The morphogenesis and origin of the skeletal pattern of the tetrapod limb

This study identifies regularities in the evolution and development of the tetrapod limb that provide a new understanding of its origin and subsequent diversification.

The development of the tetrapod limb is a problem of cell differentiation, morphogenesis and pattern formation. The first visible stage of skeletal pattern formation is the condensation of pre-cartilage cells. Theoretical, comparative, and experimental studies suggest that the possible types of cell condensation patterns are limited. Cartilage foci may appear de-novo, they may segment a single element distally, or they may branch into separate foci of cellular aggregation. The iteration of these three patterns in space and time is one of the developmental mechanisms underlying skeletal pattern diversification. These regularities allow: a detailed revision of the homology of limb skeletal elements, a revaluation of some of the classic schemes of the origin of the tetrapod limb, and an analysis of limb diversity in terms of these rules of construction.

The adaptive diversity of the amphibian carpus and tarsus is highly ordered. Experimental and comparative studies suggest that the diversity of amphibian carpal and tarsal patterns is the result of the truncation of branching and segmentation events. This developmental truncation is caused by the perturbation of some of the parameters of pre-cartilage cell aggregation, and in particular, the size of the developing limb field.

Limb skeletal elements can be identified by the branching and segmentation events that produce them. The skeletal pattern of the tetrapod limb is the result of an axis of branching events. The distal portion of this axis, the "digital arch", produces the digits by branching from the corresponding distal carpal/tarsal elements. A phylogenetic analysis of the patterns of branching and segmentation events reveals that the axis of the fish fin is homologous to the digital arch. Contrary to previous schemes, this implies that the homologue of the fin axis does not run through any one of the digits. The axis bends anteriorly to follow the distal carpal/tarsal series. A new homology scheme, developed on the basis of the patterns of cellular condensation, allows an evaluation of some of the classic interpretations of limb evolution in terms of posture and function.