Naturally occurring somatic motoneuron death in a teleost angelfish, Pterophyllum scalare

Naturally occurring somatic motoneuron death in a teleost angelfish, Pterophyllum scalare, was investigated histochemically and electron microscopically. The number of motor axons in the ventral root, which corresponds to the motoneuron number in spinal hemisegment, was rapidly increased beyond the adult value within 3 days after hatching, and then decreased to reach the adult value within a few weeks. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) histochemistry, which detects fragmented nuclear DNA characteristic to apoptotic cells, showed that the apoptotic cells are located in the motor column of the cord in the larvae at specific developmental stages. Electron microscopic observations of the spinal cells further confirmed the motoneuron apoptosis. The present data suggest that the massive death of somatic motoneurons at certain ontogenic stages which has been known to occur in higher vertebrates also takes place in fish.