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Axotomy of giant lamprey (Petromyzon marinus) central neurons (anterior bulbar cells) close to their somata results in ectopic axon-like sprouting from the dendritic tips. Such sprouts first appear as swellings at the tips of a small subset of dendrites 2-3 weeks after "close" axotomy. We report here that immunocytochemical examination of these swellings reveals a structure and composition that differs from that of conventional growth cones; incipient sprouts contain many highly phosphorylated neurofilaments (NFs), little tubulin, and virtually no stable (acetylated) microtubules (MTs). The dendrites of anterior bulbar cells after close axotomy also show pronounced changes in NF protein and tubulin staining patterns prior to the emergence of sprouts from the dendrites. The amount of tyrosinated tubulin increases greatly; this rise is tightly coupled to the appearance of highly phosphorylated NFs and the loss of nonphosphorylated NFs in the dendrites. Acetylated tubulin is generally reduced after close axotomy and is selectively lost from dendrites that gave rise to sprouts. These changes indicate that an invasion of the dendrites by phosphorylated NFs may be linked to the destabilization of dendritic MTs, and in some dendrites this may lead to a marked loss of stable MTs, which is correlated with the emergence of NF-filled sprouts from the dendritic tips.

Massive dendritic sprouting was induced in identified giant reticulospinal neurons of the lamprey by axotomy close to the soma. An axonal lesion slightly farther from the cell body induced new growth from both dendrites and axon. The amount of new growth per cell was the same whether it originated from the dendrites alone or from axonal and dendritic compartments. The location of the axonal lesion therefore determines where, in the neuron, membrane is inserted to produce the new neurites. The dendritic tree of a differentiated vertebrate central neuron was shown to have sufficient plasticity to extend new growth for several millimeters beyond the normal dendritic domain.

We have investigated some of the factors controlling the distribution of axonal and dendritic sprouting following axotomy of a subset of Muller giant interneurons (anterior bulbar cells or ABCs) in the hindbrain of the larval sea lamprey (Petromyzon marinus). Sprouts originated from different sites in the cell depending on the distance of the axonal lesion from the soma. When the axon was cut close to the soma (within 500 microns), the dendritic tips sprouted profusely, whereas the proximal axon stump showed few sprouts and frequently disappeared entirely. Axotomy further from the soma (1000-1400 microns) resulted in less sprouting from the dendrites and more from the axon stump, with the total amount of dendritic plus axonal sprouting remaining constant. Axotomy at sites distant from the soma (1 cm or more) did not result in dendritic sprouting. No sprouts were ever observed emerging from the soma proper or from the axon stump except at the lesion site. Neuritic sprouts from dendrites and axon were similar in their gross morphology. Sprouts resembled axons rather than dendrites whatever their sites of origin; they followed linear, rostrocaudally oriented paths in the "basal plate" region of the hindbrain. Dendritic and axonal sprouts grew both rostrally and caudally within the brain. Either "close" or "distant" axotomy resulted in the retraction of the dendritic tree and of both dendritic and axonal sprouts by several months postaxotomy. Reaxotomy close to the soma 30 d after a distant axotomy accelerated the onset of this evoked dendritic retraction. Reaxotomy close to the soma also induced sprouting significantly sooner than did close axotomy alone. These results suggest that axotomy close to the soma causes axonal regeneration to be shunted into ectopic locations at the dendritic tips. The emerging sprouts then follow guidance cues appropriate for regenerating ABC axons.

We have examined the dendritic and axonal ultrastructure of intact anterior bulbar reticulospinal neurons (ABCs) in the CNS of the larval sea lamprey and compared it with that of the dendrites and neuritic sprouts from ABCs examined 2 months following axotomy. Dendrites and axons of intact ABCs are distinguishable from one another by several ultrastructural criteria: (1) the predominance of microtubules in the dendritic cytoskeleton and neurofilaments in that of the axon, (2) the exclusively postsynaptic status of the dendrites versus the presynaptic status of the axon, and (3) the presence of polyribosomes and large numbers of mitochondria in the dendrites and their respective absence and scarcity in the axon. The ultrastructure of axonal sprouts evoked by axotomy of ABCs 1-1.5 mm from their somata ("intermediate axotomy") in many ways resembled that of intact axons. Axonal sprouts were presynaptic to other neurons, and their cytoskeletons consisted mainly of neurofilaments. They also exhibited some features not seen in either axons or dendrites, such as numerous clusters of small vesicles that were not associated with synapses and, in some cases, close associations with glial elements. We also examined sprouts emerging from the dendrites of ABCs following axotomy within 500 microns of their somata ("close axotomy") and found that such "dendritic" sprouts closely resembled axonal sprouts; they possessed neurofilament-dominated cytoskeletons, were presynaptic to other neurons, and were often associated with glial elements. The dendrites of ABCs undergoing dendritic sprouting retained their normal gross morphology but possessed a mixture of "axonal" and "dendritic" ultrastructural characteristics, exhibiting neurofilament-dominated cytoskeletons while remaining entirely postsynaptic to other neurons. However, there were significantly fewer synapses on the dendrites of axotomized cells than were found on the dendrites of intact ABCs. We conclude that sprouts evoked by axotomy are intrinsically axonal in character whether they originate from the axon stump or from the dendritic tree. Our results also suggest that the materials necessary for axonal regeneration may displace elements of the dendritic cytoskeleton as they are transported through the dendrites to the emerging "dendritic" sprouts following close axotomy.

In the previous paper (Hall and Cohen, 1988), we showed that axotomy of anterior bulbar cells (ABCs) in the hindbrain of the larval lamprey results in the sprouting of axonlike neurites from either the end of the proximal axon stump, the dendritic tips, or both, depending on the site of axotomy. Here we show that, unlike axotomy, dendritic amputation (dendrotomy) does not by itself induce sprouting from ABCs. However, dendrotomy does induce sprouting from dendrites in the immediate vicinity of the dendritic lesion in cells that have been previously axotomized. We found that dendrotomy acts primarily to rearrange the distribution of sprouts induced by axotomy rather than serving as an additional stimulus to neurite outgrowth. We propose that (1) dendritic sprouting in ABCs occurs because the dendritic tips become attractive sites for sprout initiation when they are either directly injured (as with dendrotomy) or are situated relatively close to the site of injury (as with axotomy close to the soma), and (2) the axon stump, dendritic stumps, and uninjured dendritic tips of the cell compete to initiate a limited total amount of sprouting induced by axotomy. The probability that a given locus will support sprouting is determined both by its proximity to the nearest lesion site and by whether there are other attractive potential sprouting sites in the cell.