Delayed synaptogenesis in the dentate gyrus of the thyroid-deficient developing rat

The synaptogenesis in the molecular layer of the dentate gyrus was studied during the development of normal and thyroid-deficient rats using a monoclonal antibody directed against synaptophysin, a marker of the synaptic vesicles. Immunostaining appeared after birth in the lateral edge of the molecular layer and spread over the entire length of the layer during the following days. Such a lateral-medial gradient has previously been reported for cell acquisition in the granule cell layer. Thyroid deficiency caused a delay of about 2 days in the progression of labeling along the layer. Synaptogenesis in the molecular layer is thus altered when thyroid hormone is lacking during the critical period of dentate gyrus development, probably affecting the nature and function of the synaptic networks that are formed in this area. This may partly account for the neurological defects observed after perinatal hypothyroidism.