Sphingomyelin lipidosis in a cat: Golgi studies.

Alterations in neuronal geometry in a feline model of sphingomyelin lipidosis were evaluated using Golgi staining. Neurons in cerebral cortex, basal ganglia, amygdala, thalamus, and cerebellum were impregnated and many were found to possess conspicuous enlargements at the axon hillock-initial segment region (meganeurites) and/or to sprout secondary neuritic processes from this same area. The latter were sometimes well developed and resembled small dendrites. These changes were cell type specific with distribution limited to certain types of neurons in select brain regions, while others remained normal, or underwent only simple somatic enlargement or generalized degenerative changes. Occasional cortical pyramidal neurons also displayed thinning of dendrites and extensive loss of dendritic spines. These observations add sphingomyelin lipidosis to other neuronal storage disorders in which aberrant neurite growth and meganeurite formation accompany lysosomal enzyme deficiency and the associated metabolic alterations and storage. Although meganeurite-like expansions have been reported to occur in many storage disorders, the appearance of axon hillock-associated neurite growth on morphologically mature neurons has been identified heretofore only in the gangliosidoses, and in feline models of alpha-mannosidosis and mucopolysaccharidosis type 1.