ULTRASTRUCTURE OF THYROID IN DWARF MICE

The thyroid gland of Snell''s dwarf mice consists of small follicles with flattened epithelium, partly differentiated cords and undifferentiated masses. Many adipocytes are found. The ultimobranchial cysts are well developed. Parafollicular cells are normal. In the partly organized cords, microfollicular cells and some small follicles limited by 2 or 3 cells are seen. The presence of these structures suggested that they represent the 1st stages of normal folliculogenesis, described as the fusion of 2 adjacent unicellular microfollicles. Their further growth is the result of the coalescing of small adjacent follicles or of cellular multiplication. The presence of undifferentiated masses and partly differentiated cords, in dwarf mice, seems due to a developmental arrest rather than to aberrant development. This disorder of organogenesis is ascribed mainly to a lack of pituitary growth hormone.