Olfactory Placode Derivatives

The paired olfactory placodes, which invaginate toward the telencephalon to form olfactory pits, give rise to the entire olfactory (odorant-sensing) and, where present, vomeronasal (pheromone-sensing) epithelia, together with the respiratory epithelium that lines the nasal passages. The olfactory and vomeronasal epithelia contain ciliated sensory receptor neurons, each of which bind odorants via a single member of an enormous family of G-protein-coupled, seven-transmembrane domain receptor molecules (reviewed in Mombaerts, 2001; Ronnett and Moon, 2002). The epithelia also contain basal cells, which generate olfactory sensory neurons throughout life (for a review on stem cells in the olfactory epithelium, see Calof et al., 1998), and supporting sustentacular cells, which share some characteristics with glia (reviewed in Ronnett and Moon, 2002). All of these cells are derived from the olfactory placode.

The cell bodies of the olfactory receptor neurons remain in the placode, while their axons extend into the brain to form the olfactory, vomeronasal, and terminal nerves (for reviews of olfactory axon pathfinding, see Mombaerts, 2001; St. John et al., 2002). These nerves are ensheathed by olfactory placode-derived glial cells (reviewed in Wewetzer et al., 2002) that leave the pla-code and migrate along the nerves into the brain. In the zebrafish, pioneer neurons, distinct from olfactory receptor neurons, differentiate early within the placode and send their axons to the telencephalon (Whitlock and Westerfield, 1998). Axons from the olfactory receptor neurons follow this initial scaffolding, and the pioneer neurons subsequently die by apoptosis (Whitlock and Westerfield, 1998). Olfactory axons are the first peripheral input to reach the brain during development. The axons of pioneer neurons in the rat induce formation of the olfactory bulbs (Gong and Shipley, 1995), which fail to form if the olfactory placodes are missing or if olfactory axons fail to reach the brain (reviewed in Baker and Bronner-Fraser, 2001). The olfactory epithelium is also required for induction of the cartilaginous nasal capsule, which is derived from the neural crest.

The olfactory placode also forms neuroendocrine cells that migrate along the olfactory nerve into the forebrain and dien-cephalon. These neurons produce gonadotropin-releasing hormone (GnRH) and form the terminal nerve-septo-preoptic GnRH system (reviewed in Dubois et al., 2002). This system regulates gonadotropin release from the adenohypophysis (anterior pituitary), another placodal derivative (section Overview of Cranial Ectodermal Placodes). Hence, the olfactory placode is not only essential for olfaction, but also for reproduction. This is seen clinically in Kallmann's syndrome, in which olfactory axons and GnRH neurons fail to migrate into the brain, resulting in anosmia and sterility (hypogonadism) (reviewed in MacColl et al., 2002). An early-stage fate-map in zebrafish, however, challenges the olfactory placode origin of GnRH neurons, suggesting that terminal nerve GnRH neurons originate from the neural crest, and hypothalamic GnRH neurons from the hypophyseal placode (adenohypophysis) (Whitlock et al., 2003). More early-stage fate-map data are needed from multiple species to resolve this controversy.

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