Development of the Olfactory System

Excerpt

Several excellent reviews have detailed the conservation of principles between insect, amphibian, and mammalian olfactory systems, and many important contributions to the field have been made by cross-phyla comparisons. However, within the limitations of this reference, in this chapter we will emphasize embryonic development and axon targeting in the rodent olfactory system, focusing on the mouse. To aide comparisons between studies in other rodents, the developmental ages in studies in the rat will be adjusted and referred to as the equivalent developmental age in mice. We apologize for any papers we may have missed or not had the opportunity to discuss in detail due to space limitations.

The olfactory system is one of the most precocious sensory systems to develop in the embryo. The primary olfactory pathway is comprised of two components, the olfactory epithelium (OE) and the olfactory bulb (OB). The secondary olfactory pathway includes multiple cortical regions, all of which are directly innervated by OB projection neurons, and are collectively referred to as the olfactory cortex. Major regions of the olfactory cortex include the anterior olfactory nucleus (AON), the olfactory tubercle, the piriform cortex, and the entorhinal cortex (reviewed in Wilson et al. 2006). These regions are innervated by mitral and/or tufted cell axons via the lateral olfactory tract (LOT).

During the earliest stages of primary olfactory pathway formation, the OE and the OB undergo simultaneous, yet independent, developmental programs (Lopez-Mascaraque and De Castro 2002). However, as development progresses and axons from the OE innervate the nascent OB, their developmental programs become interrelated (Treloar et al. 1999; Matsutani and Yamamoto 2000; Lopez-Mascaraque et al. 2005). The early separation in developmental programs is perhaps not surprising given the spatial segregation of the regions that give rise to these two structures (see below). Despite a common ectodemal origin, the OE (the peripheral component of the olfactory system) is derived from the olfactory placode, while the OB emerges from the germinal zones of the neural tube, like other central nervous system (CNS) structures.