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Early neuron differentiation in the mouse olfactory bulb. II. Electron microscopy

Differentiating mitral cells were followed in serial section analysis of mouse embryos from day 12 of gestation (E12) to E14. Animals at E11 and E15 were also examined. Olfactory nerve axons reach the presumptive olfactory bulb at E12 and many penetrate deeply into the intermediate (mantle) layer at this time. The first identified young neurons (postmitotic neuroblasts) were also seen at E12. Many had a tangential orientation, and they occurred only in the region where olfactory nerve axons were present. Olfactory nerve axons (and also mitral cell axons formed later) are characterized by numerous longitudinallyoriented microtubules and an extensive system of transversely and longitudinallyorinted cisternae of smooth endoplasmic reticulum (SER). Near the growing tips of olfactory nerve axons, the microtubules disappear and the SER becomes more extensive and irregular.