Effect of methylthiouracil and triiodothyronine on development of the central nervous system in chicken embryos.

Abstract Hypothyroidism in chick embryos was produced experimentally by injecting methylthiouracil (MTU) into eggs on Day 10 of incubation. It resulted in retarded development of structures in the cerebellar cortex and cerebrum of the embryos. Other manifestations were seen in the smaller size of the embryos, the presence of a large yolk sac even on the 21st day of incubation, and delay in hatching. Retardation was also evidenced by broadening of the external granular layer (EGL), narrowing of the molecular layer (ML), a decrease in the number of cells in the internal granular layer (IGL), and in a reduction in the size of Purkinje cells and their nuclei. In the cerebrum, the cells were more densely packed and cellular and nuclear sizes were reduced. The indices of labelled cells and mitotic divisions were higher in the EGL and in the cerebrum of MTU-treated embryos; this indicates a prolongation of the proliferative phase. The cerebrum showed slight reaction to the enzymes studied in the MTU-treated embryos. The Purkinje cells showed strong reaction to succinic dehydrogenase and NADH and NADPH diaphorases. The ML and IGL also reacted but less markedly. Embryos treated with MTU + triiodothyronine (T 3 ) or saline did not differ from the normal controls. T 3 injection was also ineffective.