Development of the diencephalon in the rat. IV. Quantitative study of the time of origin of neurons and the internuclear chronological gradients in the thalamus.

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 13 and 14 (E13 + 14) until the day before birth (E21 + 22). With this progressively delayed comprehensive labelling procedure we determined the time of origin of neurons in the nuclei of the epithalamus, thalamus, and ventral thalamus. The zona incerta, subthalamic nucleus, reticular nucleus, posterior nucleus, and ventral lateral geniculate nucleus are composed of the earliest arising neurons (E13, or before, to E15). The neurons of the lateral habenular nucleus are produced between days E13--16. The neurons of the medial geniculate and lateral geniculate nuclei, the ventrobasal and ventrolateral complexes, and the nucleus lateralis, pars posterior, arise rapidly on days E14--15; the medial geniculate nucleus with a peak on day E14, the others with a peak on day E15. Neurons of a group of nuclei, with ill-defined boundaries medial to the sensory relax nuclei, arise apparently on days E15--16, with a peak on day E15; these may represent the intralaminar nuclei. The next group is generated on days E15--16 but with peak formation time on day E16; this includes the anteroventral, anterodorsal, anteromedial and mediodorsal nuclei. The rhomboid, reuniens and paratenial nuclei, and the paraventricular nucleus, pars anterior, arise next (E16--17). The medial habenular nucleus forms last and over a protracted period (E15--19). With their lengthy generation time the lateral and medial habenular nuclei resemble more the nuclei of the hypothalamus than the nuclei of the dorsal thalamus.