Fine structure of meganeurites and secondary growth processes in feline GM1-gangliosidosis.

Electron microscope studies were carried out on neurons of the hippocampal formation in a feline mutant with beta-galactosidase deficiency and GMI-gangliosidosis. Fusiform processes with characteristics similar to meganeurites of Golgi studies were identified between cell bodies and axons of pyramidal and granule cells. The presence of dense material subjacent to the plasma membrane at the meganeurite-axon junction provides evidence that meganeurites form at the axon-hillock region and displace the initial axonal segment distally. Meganeurites of hippocampal neurons exhibited pleomorphic secondary processes with fine structural features of growth cones. Spines and spine-synapses were abundant on perikarya and meganeurites. Numerous membranous cytoplasmic bodies (MCBs) were encountered amongst otherwise normally appearing organelles of the cell body. MCBs were densely packed in meganeurites except near their peripheral area. They were less common in dendrites and rare in synapses of the neuropil. The observations provide further support for the view that meganeurites of mature cortical neurons in ganglioside storage diseases have embryonic growth characteristics.