Effect of thyroid deficiency on the growth of the hippocampus in the rat. A combined biochemical and morphological study.

The growth of the hippocampus was studied in normal and hypothyroid rats using both biochemical and morphological techniques, and the results were compared with observations on the whole forebrain or on the cerebral cortex. The longitudinal growth, area and the volume of the hippocampus was severely reduced in thyroid deficiency. In the cerebral cortex the longitudinal growth and some of the parameters of transverse development were significantly decreased at the rostral but not at the caudal level. On rehabilitation from day 35 to 160 the longitudinal growth remained decreased in both the brain parts while the transverse growth was restored to normal in the cerebral cortex but not in the hippocampus. In the normal hippocampus about 60% of the cells were formed during the first 3 postnatal weeks. This developmental increase was significantly depressed in hypothyroid rats; the final deficit in cell number was about 13%. The rate of cell acquisition was calculated from the slopes of the logistic curves fitted to the data of DNA content. At the age of maximal cell acquisition (at day 2-3) the daily deposition was 0.44x106 cells in controls and 0.34x106 cells in the hypothyroid rats. In controls the concentration of DNA decreased in the hippocampus during maturation. Thyroid deficiency did not influence this developmental trend. In contrast, a severe reduction was observed in the cellular composition of RNA and protein.