Banding Pattern and Fibrillogenesis of Ceratotrichia in Shark Fins

Study by light microscopy, scanning electron microscopy and transmission electron microscopy of the distribution, structure and growth of ceratotrichia in the anterodorsal fin of a lemon shark, Negaprion breuirostris, and in the tailfin of a nurse shark, Ginglymostoma cirratum, shows that the ceratotrichia are large collagen fibers which develop in bilateral rows within the dermis. Surrounding each ceratotrichium is a layer of peritrichial fibroblasts containing secretory vesicles, which appear to be the source of matrix constituents. The peritrichial matrix contains bundles of fine, unbanded collagen fibrils as well as larger, banded fibrils like those in the matrix of ordinary connective tissue. The structure of the peritrichial fibroblasts and of the subjacent peritrichial matrix is the same as that of the fibroblasts and matrix of the conventional connective tissue throughout the fin dermis. Ceratotrichia grow by apposition of collagen fibrils from the peritrichial matrix. In cross section the ceratotrichia appear layered, evidently because of close packing of constituent fibrils in lamellae. In longitudinal section the ceratotrichia exhibit the conventional a, b, c, dand e bands of collagen. The e bands show two distinct subbands, and the b bands three subbands. Periodicity of the banding pattern is approximately 640 A like that of conventional collagen fibrils. Beyond their endoskeletal fin rays the fins of sharks are supported bilaterally by large dermal fibers called ceratotrichia (Goodrich, '04). I t is clear that these fibers are homologous with the fibrous rays called actinotrichia, which are localized a t the distal ends of bony rays in teleosts and also within the adipose fins of salmonids, siluroids, characins, synodontids and certain other fish (Brohl, '09; Garrault, '36; Bear, '52; Fitton Jackson, '62, '68; Kemp and Park, '70; Bouvet, '74; Geraudie, '77). I t is equally clear that ceratotrichia and actinotrichia are members of the collagen class of scleroproteins (Bear, '52; Ysuchiya and Nomura, '53; Gross and Dumsha, '58; Gross, Dumsha and Glazer, '58; Sastry and Ramachandran, '65; Kimura and Kubota, '66; Dubey, '67). The name elastoidin was given to the protein of shark fibers by Krukenberg (1886) because it had properties he considered intermediate between those of collagen and elastin. He found that elastoidin fibers, unlike mammalian collagen, contained a significant amount of sulfur and did not yield gelatin when boiled with water. Unlike elastin, the elastoidin fibers were not digested by pancreatic trypsin. Elastoidin fibers (ceratrotrichia) vary with the size of a shark. They may be up to 30 cm long and several millimeters wide in large sharks (Schmidt, '24; Faurk-Fremiet, '36a,b; Damodaran et al., '56). They are shiny and transparent, and may be yellowish or brownish in contrast to the white ordinary connective tissue surrounding them. They are spindle-shaped, tapering to fine points a t either end. Their surfaces show longitudinal striations. Cross sections show peripheral grooves and layers of concentric lamellae. Evidence from X-ray diffraction, chemical analysis and electron microscopy has established the collagenous nature of ceratotrichia. They yield wide-angle or low-angle diffraction patterns characteristic of collagen (Champetier and Faure-Fremiet, '37; Bear, '52; McGavin, '62). Engeland and Bastian ('38) identified glycine, alanine, serine and hydroxyproline in an acid hydrolysate of elas187 J. MORPH., 154: 187-204.