CHONDROGENESIS OF A NON-COLLAGEN-BASED CARTILAGE IN THE SEA LAMPREY, PETROMYZON-MARINUS

Chondrogenesis of the trabeculae, non-collagen-based cartilages in pro larval stages of the sea lamprey, Petromyzon marinus, was examined by light and electron microscopy. Chondrogenesis of the trabecular cartil ages in prolarval lampreys commenced with the formation of mesenchymal condensations. Two peaks in mesenchymal cell density occurred, one pr ior to condensation formation and a second immediately before cartilag e differentiation. The possibility of inductive influences by epitheli o-mesenchymal interactions on the initiation of chondrogenesis is disc ussed. Bilateral condensations first appeared by day 17 post fertiliza tion ventromedial to the eyes in a band of tightly packed yolk-laden m esenchymal cells that represent neural crest derived tissue. Cartilage differentiation occurred by day 19 post fertilization and was indicat ed by the presence of matrix-synthesizing organelles and the first ult rastructural appearance in the extracellular matrix of lamprin, a stru ctural protein unique to lamprey cartilage. Lamprin was initially depo sited as discrete 15- to 40-nm globules. Subsequently, lamprin appeare d as fibrils aggregated into branching and parallel arrays arranged in pericellular, territorial. and interterritorial zones. Lengthening of the trabecular cartilages was primarily by appositional growth at the rostral end. The timing of the appearance of trabecular cartilages in prolarval stages likely reflects the functional importance of these s tructures for supporting the brain as the lamprey initiates burrowing behaviour.