DNA to start the process of RNA synthesis (transcription), followed by protein synthesis (translation) allowing cells to adapt. Some of those proteins may be released into the extracellular space and can, in turn, act as signals to neighboring cells.
Figure 2: Laws of physics determine the early stages of form development. As cells proliferate and their number increase, the form assumes a sphere or ball shape (A), not a cube (B) nor a pyramid shape (C). A sphere has a minimal surface area for a given body and requires less energy.
Figure 3: Similarity in the sorting behaviors of biological materials (cells) and non-biological materials (liquids). A mixture of two different cells (orange and purple spheres) with different affinities (different number or types of adhesion molecules) spontaneously sorts itself into two distinct cell populations (A), just like a mixture of droplets of two liquids (yellow and blue) with different surface tension (B) sorts itself into a liquid surrounded by another.
Figure 4: Migration of cells plays a significant role in creating the body’s general form during development. Early during development, the embryo adopts the form of a disk with two cell lawyers, ectoderm and endoderm. The migration of a group of cells from the ectoderm toward the space between the endoderm and ectoderm gives rise to the mesoderm layer. This migration changes the embryo form from a 2-layer (A) to a 3-layer (B) structure. Each of these lawyers later give rise to different body structures with different shapes and functions.
Figure 5: Cells communicated through cell-to-cell interactions and protein signaling. Interaction between the cells evolves as the number of cells increases. In addition to the cell-to-cell direct interactions, protein production increases. Released proteins signal through cell membrane receptors, to activate transcription factors and produce new proteins. This cascade of events produces a self-evolving environment with increased complexity and structural organization.
Figure 6: Gradients created by diffusion of proteins modulate cell activity. As soon as soluble proteins are released into the extracellular space, they diffuse. This diffusion produces areas with different concentrations of proteins or gradients (A). The response of cells to different concentrations of the same proteins is distinct, which causes their differentiation and activity to change affecting the final macro-state. At the same time, many proteins are produced at different locations creating overlapping gradients that interact with each other (B). These proteins may have synergistic or antagonistic effects on signaling, further modulating the activity of the cells and their differentiation.
Figure 7: Apoptosis or programmed cell death helps shaping the hand. Apoptosis plays a significant role in morphogenesis during development. For example, during limb development, apoptosis of the cells in the area between fingers allows the separation of digits into individual structures, giving rise to the characteristic hand shape.
Figure 8: Extracellular matrix production plays a crucial role defining the overall form. Proliferation increases the number of cells until local signaling instructs cells to differentiate into a particular cell type. As a group of cells differentiates, they produce a specialized extracellular matrix around themselves composed of different structural proteins that adopt a specific shape, participating in the overall form of the individual.
Figure 9: Interaction of Genetics, Entropy, and Emergence. At each stage of development, proteins can change the probability of the formation of one micro-state over the others. In this schematic, balls (units) collected in the basket randomly take one path leading to creation of a form (forms A through D). Protein synthesis changed the probability of form C occurring, and blocked the formation of form D. These changes in micro-state probability constantly evolve depending on where and how many proteins are produced. Under these conditions, complex forms are reproducible without a “DNA map” of a pre-determined form.
Figure 10: Sequential creation of a dynamic form. The first stage of shape formation is patterning and morphogenesis. In this stage, the general form of the body is defined in the protected environment of the womb. The second stage is a coordinated increase in the size of the form after birth, also considered growth and development. The third stage that continues throughout the life of the organism is the maturation of form.
