The status of the thyroid gland in relation to the development of the nervous system

Role of thyroid hormones in the maturation of interhemispheric connections in rats

Hypothyroidism causes mental retardation secondary to changes in the organization of the CNS. These changes affect higher brain functions for which interhemispheric transfer of information is crucial. In present study, the anterior commissure (AC) and corpus callosum (CC) of normal (C) and hypothyroid (H) rats has been examined using quantitative electron microscopy. H rats received an antithyroid treatment with methimazole from embryonic day 14 (E14) and surgical thyroidectomy at postnatal day 6 (P6). In the AC, the number of axons (unmyelinated and myelinated) increased from 0.17 x 10(6) axons at E18 to 1.08 x 10(6) axons 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 P180 resulting in a 66% reduction with respect to C rats (0.36 x 10(6) axons). In the CC of C rats, the number of myelinated axons increased from 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% reduction with respect to C rats. This reduction was more important in the posterior sector of the CC (95%) than in the rest (on average 63%). Therefore these results show that thyroid hormones play an important role in the processes involved in the maturation of commissural axons.

The development of the anterior commissure in normal and hypothyroid rats

The development of axon number in the anterior commissure (AC) was analyzed in 39 normal and 37 hypothyroid rats using conventional electron microscopy. Hypothyroid rats underwent antithyroid treatment with methimazole from embryonic day (E) 14 onwards, followed in a fraction of the animals by thyroidectomy at postnatal day (P) 6. In normal rats, the midsagittal cross-sectional anterior commissure area (ACA) increased throughout their life; in hypothyroid rats, ACA was stationary from P4 onwards and at P174-180 it was reduced by 39% relative to normal rats. In normal rats, the number of AC axons increased rapidly from 168,500 at E18 to, on average, 1,049,000 from P4 onwards. Similarly, in hypothyroid rats, the number of axons increased from 135,000 at E18 to, on average, 1,052,000 from P4 onwards. At all ages, the number of axons was similar in normal and hypothyroid rats. During development of the AC, the evolution of axon number observed in normal and hypothyroid rats is different from what was reported for other telencephalic commissures, including the AC of the monkey, where an important fraction of the axons are eliminated postnatally. Antithyroid treatment dissociated ACA from total number of AC axons.

Organization of auditory callosal connections in hypothyroid adult rats

Callosal connections were studied with tracers (horseradish peroxidase (HRP) and wheat germ agglutinin-horseradish peroxidase (WGA-HRP)) in normal rats and rats deprived of thyroid hormones with methimazole (Sigma) since embryonic day 14 and thyroidectomized at postnatal day 6. In hypothyroid rats, the auditory areas, in particular the primary auditory area, showed cytoarchitectonic changes including blurred lamination and decrease in the size of layer V pyramidal neurons. In control rats, callosally-projecting neurons were found between layers II and VI with a peak in layer III and upper layer IV. In hypothyroid rats, labelled neurons were found between layers IV and VI with two peaks corresponding to layer IV and upper layer V, and in upper layer VI. Quantitative analysis of radial distribution of callosally-projecting neurons confirmed their shift to infragranular layers in hypothyroid rats. Three-dimensional reconstructions showed a more continuous tangential distribution of callosally-projecting neurons in hypothyroid rats which may be due to the maintenance of a juvenile 'exuberant' pattern of projections. These changes in cortical connectivity may be relevant for understanding epilepsy and mental retardation associated with early hypothyroidism in humans and to clarify basic mechanisms of cortical development.

Forebrain development in control and hypothyroid larvae of Triturus cristatus carnifex

Some cytological and developmental aspects of the forebrain gray matter were studied in the control and the hypothyroid larvae of Triturus cristatus carnifex. The mitotic cell percentages in the periventricular germinal layer, the fusiform shape of the recently divided neuronal cells and the transitory gray matter cleavage, are characteristic of the immature status of the central nervous system (CNS) in this species. The hypothyroidism obtained during two months of thiourea treatment slows down the development in all the forebrain areas of the experimental larvae. Nevertheless, the nucleus olfactorius anterior and the corpus striatum, which have a longer mitotic activity with respect to the other neuronal structures, are more deeply affected by thyroid atrophy. Thyroid activity and forebrain maturation are discussed on the basis of the literature and our data with regard to their influence on behavior and evolution.