Developmental Study of the Postcranial Skeleton of the Sea Lamprey (Petromyzon marinus)

The development of the neurocranial and branchial skeletons during larval skeletogenesis has largely been described in lampreys. However, the development and morphology of the postcranial skeleton are still poorly documented. We studied the development and the cellular morphology of the postcranial elements (i.e., arcualia, fin rays, notochordal cartilage) of the Sea Lamprey, Petromyzon marinus. Sequences of chondrogenesis were established using Alcian blue-stained whole specimens and stained cross-sections of 79 ammocoetes, 14 metamorphosing ammocoetes and 21 adults. The development of fin rays is size dependent and occurs during the ammocoete stage. Fin rays with first and second level bifurcation were observed. Two cellular morphologies were detected: stacked/cuboid (type I) and irregular/globular (type II). The general and cellular morphologies of the fin rays are unique; these elements should be considered as true rays rather than radials. Arcualia possess two centers of development and are composed of chondrocytes embedded in a territorial extracellular matrix; this cellular morphology is similar to lamprin-based cartilages. In all studied stages, an unexpected type II cartilage was detected along the notochord in the caudal region (notochordal cartilage). Our study is the first to fully describe the development and the cellular morphology of the postcranial elements in lampreys.