Faulty development of cortical neurons in the Snell dwarf cerebrum.

In the Snell dwarf motor cortex, area 6 of Caviness, the cell number, the stratification of neurons and the portion of layer-widths were absolutely identical to those of the controls. By means of the Golgi-Cox method modified by Ramon-Moliner, however, the pyramidal neuron was found to have small perikarya, short primary dendrites with sparse branchings, and a low spine density on the dendrites. The corpus callosum of the dwarf contained a reduced number of fibers compared to that of the controls, and the staining for myelin basic protein revealed a considerable reduction of positive-fibers of radiation in this area. The content of Thy-1 antigen in the cerebrum, cerebellum and brainstem, was significantly lower than that of controls, but the monoamine content was normal in the cerebrum and brainstem. From these results, it appears that the Snell dwarf cerebrum shows retarded neuronal growth; a reduction in size of neurons, an underdevelopment of axons and dendrites, and a retarded maturation of spine, in addition to arrested glial proliferation. At present, it is unclear which hormone deficient in these mutants, growth hormone or thyroxine, is the essential potentiator for neuronal growth.