The specific growth rates of tissues: a review and a re-evaluation.

The first objective of this review and re-evaluation is to present a brief history of efforts to mathematically model the growth of tissues. The second objective is to place this historical material in a current perspective where it may be of help in future research. The overall objective is to look backward in order to see ways forward. It is noted that two distinct methods of imaging or modeling the growth of an organism were inspired over 70 years ago by Thompson's (1915, "XXVII Morphology and Mathematics," Trans. - R. Soc. Edinbrgh, 50, pp. 857-895; 1942, On Growth and Form, Cambridge University Press, Cambridge, UK) method of coordinate transformations to study the growth and form of organisms. One is based on the solid mechanics concept of the deformation of an object, and the other is based on the fluid mechanics concept of the velocity field of a fluid. The solid mechanics model is called the distributed continuous growth (DCG) model by Skalak (1981, "Growth as a Finite Displacement Field," Proceedings of the IUTAM Symposium on Finite Elasticity, D. E. Carlson and R. T. Shield, eds., Nijhoff, The Hague, pp. 348-355) and Skalak et al. (1982, "Analytical Description of Growth," J. Theor. Biol., 94, pp. 555-577), and the fluid mechanics model is called the graphical growth velocity field representation (GVFR) by Cowin (2010, "Continuum Kinematical Modeling of Mass Increasing Biological Growth," Int. J. Eng. Sci., 48, pp. 1137-1145). The GVFR is a minimum or simple model based only on the assumption that a velocity field may be used effectively to illustrate experimental results concerning the temporal evolution of the size and shape of the organism that reveals the centers of growth and growth gradients first described by Huxley (1924, "Constant Differential Growth-Ratios and Their Significance," Nature (London), 114, pp. 895-896; 1972, Problems of Relative Growth, 2nd ed., L. MacVeagh, ed., Dover, New York). It is the method with an independent future that some earlier writers considered as an aspect of the DCG model. The development of the DCG hypothesis and the mixture theory models into models for the predicted growth of an organism is taking longer because these models are complicated and the development and refinement of the basic concepts are slower.