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LINEAR GROWTH IN THE RABBIT IS CONTINUOUS, NOT SALTATORY

A recent report (Science 258:801) suggests that human growth occurs in brief bursts separated by extended periods of stasis. We tested this hypothesis in an animal model in which growth rate could be measured with greater accuracy than that achieved in the human. Metal pins were inserted into the bone immediately adjacent to the proximal tibial and distal femoral growth plates of 6-week old rabbits. Growth rates were determined by measuring the change in distance between pins on serial radiographs taken daily for 10 days. All measurements were made in quadruplicate by an observer blinded as to the day and animal number. The error of measurement was small (0.05 mm/d) compared to the mean growth rate (0.27 mm/d). The individual growth curves revealed continuous growth. To analyze statistically the pattern of growth, we examined the frequency distribution of daily growth velocities. The model of saltation and stasis predicts a majority of daily growth velocities clustered around zero, and a minority of high daily growth velocities. The pattern we observed was significantly different (p<0.001), showing instead a single Gaussian distribution about an intermediate growth velocity, indicating continuous growth. We conclude that linear growth in the rabbit is continuous, not saltatory.