Meeting Abstract
Gnathstome paired appendages consist of two distinct skeletal compartments: a proximal endoskeleton which develops via cartilaginous condensations, and a distal fin-fold containing dermal rays that form via direct membranous ossification. These different modes of development, and the loss of the appendage dermoskeleton during the fin-to-limb transition, fueled the hypothesis that these skeletons form as separate developmental modules with their own evolutionary histories. This hypothesis was further supported by evidence that dermal rays might be neural crest derived, unlike the lateral plate mesoderm (LPM) derived endoskeleton. However, recent work has demonstrated that both skeletons are derived from the same progenitor LPM population. To further investigate the emergence of distinct skeletogenic cell populations from a common developmental origin, we herein describe the earliest events of fin development in a basal actinopterygian, the American paddlefish Polyodon spathula. In paddlefish, molecularly distinct compartments are established by the earliest fin-bud stages, presaging the ultimate arrangement of fin radials and actinotrichia in mature larvae. In order to determine the histological context for these events, we compare plastic-embedded ultratome cross-sections through the pectoral fin with the labeling domains of the prechondrogenic marker Sox9 and the fin-fold actinotrichia marker And1. These data reveal cryptic molecular boundaries for nascent endoskeletal and dermoskeletal compartments. Additionally, we describe septations within the fin bud mesenchyme, which correspond with the remarkably precocious expression of And1 in the same region. We use these new observations to test the hypotheses of current models of appendage differentiation and skeletogenesis.
