Chondrogenesis of the branchial skeleton in embryonic sea lamprey, Petromyzon marinus

This study provides concise temporal and spatial characteristics of branchi al chondrogenesis in embryonic sea lamprey, Petromyzon marinus, using high resolution light microscopy, transmission electron, and immunoelectron micr oscopy. Prechondrogenic condensations representing the first branchial arch appeared first in the mid-region of the third pharyngeal arch at 13 days p ost-fertilization (pf). Cartilage differentiation, defined by the presence of the unique, fibrillar, non-collagenous matrix protein characteristic of branchial cartilage, was first observed at 14 days pf. Development of lampr ey branchial cartilage appeared unusual compared to that in jawed fishes, i n that precartilage condensations appear as a one-cell wide orderly stack o f flattened cells that extend by the addition of one dorsal and one ventral condensation. Development of lamprey gill arches from three condensations that fuse to form a single skeletal element differs from the developing gil l arches of jawed fishes, where more than one skeletal element forms from a single condensation. The initial orderly arrangement of cells in the lampr ey branchial prechondrogenic condensations remains throughout development. Once chondrification of the condensations begins, the branchial arches star t to grow. Initially, growth occurs as a result of matrix secretion and cel l migration. Later in development, the arches grow mainly by cell prolifera tion and enlargement. This study defines the morphology and timing of lampr ey branchial chondrogenesis. Studies of lamprey chondrogenesis provide not only insight into the developmental biology of a unique non-collagenous car tilage in a primitive vertebrate but also into the general evolution of the skeletal system in vertebrates. Anat Rec 260: 252-267, 2000. (C) 2000 Wile y-Liss, Inc.