The development of the diencephalon in Xenopus. An autoradiographic study

Abstract

The development of the diencephalon and the time of origin of neurons of thalamic nuclei were determined in Xenopus with 3H-thymidine autoradiography. Isotope was administered into embryos, tadpoles and metamorphic animals and sacrificed after survival time between 24 hours and 5 months. The position and the number of heavily labeled cells, corresponding to terminal mitoses at the time of isotope injection were established on transverse and sagittal sections of the frog brain. Neurons in the diencephalon were distributed in a spatiotemporal manner such that cells generated earliest were located in the caudo-ventro-lateral portion of the diencephalon followed in a rostro-dorso-medial sequence by cells formed at later stages. The waves of cell generation resulted in three apparent developmental gradients in the caudo-rostral, latero-medial and in the ventro-dorsal directions in the diencephalon. Consequently neurons generated latest were found in the rotro-dorso-medial portion of the diencephalon. The overall rostro-dorso-medial diencephalic growth and the spatiotemporal generation of its neurons are the reverse of the tectal growth and cell generation reported in Xenopus which occurs in a rostrolateral to caudomedial direction. The findings of the present observations appear to indicate that the mirror-image reversal of the retinotectal and retinodiencephalic projections along the temporo-nasal retinal axis is the consequence of the divergent growth of the diencephalon and the tectum from the common embryonic di-mesencephalic junction. These observations furthermore suggest that the orientation of the retinal maps is ensured by the differential maturity gradients in the tectum and diencephalon, respectively, presumably expressed in molecular terms.