The Development of the Teleost Fin and Implications for Our Understanding of Tetrapod Limb Evolution

Mechanism of pectoral fin outgrowth in zebrafish development

“…S1), which explains fin-to-limb transition in evolution as a heterochronic shift of the timing of AER-to-AF transition (Thorogood, 1991;Sordino et al, 1995;Freitas et al, 2007;Yano and Tamura, 2012). Our data from prolonged experimental AER signal exposure in the fin bud support the clock model.…”

“…In teleost paired-fin development, the epithelial tissue covering the fin ray region derives from an elongated epidermal structure, the apical fold (AF), which does not form in developing limbs (Dane and Tucker, 1985;Thorogood, 1991). In early paired-fin development, the apical ectodermal ridge (AER) rims the distal edge of the fin bud along the dorsoventral boundary and promotes cell proliferation and mesenchyme outgrowth, which differentiates into endoskeletal elements as seen in limb development (Grandel and SchulteMerker, 1998;Grandel et al, 2000;Kawakami et al, 2003).…”

“…Nevertheless, one model of limb evolution, the clock model (supplementary material Fig. S1), suggests that a heterochronic shift of the transformation from AER to AF determined the evolutional fin-to-limb transition (Thorogood, 1991). In this model, the AER-AF transformation occurs early in teleost fin development, later in sarcopterygian fin development, and never in tetrapod limb development.…”

Next generation limb development and evolution: old questions, new perspectives

“…During fin bud development in pairedfin fishes, an AER develops and promotes cell proliferation and the outgrowth of mesenchyme, which differentiates into endoskeletal elements as seen in limb development (Grandel and SchulteMerker, 1998). In contrast to the AER in tetrapods, which is maintained during limb bud outgrowth, the AER in fish fin buds differentiates into an extended structure, the apical ectoderm fold (AEF), which lifts off the mesenchyme at an early stage and elongates to give rise to the dermoskeleton (Thorogood, 1991). Studies have shown that the repeated experimental removal of the AEF from zebrafish fin buds results in increased mesenchymal cell proliferation and excessive elongation of the mesenchyme, suggesting that prolonged exposure to AER signals induces distal elongation of mesenchyme and the endoskeleton (Yano et al, 2012).…”

“…Lobe-finned fish, which share a common ancestor with tetrapods, have a similar but more elaborate endoskeleton ( Fig. 4A,B; Thorogood, 1991). Although palaeontological data suggest progressive evolution of the bony elements of fins towards limb skeletal elements, it has proven difficult to determine by molecular analysis whether the tetrapod distal limb skeletal structures (carpals and digits) are homologous to or derived from teleost radials, or whether they arose de novo.…”