Macroscopic structure of the fin-hays and their annuli in pectoral and pelvic fins of chinook salmon, Oncorhynchus tshawytscha

Light microscopy, enzyme clearing, and staining techniques were used to des cribe the structure of fin-rays in pectoral and dorsal fins of adult and ju venile chinook salmon, Oncorhynchus tshawytscha. In addition, several decal cification agents, fixatives, and staining methods were employed to demonst rate 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 regio ns along the length of fin-rays in both fins. Each ray consists of parallel halves, medial and lateral hemisegments, which diverge proximally and embr ace underlying radial cartilages and are segmented distally. Fibrocartilage masses are associated with bone and hyaline cartilage in the proximal regi ons. There is evidence of periosteal, and perhaps endochondral, osteogenesi s 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-li ght purple bands following prolonged decalcification/fixation in Bouin's fl uid only with subsequent hematoxylin-eosin staining. The dark and light ban ds correspond to translucent (annuli) and opaque zones, respectively, in dr ied, unstained sections in transmitted light and in backscatter SEM. In con junction with different light microscopic staining procedures, etched, tran sverse sections of fin-ray bone in SEM suggest that annuli are a manifestat ion of an ordered regionalization of the extracellular matrix of the bone i n the fin-rays. (C) 1999 Wiley-Liss, Inc.