[SY-J2] Amelogenesis: Nature’s 3D printing system for multi-scale laminates
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Ameloblasts are pencil-shaped cells with writing (secretory) tips at one end, which mass together in 2D sheets, standing on their writing ends. As they write (secrete protein scaffolds), they slide past one another and/or swap places, so that the secretions of the whole cell population form complex patterns in 3D space as the secreted material accumulates (amelogenesis is additive manufacturing). The secreted protein is the spatial template for the chemical reactions that result in mineralized dental enamel. By forming patterned microstructures, the cells impart exceptional fracture properties to the enamel.
How does each ameloblast know where and how fast to move at any instant during the writing process, so that it will contribute correctly to the desired pattern? There is no analogue of an external computer issuing instructions to all cells (the human engineering approach to 3D printing). Instead, we postulate that cells acquire the timing and positional information they need by sensing their evolving local strain environment: as the population moves, global shape changes map onto local strains around individual cells, and vice versa. Analyzing the mapping from local to global is a complex 3D problem in nonlinear wavelike cell motions. Its solution leads to deduction of cell “response functions” (rules that state cell actions for a given strain state) that will lead, when assigned to all cells, to correct patterns.
Having discovered the right response functions, we can account for much of the movement of pattern-forming ameloblasts in, e.g., the mouse incisor, including: the complex 3D trajectory of each cell during amelogenesis as layered structures are formed; switching on and off of secretion; the matching of the rate of enamel manufacture to the rate of eruption of the mouse incisor; and the appearance and spatial wavelength of “cohorts” of ameloblasts, which are an instance of spontaneous segmentation of a homogeneous population into a periodic structure.
How does each ameloblast know where and how fast to move at any instant during the writing process, so that it will contribute correctly to the desired pattern? There is no analogue of an external computer issuing instructions to all cells (the human engineering approach to 3D printing). Instead, we postulate that cells acquire the timing and positional information they need by sensing their evolving local strain environment: as the population moves, global shape changes map onto local strains around individual cells, and vice versa. Analyzing the mapping from local to global is a complex 3D problem in nonlinear wavelike cell motions. Its solution leads to deduction of cell “response functions” (rules that state cell actions for a given strain state) that will lead, when assigned to all cells, to correct patterns.
Having discovered the right response functions, we can account for much of the movement of pattern-forming ameloblasts in, e.g., the mouse incisor, including: the complex 3D trajectory of each cell during amelogenesis as layered structures are formed; switching on and off of secretion; the matching of the rate of enamel manufacture to the rate of eruption of the mouse incisor; and the appearance and spatial wavelength of “cohorts” of ameloblasts, which are an instance of spontaneous segmentation of a homogeneous population into a periodic structure.