Macroscopic structure of the fin-rays and their annuli in pectoral and pelvic fins of Chinook Salmon, Oncorhynchus tshawytscha.

Light microscopy, enzyme clearing, and staining techniques were used to describe the structure of fin-rays in pectoral and dorsal fins of adult and juvenile chinook salmon,Oncorhynchus tshawytscha. In addition, several decalcification agents, fixatives, and staining methods were employed to demonstrate and determine the nature of the fin-ray annulus (yearly growth ring). Etched, transverse sections of fin-rays were examined by scanning electron microscopy (SEM). A description is provided of specific morphological regions along the length of fin-rays in both fins. Each ray consists of parallel halves, medial and lateral hemisegments, which diverge proximally and embrace underlying radial cartilages and are segmented distally. Fibrocartilage masses are associated with bone and hyaline cartilage in the proximal regions. There is evidence of periosteal, and perhaps endochondral, osteogenesis in the fin-rays. A novel fixation-staining method is described to depict the fin ray annulus in adult chinook salmon. Transverse sections of rays of adults, but not juveniles, demonstrate alternating narrow-dark and wide-light purple bands following prolonged decalcification/fixation in Bouin's fluid only with subsequent hematoxylin-eosin staining. The dark and light bands correspond to translucent (annuli) and opaque zones, respectively, in dried, unstained sections in transmitted light and in backscatter SEM. In conjunction with different light microscopic staining procedures, etched, transverse sections of fin-ray bone in SEM suggest that annuli are a manifestation of an ordered regionalization of the extracellular matrix of the bone in the fin-rays. J. Morphol. 239:297-320, 1999. © 1999 Wiley-Liss, Inc.