The prenatal development of the pulvinar in the monkey: 3H-thymidine autoradiographic and morphometric analyses.

Abstract

The genesis and development of the pulvinar in monkey (Macaca mulatta) was studied by means of tritiated thymidine (3H-TdR) autoradiography and by examination and morphometric analysis of Nissl stained histological sections. The time of origin was determined by analysis of labeled cells in the pulvinar of monkeys that had been exposed to 3H-TdR at various embryonic (E) days and sacrificed 2–3 months after birth, which in this species occurs around E165. Heavily labeled pulvinar neurons were observed only in animals that had been exposed to 3H-TdR between E36 and E45. Onset of genesis for neurons that comprise the inferior and adjacent areas of lateral pulvinar precedes the onset in the remainder of lateral and medial pulvinar by two days. The site of origin and migratory pathways of pulvinar neurons were determined in a series of animals that had been exposed to 3H-TdR between E36 and E45 and sacrificed 1 h or 1, 3, 7, 14, and 42 days after injection. No neurons are produced within the pulvinar. Rather, they arise exclusively from the diencephalic ventricular and subventricular zones that border the third ventricle between sulcus medius and sulcus dorsalis. Postmitotic cells migrate laterally and in less than 3 days attain their position at the lateral aspect of the developing thalamus where they remain uniformly distributed during the first half of gestation. The cytoarchitectonic boundaries of the pulvinar are distinguished from adjacent thalamic nuclei by E60 whereas its own subdivisions emerge around E80. Morphometric analysis shows that the increase in the size of the pulvinar of the monkey during the last two thirds of gestation is due to cell growth, elaboration of cell processes, and arrival of various afferent inputs rather than to the influx of newly generated neurons, as in the human brain (Rakic and Sidman 1969). Instead the present data indicate that the pulvinar in monkey is generated much earlier than expected from previous studies and that unlike the human pulvinar, does not contain a subpopulation of neurons of telencephalic origin.