Telencephalic origin of pulvinar neurons in the fetal human brain

Summary

Our previous autoradiographic study of fetal human brain fragments exposed supravitally to thymidine-H³ had suggested that the primitive ependyma of the third ventricle ceases neuron production at a time when the pulvinar is only slightly developed. We sought a source of additional pulvinar neurons by examining serial horizontal sections through brains of 5- to 40-week fetuses. In the nearby telencephalon lies the ganglionic eminence, composed of germinal and immature cells coating the fetal corpus striatum and bulging into the lateral ventricle. It contains numerous proliferating cells to the end of gestation. Young cells appear to stream from it, cross beneath the sulcus terminalis to enter the diencephalon, and form a hitherto-undescribed layer, the corpus gangliothalamicus. This structure is found consistently just under the external surface of the developing pulvinar of fetuses from the 18th to the 34th weeks of gestation. A 22-week specimen freshly prepared by the rapid Golgi method shows a progression of cell forms from simple elongate bipolar cells in the part of the corpus gangliothalamicus closest to the telencephalon, through a series of gradations to multipolar young neurons in the most medial and deep parts of the structure, where it merges into the pulvinar proper. We conclude that many of the neurons of the pulvinar, a very large component of the human thalamus, arise in the telencephalon and migrate to the diencephalon during the fifth to eight lunar months of gestation.