Mitral cell development in the mouse olfactory bulb: Reorientation of the perikaryon and maturation of the axon initial segment

Abstract

The proximal portion of mitral cell axons was examined in electron microscopic serial sections and Golgi impregnations of mouse embryos on the fifteenth day of gestation (E15), E16, postnatal day one (P1), and P4. During this time the cell body of developing mitral cells appears to transform from tangential to radial orientation and then migrate peripherally while the tangential axon remains relatively fixed. In E15, E16 and P1 animals, electron microscopy of most radial mitral cells reveals that the microtubules of the axon proper are continued as a bundle of approximately the same diameter through the voluminous subnuclear cytoplasm to one side of the nucleus. The rest of the subnuclear cytoplasm is nearly devoid of microtubules and contains only free ribosomes and an occasional mitochondrion. It is suggested that the position of the axon is determined within the perikaryon by a bundle of microtubules; the nucleus and rest of the subnuclear cytoplasm then migrates peripherally leaving only a relatively thin axon. If some of the perikaryal cytoplasm failed to migrate peripherally, a bleb or process might be left, as in fact has been observed in Golgi impregnations and electron microscopy. We have also examined the ultrastructure of the developing axon initial segment, in cross‐sections of the axons from E15 to P4, as well as in longitudinal serial sections. Fascicles of closely apposed microtubules have been noted in some advanced mitral cells at E15 and E16, and by P1 such fascicles are common and well developed in most mitral cells. In P1 animals some mitral cell initial segments show a faint and patchy dense undercoating of the axolemma, but many axons at this time appear to completely lack this feature. By P4 nearly all initial segments show a fairly well developed dense undercoating. In general, therefore, the development of fascicles of closely apposed microtubules precedes the elaboration of the dense undercoating of the axolemma.