Barber P C
Bibl Anat. 1982(23):12-25.
The primary sensory neurons in the olfactory and vomeronasal mucosae develop outside the neuraxis from ectodermal placodes, and their axons enter the central nervous system (CNS) during embryonic life, to terminate in the olfactory and accessory olfactory bulbs. The sensory neurons are unique in that they are produced and differentiate continuously from a stem cell in the sensory epithelium, throughout the life of the animal. After loss of sensory neurons following injury, the stem cell is able to increase its rate of division and replace the lost neurons. Sensory neurons thus formed during adult life, in the normal animal or after injury, possess another important and unique property: they are able to grow axons which can re-enter the CNS. Once in the CNS they can re-establish synaptic contact with central neurons. Such regenerative growth can result in re-establishment of connections between sensory mucosa and olfactory bulb in the adult animal, and a consequent recovery of olfactory function after injury to the pathway. However, a 'normal' pattern of connections is not always re-established, and olfactory axons may regenerate into areas of the CNS which they never enter during normal development. The special qualities which allow olfactory axons to re-enter the CNS are not known. Current work is aimed at examining more closely the interface where the peripheral nerve meets the CNS. Preliminary results suggest that this interface may be hard to define: the olfactory nerves themselves, although classically regarded as peripheral nerves, possess some features of central tracts, since their glia contain the astrocyte-specific protein GFAP, and resemble astrocytes in fine structure.
嗅觉和犁鼻器黏膜中的初级感觉神经元由外胚层基板在神经轴外发育而成，其轴突在胚胎期进入中枢神经系统（CNS），并终止于嗅球和副嗅球。感觉神经元的独特之处在于，在动物的整个生命周期中，它们由感觉上皮中的干细胞持续产生并分化。在损伤导致感觉神经元丧失后，干细胞能够加快分裂速度，替代丢失的神经元。在成年动物正常状态下或受伤后形成的感觉神经元具有另一个重要且独特的特性：它们能够长出轴突，这些轴突可以重新进入中枢神经系统。一旦进入中枢神经系统，它们就能与中枢神经元重新建立突触联系。这种再生性生长能够在成年动物中重新建立感觉黏膜与嗅球之间的连接，进而在嗅觉通路受损后恢复嗅觉功能。然而，连接的“正常”模式并不总是能够重新建立，嗅轴突可能会再生进入中枢神经系统中它们在正常发育过程中从未进入的区域。目前尚不清楚允许嗅轴突重新进入中枢神经系统的特殊性质。当前的研究工作旨在更仔细地研究外周神经与中枢神经系统的交界处。初步结果表明，这个交界处可能难以界定：嗅神经本身虽然传统上被视为外周神经，但具有一些中枢神经束的特征，因为其胶质细胞含有星形胶质细胞特异性蛋白GFAP，并且在精细结构上类似于星形胶质细胞。
