Development of Olfactory Glomeruli

Abstract

Insects have an olfactory system able to detect and to discriminate a remarkable number of chemically different odorants in their environment (Chaps. 2–6). The process of odorant perception begins in the antenna with the interaction of odour molecules with odorant receptor proteins and subsequent signal transduction in the dendrites of olfactory receptor neurons (ORNs) (Chaps. 2–3). Their axons project into the antennal lobe (AL) and thus bring the information about the olfactory stimulus to the first central processing stage in the olfactory pathway (Chaps. 4–5). The AL is part of the deutocerebrum, the second ganglion in the insect brain. The AL and also the olfactory bulb, the target area of ORN axons in vertebrates, have in common that groups of sensory axons converge into characteristic compartments, the olfactory glomeruli (reviewed in: Boeckh et al. 1990; Boeckh and Tolbert 1993; Shepherd 1994; Hildebrand 1995, 1996; Hildebrand and Shepherd 1997; Chap. 4). Glomeruli are anatomical spheroidal neuropil subunits, surrounded by glial cells, in which the ORN axons are connected in complex synaptic circuits with processes of central neurons (Halász and Shepherd 1983; Distler 1990a,b; Malun 1991a,b; Shepherd 1993; Distler and Boeckh 1996, 1997a,b; Sun et al. 1997). In insects, there are two main populations of central target neurons: the projection neurons (PNs) and the local interneurons (LNs).