ROLIAN, C; Harvard University: Comparative growth plate kinetics in rodents: insights into the evolution and development of limb length allometry.

The primate appendicular skeleton shows significant diversity in the proportions of fore- and hindlimbs. Humans have proportionately longer hindlimbs (intermembral index = 0.70), quadrupeds have limbs approximately equal in length (IMI=0.95), and brachiating apes have the longest relative forelimb length (IMI >1.20). These differences are thought to have evolved in response to selective pressures relating to the energetics and biomechanics of locomotion. However, the developmental mechanisms that selection acts upon to generate these interspecific differences have been poorly studied. For example, differences may be due to changes in rates affecting endochondral bone growth, or they may result from variation in the cellular properties of physeal growth plates, such as the size and number of proliferative and hypertrophic chondrocytes. Here I present a rodent model to address these hypotheses, using cross-sectional ontogenetic series of two species, the house mouse (Mus musculus, n=14, IMI=0.73) and Mongolian gerbil (Meriones unguiculatus, n=16, IMI=0.63). In each individual, left limbs were used to determine bone growth rates; and right limbs were thin-sectioned for analysis of cellular properties in the growth plates of the proximal humerus, distal radius, distal femur, and tibia. Results indicate that differences in rates of chondrocyte proliferation are significantly correlated with interspecific differences in bone growth rates and interlimb proportions. Cellular properties of the growth plates do not contribute significantly to differences in growth or limb proportions between species or ontogenetic stages. These findings are discussed in the context of the developmental genetics and evolution of primate limb proportions.