Early neuron differentiation in the mouse olfactory bulb. I. Light microscopy

Olfactory bulb development in mouse embryos from day 10 of gestation (E10) to E15 was studied with 1 μ plastic sections and Golgi impregnations. Olfactory nerve axons reach the presumptive olfactory bulb region of the cerebral vesicle at E12, the same time that the first young neurons (postmitotic neuroblasts) are seen in the newly‐formed intermediate (mantle) layer. By E14 many of the presumptive mitral cells are oriented tangential to the surface and have grown a long axon that enters the lateral olfactory tract on the surface of the presumptive piriform cortex. The following probable sequence of differentiation of mitral cells was determined from Golgi impregnations of E14 and E15 animals: (1) After regrowth of an internal and external process following rounding up at mitosis, the nucleus and perikaryon of certain ventricular cells migrate into the deepest portion of the intermediate layer and start to re‐orient into a tangential orientation. This stage is called the primitive radial stage .(2) The growth of tangential processes combined with withdrawal or atrophy of external and internal primitive radial processes follows to produce cells in the pre ‐ axonic tangential stage . (3) The tangential cells then acquire a long, thin, unbranched process (axon) oriented towards the lateral olfactory tract and a larger, branching process (dendrite) running in the opposite direction. This stage is called the tangential mitral cell stage . (4) Radial re‐orientation of the perikaryon and dendrites of these cells and external migration of the perikaryon results in cells in the radial mitral cell stage .