A quantitative comparison of the growth of the embryonic chick tibiotarsus in vivo and in vitro

An apparently phylogeny-independent method for identification of skeletal (longitudinal) growth cessation (skeletal maturity) in birds

Identification of skeletal maturity is of interest as a measure of species longevity and for identifying its maximal achievable size/mass. Measurement of age on the basis of growth arrest/accentuation lines (LAGs) and external fundamental system (EFS) evidences cessation or at least extreme slowing of circumferential bone growth. Such intramembranous (periosteal)-derived growth is distinct from the endochondral ossification responsible for longitudinal growth and therefore achievable organismal size/mass. As subchondral trans-cortical channels are required for nourishment, their loss should identify cessation of longitudinal growth. Predicated on phylogenetic bracketing/relationship and shared anatomical structures with and without growth plates, birds represent an appropriate model for the study of dinosaur ontogeny. Persistence of trans-cortical subchondral channels in the long bones of birds are examined at 100-200X magnification and correlated with bone length. Trans-cortical channels are present in subchondral articular surfaces, but disappear when terminal longitudinal growth is achieved. Articular vascular channels perforating articular surfaces from within the bone are detected. Loss of penetrating channels is interpreted as evidence of skeletal growth cessation, identifying the longitudinal bone length at which skeletal growth cessation has been achieved. The current study provides evidence that maximal bone length does correlate with endochondral cessation growth. Failure of circumferential growth reduction/cessation to correlate with bone length may be related to lack of synchronicity of periosteal-based circumferential growth with the endochondral process responsible for bone lengthening. Loss/closure of articular vascular channels may be the most reliable measure of a bird's achievement of maximal growth (indicating cessation of appendicular element lengthening). This article is protected by copyright. All rights reserved.