Neurogenesis and cell death in the isthmic nuclei of the frog Limnodynastes dorsalis.

In Limnodynastes dorsalis, neurogenesis of the isthmic nuclei was determined by 3H-thymidine autoradiography. The nuclei developed in a rostroventral to caudodorsal direction. Peak neurogenesis occurred in midlarval life, resulting in the formation of a substantial portion of each nucleus. Cell death was evidenced by the presence of pyknotic cells within the nucleus in all except the earliest stages. Peak cell death occurred around metamorphic climax. It was during this period that a rim neuropil appeared that segregates within the nucleus the contralateral and ipsilateral components of the tecto-isthmo-tectal pathways underlying binocular vision. It is also a time just prior to the onset of electrophysiologically detectable ipsilateral visual responses. However, no systematic cell death was found. Therefore the role of cell death in the formation of the isthmic nuclei remains obscure, although it may be involved in the establishment of tecto-isthmo-tectal connections.