Anomalous anatomy of identified neurons in the larval prawn: spontaneous and induced by microlesions.

The abdominal ganglia of the prawn Macrobrachium rosenbergii undergo developmental changes of fundamental interest between the time of hatching and metamorphosis. These changes include an increase in cell numbers and changes in the connectivity between identified neurons. The giant motoneurons involved in the escape response, which form a syncytium in the adult, are observed as separate neurons with crossed axons in early larvae. Anomalous growth and connections of identified neurons were studied in order to gain some understanding of the rules and mechanisms governing normal development. Spontaneous anomalies included: supernumerary axons and abnormal axonal trajectories. The plasticity and specificity of identified neurons were studied by following the anatomical effects of deletions of giant neurons. Microlesions were inflicted reproducibly by means of a focused beam of visible and ultraviolet light. Within a day, irradiated cell bodies are eliminated; complete disappearance of the axon takes about 10 days, indicating that the remarkable ability of some invertebrate neurons to survive without a soma is not present in the larval prawn. As a result of the removal of an axon, the most common effect found in central connections was the absence of the collaterals or axons deprived of their targets. No collateral sprouting was detected in the central nervous system. In about a third of the ganglia where a giant motoneuron was killed and structure was analyzed 2 or more weeks after irradiation, anomalous connections were found. They usually involved contacts between an interneuron deprived of its normal target and the contralateral motoneuron which remained intact. The restricted types of anomalies observed support the notion of a hierarchical order in the rules governing formation of central synapses, in which neuron type ranks higher than laterality.