Process heterochronies in endochondral ossification.

Heterochrony, evolutionary changes in developmental rates and timing, is a key concept in the construction of a synthesis of development and evolution. Heterochronic changes in vertebrate evolution have traditionally been identified through plesiomorphic-apomorphic comparisons of bone growth. This methodological framework assumes that observed heterochronies are the outcome of dissociations of developmental processes in time. Recent findings of non-heterochronic developmental changes underlying morphological heterochrony invalidate this assumption. In this paper, a function for bone growth (at the organ level) has been mathematically deduced from the underlying developmental mechanisms. The temporal domain of the model spans from the time at maximum growth rate, after the formation of growth plates, to the time at atrophy of the proliferating stratum of cells. Three organizational levels were considered: (a) cell kinetics of endochondral ossification, (b) variation of bone growth rates and (c) variation of accumulated bone growth with increasing age. This quantitative model provides an excellent tool to deal with the problem of the developmental basis of morphological change. I have modelled potential evolutionary changes on the system at different levels of biological organization. This new framework involves an epistemological shift in heterochronic analysis from a pattern-oriented inductive way to a process-oriented deductive way. The analysis of the relationships between the evolutionary alterations of endochondral ossification and the morphological expression of these changes reveals that observed pattern heterochronies can be the outcome of different process heterochronies. Moreover, I discuss at length the heteroposic hypothesis, that evolutionary changes in the tight regulation of the amount of protein synthesized by a cell population during development would underlie acceleration or deceleration in cases of evolutionary changes in the initial number of proliferating cells at growth plates. Future research on the genetic basis of process heterochronies and heteroposies will complete our understanding of these evolutionary phenomena.