LINEAR GROWTH IN THE RABBIT IS CONTINUOUS, NOT SALTATORY

A recent report in Science suggests that human growth occurs in brief bursts, up to 1.65 cm in a single day, separated by extended periods o f stasis, lasting up to 63 days. Thus, the organism is proposed to alt ernate between two states, one with a growth velocity of zero, the oth er with a mean annualized growth velocity greater than 350 cm/yr. Thes e observations, if correct, suggest the existence of a previously unsu spected hormonal mechanism capable of abruptly switching growth plate cell division on and off and of synchronizing cellular growth not only throughout the growth plate, but presumably throughout all the growth plates in the organism. However, the experimental assessment of short -term growth velocity in the human faces the formidable obstacle of a technical error of measurement that exceeds the mean daily growth rate . Accordingly, we tested the saltatory growth hypothesis by measuring proximal tibial growth in the rabbit, a model in which daily growth ra te could be measured more than 15 times more accurately than in the hu man. The model of saltation and stasis predicts a majority of daily gr owth velocities clustered around zero, and a minority of high growth v elocities, that is, a bimodal distribution. The frequency distribution of observed daily growth velocities instead approximated a single Gau ssian distribution, indicating continuous growth. We conclude that lin ear growth, in the most accurate mammalian system yet studied, is cont inuous, not saltatory.