Embryogenesis of the inferior olivary nucleus in the rat: A radioautographic study and a re-evaluation of the rhombic lip

Interkinetic nuclear migrations studied by redioautography in the medullar neuroepithelium of the 14‐day rat embryo are similar to those in other areas of the neuraxis in other species. The cell cycle lasts approximately 12 hours. Olivary neurons are produced along the length of the alar plate on embryonic days 14 and 15, mostly on day 14. Between 36 and 48 hours after injection of thymidine‐ H3, labeled cells migrate into the mantle layer and are more concentrated in the alar lamina than in the basal lamina. The olivary primordium appears in the ventromedial medulla on day 17 and is almost mature by day 21. Labeled neurons in the alar lamina migrate to the primordium in two bands, the marginal and the submarginal migration strands. From these strands, labeled cells move into the ventrolateral and ventromedial portions of the olivary complex, Newly arrived neurons push the existing cells dorsally. By embryonic day 21, as proliferation of cells in the neuroepithelium ceases, the migration strands disappear, and the olive in fully formed. In the series injected on day 14, heavily labeled neurons become located dorsally in the medial accessory, dorsal accessory and principal nucleus; lightly labeled cells are found in the ventral half of the complex. In the series injected on day 15, labeled cells are found only in the ventral portions of the olive, in the medial accessory nucleus. His described 2 rhombic lips. The first may be an artifact. The second is not seen in the medulla of the rat. If the term is used for any species, it is best viewed as the lateral part of the alar plate.