Chondrogenesis of the branchial cartilage during embryogenesis of the sea lamprey, Petromyzon marinus

The cartilaginous skeleton of the lamprey, unlike all other vertebrate cartilages, is non-collagenous. Previous studies have shown that there are two distinct types of cartilage present in lamprey characterized by their unique extracellular matrix (ECM) proteins. Most skeletal elements contain lamprin as the major structural protein. However, branchial cartilages contain a second, as yet unnamed non-collagenous protein. The objective of this study was to provide concise temporal and spatial characteristics of branchial chondrogenesis in embryonic lamprey. Morphological and ultrastructural aspects of branchial cartilage chondrogenesis in the embryonic sea lamprey, Petromyzon marinus, were examined using light and transmission electron microscopy to determine if this unique cartilage followed similar stages of development as the lamprin-based cartilage and the collagen-based cartilage of all other vertebrates. Branchial cartilage condensations first appeared in the mid-region of the third pharyngeal arch representing the first branchial arch at 13 days post-fertilization (pf). Unlike other vertebrate cartilage condensations, condensing cells in the lamprey branchial arches initially appeared as a one cell wide orderly stack, resembling a stack of coins, with cells closely apposed to each other. This ordered pattern was retained throughout development. By 20 days pf, the extracellular matrix surrounding the branchial chondrocytes was arranged in two layers: a subperichondrial layer of mainly collagen fibrils and a territorial layer of ECM protein in the form of beaded fibrils concentrically arranged around the chondrocytes. (Abstract shortened by UMI.).