An immunocytochemical and electron microscopic study of the hyt mouse anterior pituitary gland.

The hyt mutant mouse used in this study has a hypoplastic thyroid gland and is characterized by retarded somatic growth, very low to undetectable levels of plasma thyroxine (T4), and increased levels of plasma thyroid-stimulating hormone (TSH). This congenital hypothyroid mouse is therefore an ideal model for studying the effects of thyroid hypofunction on the adenohypophysis. The anterior pituitary of the hyt mouse appeared less granular than that of the normal control when viewed by light microscopy, owing to a decrease in the population of somatotrophs. Many cells, in various stages of transformation into 'thyroidectomy cells', were recognized by the appearance of the characteristic granules and dilated rough endoplasmic reticulum. In some cases, the enlarged rough endoplasmic reticulum also contained spherical electron-dense secretory granules. In addition there were many cells undergoing mitosis and these were identified as thyrotrophs by their characteristic granules. Administration of T4 during the first 40 days of life prevented the abnormal changes in the hyt anterior pituitary. A reduction in immunoreactive thyrotrophin-releasing hormone (TRH) levels was seen in the median eminence of the hyt mouse. Treatment with T4 restored this to normal, suggesting that the reduced TRH content of the hypothalamus of the mutant mouse may be due to T4 deprivation.