Hypothyroidism-Induced Ultrastructural Peculiarities of Neuronal and Glial Elements in the Neocortex and Peripheral Structures of the Nervous System

Electron microscopic study of the structural components of the frontal cerebral cortex, lumbar spinal ganglia, and sciatic nerve in rats with hypothyroidism induced by thyroidectomy was carried out. A 100-day-long period of deficiency of hormones produced by the thyroid gland evoked significant ultrastructural modifications in the examined regions of the nervous system; the pattern of these changes was mosaic and, to a certain extent, structure-specific. In the neocortex, many neurons were modified according to a light type; they contained a decreased number of the organelles and demonstrated manifestations of swelling. Other neurons were characterized by a high electronic density of the cytoplasm, and the integrity of the plasma membranes of such units was disturbed. Long-term hypothyroidism was accompanied by manifestations of swelling in cortical oligodendrocytes and astrocytes. Perivascularly localized astrocytes were found to be most sensitive to the negative influences of hypothyroidism; the respective shifts should inevitably influence the functioning of the blood-brain barrier. Nerve fibers in the cortex were considerably subjected to demyelination; swelling of the axons and synaptic terminals was observed. The number of vesicles in synaptic structures clearly decreased. Unidirectional changes were found in the somata of spinal ganglia neurons. Hypoplasia of all organelles, fragmentation of tubules of the endoplasmic reticulum, and generalized lysis of cristae and matrix in the mitochondria were observed. This was accompanied by an increase in the number of phagosomes, which was indicative of activation of the autolytic processes. Mantle gliocytes, unlike glial cells in the brain, demonstrated both dark- and light-type modifications. Processes of demyelination developed also in the structures of the peripheral nervous system, and the intensity of these changes was greater than in the brain. In both spinal ganglia and sciatic nerve, lamellae of the myelin sheaths separated from each other and became homogenized; axial cylinders were replaced by a degenerative thickened sheath. These changes were most clearly manifested in large-diameter myelinated fibers.