Time of neuron origin in the diencephalon of the mouse. An autoradiographic study

Time of origin of neurons of thalamus ventralis, thalamus dorsalis, and epithalamus was determined autoradiographically at three transverse levels in adults which received thymidine‐H3 once during gestation. A ventrodorsal gradient in the entire region demonstrates a wave of neuron origin in the forebrain, probably rostrocaudal originally but revectored by rearrangement of neuromeres I–III. A caudorostral gradient in dorsal thalamus similarly may represent redirection of an earlier ventrodorsal one. A lateromedial gradient in dorsal thalamus and epithalamus (opposite that in cortex) may reflect lateral displacement of neuroblasts (or their nuclei) by younger elements. Large neurons generally arise before small ones, as seen in epithalamus; the two habenular nuclei illustrate programmed origin of large and small neurons and glia consecutively, with progressively restricted potentiality by the primitive ependyma. Phylogenetically, slight differences in duration of proliferation might radically alter the numbers of small neurons. Although dorsal thalamic nuclei “appear” in external germinal and mantle layers, their neuroblasts originate in primitive ependyma; time of origin accords with subsequent recognition of nuclei and appearance of thalamocortical connections. Massive proliferation (from the 10th to 15th days of gestation) and the gradients overshadow nuclear differences and demonstrate a developmental unity in the dorsal thalamus. The findings further validate Herrick's diencephalic zones and suggest some orderly mechanism, probably at the molecular level, underlying generation of the neurons in this complex brain region.