Transient cartilages and interdigital vasculature organization in the embryonic duck foot.

The evolution of webbed limbs represents a recurrent adaptation to aquatic life in tetrapods, facilitated by the dynamics of embryonic digit development. Digits form at the distal tip of the limb bud as cartilage condensations surrounded by mesenchymal cells, a process described as a Turing-type reaction between activator and inhibitor molecules. In species with free digits (lacking interdigital membranes), extensive programmed cell death occurs in the interdigital tissue, whereas this process is reduced in species with webbed limbs. Other factors potentially involved in the development of webbed limbs are fluctuations in oxygen tension, production of reactive oxygen species, and vascular remodelling, all of which have been documented in the embryonic interdigital tissue. Whether and how these phenomena are interrelated to the regression or persistence of the interdigital membrane remains poorly understood. On the basis of the analysis of skeletal and vascular development in ducks, using whole-mount immunofluorescence, we identified transient populations of chondrocytes that appear to influence the maintenance of interdigital vasculature and permit the continued growth of the membrane. Finally, we propose that current Turing models can explain the formation of these transient chondrocytes in ducks.