FRANSSEN, RA; Univ. of Chicago: Chondrogenesis in the developing limbs of urodeles Ambystoma and Desmognathus

Salamanders are infrequently mentioned in models of tetrapod limb formation as their development varies considerably from that of amniotes. However, urodeles provide an opportunity to study two modes of limb formation within a single lineage – development through a larval stage (Ambystoma mexicanum), and via direct development (Desmognathus aeneus). Both modes result in a limb that is morphologically indistinct from an amniote limb. Understanding the variation in salamanders will contribute to the tetrapod model of limb development. Developmental series of A. mexicanum and D. aeneus have been stained using Type II collagen immunochemistry and Alcian Blue techniques to reveal patterns of limb formation. Ambystoma mexicanum digits extend from the limb as individual buds. For each digit, Type II collagen and proteoglycan (visualized with Alcian Blue) secretion occurs almost simultaneously with mesenchyme condensation. Conversely, D. aeneus digits develop within an amniote-like paddle. The mesenchymal template for all digits forms first. Then, within the resulting paddle, the cartilage model for each digit is created. Despite these significant differences in limb formation, the timing of digit development is staggered in both species. Ambystoma mexicanum digits 1 and 2 bud first, followed by digit 3. In D. aeneus, rather than all digits developing simultaneously, digits 2 and 3 form, followed by digit 1. Within each species, Type II collagen expression patterns are similar to those of proteoglycans. Both clearly show distal structures (phalanges and metacarpals) forming before more proximal structures (carpals). This observation is contrary to the proximal-to-distal developmental pattern seen in other tetrapods and fishes and may be unique to urodeles.