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

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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.