Vertebrate olfactory neurons are unique among neulons in that they are continually replaced throughout the life of the animal. The rate of neurogenesis can be regulated by manipulating the system to abbreviate or prolong the average life of a sensory neuron. Moreover, the neuron may die before or after reaching full maturity. When compared with other neurons, the fully mature olfactory neuron retains juvenile characteristics, it is probable that genetic controls operate to maintain this relatively immature state.

Replacement of neurons is, with very few exceptions, essentially unknown in the postnatal vertebrate. Most neurons that are generated postnatally, such as granule cells in the cerebellum and olfactory bulb, are formed during infancy as a part of normal growth and development. In poikilothermic animals, such as goldfish, which exhibit continuous growth throughout life, continuing proliferation of neurons in the CNS is a reflection not of cell turnover but of growth in cell numbers to accommodate the increase in body size. However, neuron replacement is known to occur in the vocalization centers in the brain of the male songbird 1.2 and in the olfactory and vomeronasal systems of adult vertebrates in general (reviewed in Ref. 3). Neurogenesis in the avian brain is usually tied to reproductive cycles. In contrast, in olfactory and vomeronasal epithelium it is continuous throughout life, thus endowing the animal with the ability to replace neurons that might be lost as a consequence of physical, chemical, or infectious trauma.