Ontogeny of the collar cord: neurulation in the hemichordate Saccoglossus kowalevskii

Abstract

The chordate body plan is characterized by a central notochord, a pharynx perforated by gill pores, and a dorsal central nervous system. Despite progress in recent years, the evolutionary origin of each of theses characters remains controversial. In the case of the nervous system, two contradictory hypotheses exist. In the first, the chordate nervous system is derived directly from a diffuse nerve net; whereas, the second proposes that a centralized nervous system is found in hemichordates and, therefore, predates chordate evolution. Here, we document the ontogeny of the collar cord of the enteropneust Saccoglossus kowalevskii using transmission electron microscopy and 3D-reconstruction based on completely serially sectioned stages. We demonstrate that the collar cord develops from a middorsal neural plate that is closed in a posterior to anterior direction. Transversely oriented ependymal cells possessing myofilaments mediate this morphogenetic process and surround the remnants of the neural canal in juveniles. A mid-dorsal glandular complex is present in the collar. The collar cord in juveniles is clearly separated into a dorsal saddle-like region of somata and a ventral neuropil. We characterize two cell types in the somata region, giant neurons and ependymal cells. Giant neurons connect via a peculiar cell junction that seems to function in intercellular communication. Synaptic junctions containing different vesicle types are present in the neuropil. These findings support the hypotheses that the collar cord constitutes a centralized element of the nervous system and that the morphogenetic process in the ontogeny of the collar cord is homologous to neurulation in chordates. Moreover, we suggest that these similarities are indicative of a close phylogenetic relationship between enteropneusts and chordates.