Differentiation of mitral cell dendrites in the developing main olfactory bulbs of normal and naris-occluded rats

The morphological differentiation of mitral cell dendrites during embryonic and early postnatal development was examined in the main olfactory bulb of rats to determine a possible role of afferent activity in the development of the dendrites. Mitral cells and olfactory nerve fibers were labeled with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) a nd fluorescein-conjugated lectin (Ulex europeus agglutinin-I), respectively . Morphogenesis of mitral cell dendrites proceeded as previously described (Malun and Brunjes [1996] J. Comp. Neurol. 368:1-16); that is, undifferenti ated dendrites with radial orientation were transformed into a single prima ry dendrite having a glomerular tuft and secondary dendrites extending tang entially into the external plexiform layer. Quantitative examinations in bo th pre- and postnatal rats revealed that the differentiation of primary den drites, including tuft formation, increases in diameter and decreases in br anching, started before birth, whereas differentiated secondary dendrites w ere only observed in postnatal animals. Mitral cells with more than two pri mary dendrites were found after embryonic day 21. The proportion of the mit ral cells with differentiated dendrites increased postnatally. At postnatal day 10, almost all mitral cells had fully differentiated dendrites, and mi tral cells with multiple primary dendrites were no longer seen. No signific ant change was found during development in the number of stem dendrites tha t arose directly from the cell body. Unilateral naris occlusion started on postnatal day 1 retarded differentiation of primary and secondary dendrites , and increased the proportion of mitral cells with multiple primary dendri tes. These finding revealed that differentiation of mitral cell primary den drites precedes that of secondary dendrites, and suggested that the differe ntiation of secondary dendrites proceeds in an activity-dependent manner. ( C) 2000 Wiley-Liss, Inc.