Early pectoral fin development and morphogenesis of the apical ectodermal ridge in the killifish, Aphyosemion scheeli.

Using scanning electron microscopy it has been found that in the killifish the distal epithelium of the pectoral fin bud forms a morphologically distinct apical ectodermal ridge along its entire distal margin. This ridge is a relatively more prominent feature than its counterpart in various tetrapod systems. At a later stage the ridge becomes folded internally and elongates distally marking the transformation of the ridge into a fin fold. By continued distal growth the fin fold develops into a semicircular swimming-paddle. Collagenous rays or actinotrichia differentiate in a highly ordered arrangement close to the epithelial basement membrane within the space enclosed by this elongating fin fold and apparently support the early swimming-paddle. During the ridge-to-fold transition the fin bud displays a dorsal deflection of approximately 45 degrees and a caudalward rotation about the axis of the fin itself. This change in orientation results in close apposition of the developing fin structure to the flank.