Pathfinding by zebrafish motoneurons in the absence of normal pioneer axons.

Individually identified primary motoneurons of the zebrafish embryo pioneer cell-specific peripheral motor nerves. Later, the growth cones of secondary motoneurons extend along pathways pioneered by primary motor axons. To learn whether primary motor axons are required for pathway navigation by secondary motoneurons, we ablated primary motoneurons and examined subsequent pathfinding by the growth cones of secondary motoneurons. We found that ablation of the primary motoneuron that pioneers the ventral nerve delayed ventral nerve formation, but a normal-appearing nerve eventually formed. Therefore, the secondary motoneurons that extend axons in the ventral nerve were able to pioneer that pathway in the absence of the pathway-specific primary motoneuron. In contrast, in the absence of the primary motoneuron that normally pioneers the dorsal nerve, secondary motoneurons did not pioneer a nerve in the normal location, instead they formed dorsal nerves in an atypical position. This difference in the ability of these two groups of motoneurons to pioneer their normal pathways suggests that the guidance rules followed by their growth cones may be very different. Furthermore, the observation that the atypical dorsal nerves formed in a consistent incorrect location suggests that the growth cones of the secondary motoneurons that extend dorsally make hierarchical pathway choices.

The spt-1 mutation alters segmental arrangement and axonal development of identified neurons in the spinal cord of the embryonic zebrafish.

We examined the arrangement and development of identified neurons in zebrafish embryos homozygous for the mutation spt-1, which acts autonomously and specifically to alter the development of precursors of trunk segmented mesoderm, resulting in muscle-deficient myotomes. We found that the mutation alters the morphology, number, and arrangement of identified motoneurons. By transplanting identified motoneurons between wild-type and mutant embryos, we found that the effect of the mutation was nonautonomous. We suggest that the segmental arrangement and proper axonal development of motoneurons may result from interactions with segmented mesoderm.

An identified motoneuron with variable fates in embryonic zebrafish.

Every trunk hemisegment of the zebrafish is innervated by 3 identified primary motoneurons whose development can be observed directly in living embryos. In this paper, we describe another identified neuron that is part of this system. Unlike the other primary motoneurons which are present in all trunk hemisegments, this cell is present in slightly less than half of the trunk hemisegments. Additionally, this cell has at least 2 different fates: it may become a primary motoneuron and arborize in an exclusive muscle territory, or it may die during embryonic development. We have named this cell VaP, for variable primary. We show that the presence of VaP does not affect the early development of the other primary motoneurons in the same hemisegment. Moreover, we show that ablation of both VaP and caudal primary does not alter pathfinding by another identified primary motoneuron.

Identified primary motoneurons in embryonic zebrafish select appropriate pathways in the absence of other primary motoneurons.

Accurate pathfinding is a crucial step in formation of a functional nervous system. Individually identified zebrafish primary motoneurons undergo a stereotyped temporal sequence of axonal outgrowth and pathway selection during which their growth cones follow a common pathway to a "choice point" and then select divergent cell-specific pathways that lead to separate muscle territories. The characteristic sequence of cell-specific pathway selection raises the possibility that the sequence of growth cone arrival at the choice point might determine pathway selection. To test this idea, we ablated identified primary motoneurons by laser irradiation, labeled the remaining primary motoneuron in the same hemisegment with a fluorescent dye, and followed its development through the end of embryogenesis. We found that the growth cone of each primary motoneuron has an independent ability to pioneer the common pathway and select its appropriate cell-specific pathway, even in the absence of all other primary motoneurons in the same hemisegment.

Nicotinic acetylcholine receptor-like molecules in the retina, retinotectal pathway, and optic tectum of the frog.

Forty-two monoclonal antibodies (mAbs) generated against nicotinic acetylcholine receptors (AChRs) from electric organ were tested for their ability to cross-react in the optic tectum of the frog Rana pipiens. Twenty-eight of the mAbs tested (67%) bound to the optic neuropil of the tectum as revealed by immunoperoxidase cytochemistry. The pattern of peroxidase stain for cross-reacting mAbs corresponded in position to a subset of the retinotectal projections. Electron microscopic examination revealed that peroxidase reaction product was associated with the surface of vesicle-containing profiles but not with synaptic sites. Removal of one retina resulted in the loss of immunoreactivity in the contralateral tectum. AChR-like immunoreactivity was also associated with the optic tract and optic nerve and with retinal ganglion cells. These results indicate that some classes of retinal ganglion cells bear AChR-like molecules on their surface. The existence of these molecules on ganglion cell axons and terminals seems the most likely explanation for the AChR-like immunoreactivity present in the tectum.

The growth cones of identified motoneurons in embryonic zebrafish select appropriate pathways in the absence of specific cellular interactions.

Developing motoneurons in zebrafish embryos follow a stereotyped sequence of axonal outgrowth and accurately project their axons to cell-specific target muscles. During axonal pathfinding, an identified motoneuron pioneers the peripheral motor pathway. Growth cones of later motoneurons interact with the pioneer via contact, coupling, and axonal fasciculation. In spite of these interactions, ablation of the pioneer motoneuron does not affect the ability of other identified motoneurons to select the pathways that lead to appropriate target muscles. We conclude that interactions between these cells during pathfinding are not required for accurate pathway selection.