Neuronal changes induced by neonatal hypothyroidism: an ultrastructural study.

The postnatal development of the neurons of the cuneate nucleus was examined ultrastructurally in euthyroid and hypothyroid rats from birth to the sixth postnatal week. In the euthyroid animals, the neurons at birth displayed mild nuclear invaginations and a scanty cytoplasm with few organelles. By 2 weeks, there was a considerable increase in Nissl bodies. At 3 weeks, the neurons contained short lamellar arrays of endoplasmic reticulum. Between 3 and 6 weeks there was a reduction in the Nissl substance. In the hypothyroid animals, although the sequence of maturational changes generally resembled that of the controls, a number of differences were noted. The neurons at 1 week displayed dilations of perinuclear space, rough endoplasmic reticulum, Golgi complexes, and mitochondria. At 4 weeks both perikaryon and myelinated axons contained glycogen. Several neurons with cytoplasmic inclusions considered to be nonfunctional RNA were seen. The 6-week hypothyroid neuron exhibited large, clear, cytoplasmic vacuoles associated with a drastic reduction in cytoplasmic organelles. Presynaptic terminals showed a 50% reduction in mitochondrial numbers associated with the presence of glycogen granules. Three changes observed in neurites in all the age groups included: (1) large accumulation of glycogen in presynaptic terminals; (2) clear vacuoles; and (3) the presence of numerous lamellar bodies within reactive axons. Aberrant myelination, such as a single myelin sheath enclosing multiple processes, and instances of collapsed and redundant myelin were encountered.