The fine structure of the developing pelvic fin dermal skeleton in the trout Salmo gairdneri

Abstract

The morphogenetic and ultrastructural features of the dermal skeleton in the pelvic fin bud of a teleost, the rainbow trout Salmo gairdneri, have been examined by light and electron microscopy. The principal structural components observed are lepidotrichia and actinotrichia. Lepidotrichia consist of two parallel and symmetrical bony demirays that form jointed segments within the fin. The demirays calcify in a proximodistal direction within the extracellular collagen network of the basal lamella belonging to the epidermal-dermal interface of the fin. Needle- and plate-like particles of a solid mineral phase appear to be associated with the collagen fibrils and with a fine, granular, interfibrillar material central to the demirays. Cellular processes and membrane-bound vesicles are absent from the regions of calcification. During fin growth, the bony, acellular lepidotrichia are separated from the epidermal-dermal interface by infiltrating mesenchymal cells in proximal fin regions; in distal areas, the lepidotrichia remain within the basal lamella. The actinotrichia are extensive unmineralized rods of elastoidin that occupy the distal margin of the fin and precede the differentiation of lepidotrichia. Once the lepidotrichia form, actinotrichia lie preferentially between their demirays. In some instances, structural interactions are suggested between actinotrichia and lepidotrichia. Considerations of embryologic and structural features of fin components fail to support the hypothesis that individual segments of lepidotrichia are modified scales in all fish.

Regeneration of Fin Rays in Teleosts: A Histochemical, Radioautographic, and Ultrastructural Study.

“…However, a later report stated that, during embryonic development, the lepidotrichium arises without an endochondral sequence (GERAUDIE and LANDIS, 1982); this finding, associated with the fact that cartilage is absent in lepidotrichia during phylogenetic evolution, argues strongly against the cartilaginous nature of lepidotrichia. Analyzing the type of collagen present in regenerating lepidotrichia and actinotrichia was beyond the scope of the present study.…”

Differentiated skeletal cells contribute to blastema formation during zebrafish fin regeneration

“…This expression is, at first glance, surprising as it is normally associated with cartilage differentiation (Yan et al, 2002;Grandel and Schulte-Merker, 1998). However, the caudal fin rays are dermal bone (Geraudie and Landis, 1982;Santamaria et al, 1992) and are an example of a skeletal tissue in which chondrocyte markers, such as sox9a, are expressed (Smith et al, 2006). It is possible that the expression of sox9a in the intraray mesenchyme may indicate another source of skeletal progenitor cells that could contribute to regeneration; a hypothesis that we have not explored in this work.…”

Structure and function of the median finfold in larval teleosts

“…Elastoidin is thought to have mechanical properties intermediate between those of collagen and elastin. The ultrastructure of actinotrichia shows a cross-striation of about 65nm, similar to collagen (Géraudie and Landis, 1982). During ontogeny, individual actinotrichia increase both in length and in width as a result of apposition of fibrils (Géraudie, 1984).…”