In this study, we use three monoclonal antibodies that recognise antigens present in the central nervous system of the ascidian Ciona intestinalis to study regeneration and post-metamorphic development of the neural ganglion. We have also used bromodeoxyuridine labelling to study generation of the neuronal precursor cells. The first antibody, CiN 1, recognises all neurones in the ganglion, whereas the second, CiN 2, recognises only a subpopulation of the large cortical neurones. Western blotting studies show that CiN 2 recognises two membrane-bound glycoproteins of apparent Mr 129 and 100 kDa. CiN 1 is not reactive on Western blots. Immunocytochemical studies with these antibodies show that CiN 1-immunoreactive neurone-like cells are present at the site of regeneration as early as 5-7 days post-ablation, a sub-population of CiN 2-immunoreactive cells being detected by 9-12 days post-ablation. The third antibody, ECM 1, stains extracellular matrix components and recognises two diffuse bands on Western blots of whole-body and ganglion homogenates. The temporal and spatial pattern of appearance of CiN 1 and CiN 2 immunoreactivity both during post-metamorphic development and in regeneration occurs in the same sequence in both processes. Studies with bromodeoxyuridine show labelled nuclei in some neurones in the regenerating ganglion. Plausibly these originate from the dorsal strand, an epithelial tube that reforms by cell proliferation during the initial phases of regeneration. A second population of cells, the large cortical neurones, do not incorporate bromodeoxyuridine and thus must have been born prior to the onset of regeneration. This latter finding indicates a mechanism involving trans-differentiation of other cell types or differentiation of long-lived totipotent stem cells.

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