A Transient Population of Neurons Pioneers the Olfactory Pathway in the Zebrafish

Abstract

Mechanisms guiding the first axons from the olfactory placode of the peripheral nervous system (PNS) to the olfactory bulb in the vertebrate CNS are unknown. We analyzed the initial outgrowth of axons from the olfactory placode in zebrafish and found a precocious transient class of pioneer neurons that prefigure the primary olfactory pathway before outgrowth of olfactory sensory axons or expression of olfactory receptor genes. Not only are the pioneers antigenically, morphologically, and spatially distinct from olfactory sensory neurons, they are also developmentally distinct; via fate mapping, we show that they arise from a more anterior region of the lateral neural plate than do the first sensory neurons. After the axons of the sensory neurons grow into the CNS, the pioneer neurons undergo apoptotic cell death. When we ablated the pioneers before axonogenesis, the following sensory axons showed severe misrouting. We propose that the pioneers provide the first necessary connection from the PNS to the CNS and that they establish an axonal scaffold for the later-arriving olfactory sensory neurons.

Isolation and characterization of the laure olfactory behavioral mutant in the zebrafish, Danio rerio

“…Loss of pioneer neurons results in misrouting of the axons of the olfactory sensory neurons into neighboring commissures in the zebrafish (Whitlock and Westerfield, 1998). lre mutant animals had pioneer neurons and the axons of the olfactory sensory neurons did not misroute into neighboring commissures.…”

“…In the olfactory system, axon guidance is a multi-step process with pioneer neurons initiating the connection between the developing olfactory placode and the telencephalon (Whitlock and Westerfield, 1998) and olfactory receptors contributing to the targeting of the axons to the glomeruli in the developing olfactory bulb (Lin and Ngai, 1999;Mori et al, 1999). Loss of pioneer neurons results in misrouting of the axons of the olfactory sensory neurons into neighboring commissures in the zebrafish (Whitlock and Westerfield, 1998).…”

“…To recognize the pioneer neurons (Whitlock and Westerfield, 1998), we used the zns-2 mouse monoclonal antibody (Trevarrow et al, 1990). Embryos were collected and maintained at 28.5°C in embryo medium (Westerfield, 1993).…”

“…As the sensory system develops, the axons of the OSNs use multiple cues to guide their outgrowth from the sensory epithelium to the bulb. These cues include pioneer neurons (Klose and Bentley, 1989;McConnell et al, 1994;Whitlock and Westerfield, 1998), cell surface molecules (Yoshihara et al, 1997; Alenius Response of wildtype fish at 3 days after fertilization, tested in 35-mm Petri dishes, to distilled water (A) and L-cysteine in distilled water (B). C,D: Responses were quantified by counting the number of fish that moved outside the center ring when exposed to water (A, C) or L-cysteine (B, D).…”

Anosmin-1a is required for fasciculation and terminal targeting of olfactory sensory neuron axons in the zebrafish olfactory system

“…Ablation experiments have demonstrated that pioneer neuron axons play an essential role for proper elongation and guidance of OSN axons suggesting these pioneer axons may serve a guidance function for later developing olfactory axons (Whitlock and Westerfield, 1998). Furthermore, as pioneer axons do not express odorant receptors in zebrafish, Whitlock and Whiterfield (1998) put forward the hypothesis that these receptors are not involved in the guidance of pioneer axons and that other cues, such as cell-surface glycoproteins, guide these axons to the telencephalon.…”