The Mushroom Bodies
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Mushroom bodies are lobed neuropils that are involved in olfactory learning and memory (Zars et al., 2000). Mushroom bodies may be common to all arthropods (Strausfeld et al, 1998) and genes expressed early in their development share homology with genes involved in the early development of comparable regions in mammalian forebrain (Kurusu et al., 2000). Studies on large taxa, such as cockroaches and honey bees, ascribe to the mushroom bodies other learning and memory functions such as place memory, associative memory, context dependent sensory filtering, and roles in motor control (Mizunami et al., 1998a,b; Rybak and Menzel, 1998; Li and Strausfeld 1997, 1999, Liu et al., 1999).
In Drosophila, mushroom bodies consist of a calyx neuropil, situated postero-dorsally in the protocerebrum, which is confluent with a forward projecting pedunculus that, anteriorly, divides into an dorsal lobe extending upwards, and into medial lobe that extend towards the midline. The matrix of the mushroom body consists of ensembles of intrinsic neurons, called Kenyon cells (or here). These are derived from globuli cells situated above the calyx. Globuli cells are the smallest and most basophilic perikarya in the brain. Some species possess hundreds of thousands of Kenyon cells but a fruitfly mushroom body comprises about 3,000 Kenyon cells
In insects, mushroom bodies can be generally grouped into two types: calyxless and calycal. Mushroom bodies in paleopteran and thysanuran insects (dragonflies, damselflies, and silverfish) lack calyces. Their globuli cells are situated dorsally and posteriorly, as they are in other insects, but they merely provide many thousands of thin neurites (cell body fibers) that extend to the front of the brain. There, the neurites enlarge and form long processes that together provide the medial and vertical lobes. The lobes receive inputs (afferents) and they also give rise to outputs (efferents). The intrinsic processes from globuli cells provide systems of local circuits between the afferents and efferents. In neopteran insects (such as flies, bees, wasps, beetles, butterflies and moths, grasshoppers), mushroom bodies possess calyces. Their mushroom body lobes, however, retain the primitive features of the paleopteran mushroom body. Their Kenyon cells give rise to axons that provide local circuits between afferents and efferents. Thus, the primitive condition is for afferents to the mushroom to supply the lobes. The calyces are a relatively recent evolutionary acquisition.
In the Neoptera, the calyces receive afferents mainly from the olfactory (antennal) lobes and, usually, a negligible supply from the optic lobes (Strausfeld and Li, 1999a). But in many Hymenoptera (ants, bees, wasps) the calyces also receive major inputs from the optic lobes (Gronenberg, 1999). Thus, it would appear that levels of activity by Kenyon cell axons within the lobes might be controlled by inputs to the calyces.
All insects have two calyces each side of the midline although in Drosophila, as in many other taxa, the two calyces appear to be fused into a single entity. Nevertheless, clonal analysis has demonstrated that four neuroblasts provide four identical subsets of Kenyon cells. Thus, the mushroom body is a quadripartite structure (Ito et al., 1997) where each hemicalyx consists of two halves, each possessing the same types of Kenyon cells, the axons of which segregate out into the subdivisions of the lobes (see below).
Mushroom bodies are longitudinally subdivided into parallel divisions. Early anatomical studies of the brain of the cockroach P. americana by Bretschneider (1914) demonstrate longitudinal subdivisions in the pedunculus and lobes (see also; Mizunami et al., 1998c). Each subdivision consists of a pair of laminae, one which stains light the other dark by the Bodian method (Strausfeld and Li, 1999b). Light laminae have affinities to taurine, NOS, and certain modulatory peptides. Each lamina is composed of several smaller subunits (called leaves), each composed of an ensemble of axons from one morphological type of Kenyon cells, as defined by its dendritic morphology in the calyx (Strausfeld and Li, 1999b). Immunocytochemical stains of the bee mushroom body also show longitudinal divisions through the lobes. These correspond to specific subdivisions of the calyx (Strausfeld et al, 2000). Likewise, GAL4 enhancer trap lines of Drosophila demonstrate longitudinal subdivision within the pedunculus, and lobes (Yang et al., 1995). As shown by Crittenden et al. (1998), antibodies to eight proteins reveal three medially projecting subdivisions called b, b', and g. The b division corresponds to the a division of the vertical lobe and the b' division corresponds to the vertical lobes a' division. Antibodies have not revealed a vertical division corresponding to g . Developmental studies, which employ mosaic analysis to visualize between one and a few Kenyon cells from the same ganglion mother cell at different times during postembryonic development (Lee and Luo, 1999), confirm that Kenyon cells to the g division are unbranched in the adult whereas Kenyon cells that invade the b and b' divisions send axon collaterals to a and a' (Lee et al., 1999). However, Golgi impregnations suggest that the g division may be further subdivided (Ito et al., 1998): a subset of K-cells that supply the g division indeed send a branch to the vertical lobe and may provide a third subdivision there.
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