Neurogenesis in sea urchin embryos and the diversity of deuterostome neurogenic mechanisms

Abstract

A single origin to the diverse mechanisms of metazoan neurogenesis is suggested by the involvement of common signaling components and similar classes of transcription factors. However, in many forms we lack details of where neurons arise, patterns of cell division, and specific differentiation pathway components. The sea urchin larval nervous system is comprised of an apical organ, which develops from neuroepithelium and functions as a central nervous system, and peripheral neurons, which differentiate in the ciliary band and project axons to the apical organ. To reveal developmental mechanisms of neurogenesis in this basal deuterostome, we developed antibodies to SoxC, SoxB2, ELAV, and Brn1/2/4 and used neurons that develop at specific locations to establish a timeline for neurogenesis. Neural progenitors express, in turn, SoxB2, SoxC, and Brn1/2/4 before projecting neurites and expressing ELAV and SynB. Using pulse-chase labeling of cells with a thymidine analogue to identify cells in S-phase, we establish that neurons identified by location are in their last mitotic cycle at the time of hatching, and S-phase is coincident with expression of SoxC. The number of cells expressing SoxC and differentiating as neurons is reduced in embryos injected with anti-sense morpholino oligonucleotides to SoxC, SoxB2, or Six3. Injection of RNA encoding SoxC in eggs does not enhance neurogenesis. In addition, inhibition of FGF receptors (SU5402) or a morpholino to FGFR1 reduces expression of SoxC. These data indicate there are common features of neurogenesis in deuterostomes, and that urchins employ developmental mechanisms that are distinct from other ambulacraria.