Developmental and Evolutionary Allometry of the Mammalian Limb Skeleton

Abstract

The variety of limb skeletal proportions enables a remarkable diversity of behaviors that include powered flight in bats and flipper-propelled swimming in whales using extremes of a range of homologous limb architectures. Even within human limbs, bone lengths span more than an order of magnitude from the short finger and toe bones to the long arm and leg bones. Yet all of this diversity arises from embryonic skeletal elements that are each a very similar size at formation. In this review article, I survey what is and is not yet known of the development and evolution of skeletal proportion at multiple hierarchical levels of biological organization. These include the cellular parameters of skeletal elongation in the cartilage growth plate, genes associated with differential growth, and putative gene regulatory mechanisms that would allow both covariant and independent evolution of the forelimbs and hindlimbs and of individual limb segments. Although the genetic mechanisms that shape skeletal proportion are still largely unknown, and most of what is known is limited to mammals, it is becoming increasingly apparent that the diversity of bone lengths is an emergent property of a complex system that controls elongation of individual skeletal elements using a genetic toolkit shared by all.

Fig. 1

Cellular parameters that contribute to differential growth in space and over time. Left, a rapidly elongating growth plate, such as the proximal tibia, in a young animal. Right, an older tibia or a slower elongating young growth plate, such as the distal phalanx. Resting zone (RZ), proliferative zone (PZ), and hypertrophic zone (HZ) chondrocytes are denoted in each. Dots in resting and proliferative chondrocytes indicate cells in S-phase that would be marked with BrdU, representative of relative proliferation, which is faster in more rapidly elongating skeletal elements. Note the larger number of cells, greater fraction of cells in S-phase, and larger size of hypertrophic chondrocytes in the more rapidly elongating growth plate.

Fig. 2

Potential scenarios for enhancer configurations that would enable integrated or independent limb development and evolution. Arrows depict gene transcription start sides. Boxes depict enhancer sequence sites that are annotated as forelimb (FL) or hindlimb (HL) restricted or pleiotropic for both limbs (limb). The hexagon and circle in E depict transcription factors with forelimb or hindlimb specific expression that are each able to bind to a pleiotropic limb enhancer.