Influence of thyroid hormones on neuronal death and differentiation in larval Rana pipiens

Abstract

The present experiments demonstrate that thyroid hormone mediates neuronal death as well as differentiation. Hypophysectomized Rana pipiens larvae exposed to high titres of hormone (50 μg/l) rapidly undergo metamorphosis. In the lateral motor column (LMC) approximately two-thirds of the motor neurons degenerate in response to hormonal treatment. At high hormone levels, degenerating cells exhibit little acid hydrolase activity, whereas at lower doses (≤10 μg/l) many secondary lysosomes are apparent in dying neurons. In both instances, phagocytic microglia apparently remove neuronal debris by heterophagocytosis. Although the majority of the cells die, others differentiate into biopolar neurons following the administration of hormone. These cells display vivid deposits of acetylcholinesterase reaction product in putative Nissl bodies. The role of thyroid hormone (or hormones) in simultaneously regulating neuronal death and differentiation is discussed in light of our current knowledge on the significance of hormones to neural maturation.