THE JOURNAL OF COMPARATIVE NEUROLOGY 257:490-512 (1987) Development of the Precerebellar Nuclei in the Rat: 11. The Intramural Olivary Migratory Stream and the Neurogenetic Organization of the Inferior Olive

Sequential thymidine radiograms from rats labeled on days El3 and E14, and killed at daily intervals thereafter, were analyzed to trace the migratory route and settling pattern of neurons of the inferior olive. Longsurvival thymidine radiograms from perinatal rats injected on day El4 were used to subdivide the inferior olivary complex on the basis of neurogentic criteria. The inferior olivary neurons originate on days El3 and El4 in the primary precerebellar neuroepithelium. The olivary neurons labeled on day El4 (the late generated components) translocate into the inferior olivary premigratory zone on day E15. On day El6 these cells join the olivary migratory stream, which follows an intramural circumferential path between the gray and white matters of the medulla. By day El7 the olivary migratory stream is reduced to a small band near the corpus of the inferior olive, which has been settled by this time by neurons generated on day E13. As a result, the unlabeled cells are situated on day El7 dorsomedially and the labeled cells ventrolaterally. The regional segregation of neurons forming subdivisions of the inferior olive begins on day E18, and by day El9 the major subdivisions are all recognizable. In thymidine radiograms from perinatal rats injected on day E14, four neurogenetic components can be distinguished in the inferior olive, those composed: (1) of unlabeled cells (generated on day E13), (2) of predominantly unlabeled cells, (3) of predominantly labeled cells (generated on day E14), and (4) of labeled cells. By combining these neurogenetic differences with the morphological features of the inferior olivary complex, we propose a modification of the currently accepted classification. The four major divisions of the inferior olive are the successively produced posterodorsal olive, anterolateral (principal) olive, posteroventral olive, and anteroventral olive. The location and configuration of these divisions are illustrated in relation to the traditional classification both in the coronal and the sagittal plane.