ROLE OF THYROID-HORMONES IN THE MATURATION OF INTERHEMISPHERIC CONNECTIONS IN RATS

Hipothyroidism causes mental retardation secondary to changes in the o rganization of the CNS. These changes affect higher brain functions fo r which interhemispheric transfer of information is crucial. In presen t study, the anterior commissure (AC) and corpus callosum (CC) of norm al (C) and hypothyroid (H) rats has been examined using quantitative e lectron microscopy. H rats received an antithyroid treatment with meth imazole from embryonic day 14 (E14) and surgical thyroidectomy at post natal day 6 (P6). In the AC, the number of axons (unmyelinated and mye linated) increased from 0.17 x 10(6) axons at E18 to 1.08 x 10(6) axon s at P4 and it was almost the same at P180 (1.01 x 10(6) axons). In H rats the number of axons between P14 and P180 was similar to that of C rats. In contrast, there were only 0.11 x 10(6) myelinated axons at P 180 resulting in a 66% reduction with respect to C rats (0.36 x 10 axo ns). In the CC of C rats, the number of myelinated axons increased fro m 1.76 x 10(3) axons at P12 to 3.34 x 10(6) axons at P184. In H rats, there were only 0.84 x 10(6) axons at P184 resulting in a 76% reductio n with respect to C rats. This reduction was more important in the pos terior sector of the CC (95%) than in the rest (on average 63%). There fore these results show that thyroid hormones play an important role i n the processes involved in the maturation of commissural axons.