Both the Snell dwarf mouse (dw) and the growth hormone-deficient Little mouse (lit) exhibit a microcephalic cerebrum with hypomyelination, retarded neuronal growth, and underdevelopment of axons and dendrites. 2. The hypomyelination is due to arrested glial proliferation, suggesting that the action of growth hormone on the proliferation and maturation of both glial and neuronal cells is a necessary precondition of myelin formation, apart from the complementary or synergistic actions of thyroxine. 3. In contrast, the cerebral hypomyelination present in the inherited hypothyroid mouse (hyt) is not related to arrested glial proliferation, demonstrating that thyroid hormones can act independently on myelinogenesis.
