Postnatal proliferation and maturation of olfactory bulb neurons in the rat.

Mitral cells are formed prenatally whereas most granule cells originate postnatally. Material was taken from 2-day-old, 14-day-old, 28-day-old, and adult rat olfactory bulbs and processed for rapid Golgi or Cresyl Violet staining. We show that the number of granule cell bodies/mitral cell body increases from 7.0 to 46.3 during the first two weeks of life; most mitral cells appear morphologically functional during the first postnatal week; few granule cells appear to be functional until the second postnatal week; and the number of short axon interneurons increases dramatically during the second postnatal week. We conclude the newborn rats have an intact afferent pathway from olfactory receptors to primary cortex that lacks the extensive interneuronal circuitry characteristic of adults.