Rostrocaudal nuclear relationships in the avian medulla oblongata: A fate map with quail chick chimeras

We present a correlative fate map of the nonsegmented caudal hindbrain down to the medullospinal boundary (medulla oblongata), as a companion to a pre vious fate mapping study of the hindbrain rhombomeres r2-r6 in quail chick chimeras at stages HH10/11 [Marin and Puelles (1995) Fur J Neurosci 7:1714- 1738]. For reproducibility and equivalent precision of analysis, successive portions of the medulla-called pseudorhombomeres "r7" to "r11"-were delimi ted by transverse planes through the center of adjacent somites at stages H H10/ 11. These units were each grafted homotopically and isochronically fro m quail donors into chick hosts. The chimeric specimens were fixed at stage s HH35/36 and alternate Nissl-stained sagittal sections were compared to ad jacent sections in which quail cells were detected immunocytochemically. Th is analysis in general showed that there is little intermixing between adja cent pseudorhombomeric domains, although some neuronal populations in the v estibular and trigeminal columns, as well as in the reticular formation and pontine nuclei, do migrate selectively into the host hindbrain. Contralate ral migration was scarce up to the stages examined. Several motor nuclei, i .e., the vagal motor complex, or sensory nuclei, i.e., the medial vestibula r nucleus, show cytoarchitectonic limits that coincide with pseudorhombomer ic ones; however, most conventional grisea were found to originate across s everal pseudorhombomeres. The inferior olivary complex originated between " r8" and "r11" (between the centers of somites 1 and 5). The medullospinal b oundary coincided precisely with the center of the fifth somite, slightly c audal to the obex and the end of the choroidal roof, and correlated with th e end of many medullary cytoarchitectonic units. In contrast, the dorsal co lumn nuclei and the caudal subnucleus of the descending trigeminal column f ell within the spinal cord. On the whole, the patterns observed were very s imilar to those found before within the overtly segmented part of the hindb rain, suggesting that some underlying common mechanism may account for the transverse cytoarchitectonic boundaries. J. Comp. Neurol. 427:522-545, 2000 . (C) 2000 Wiley-Liss, Inc.