Development and evolution of the insect mushroom bodies: Towards the understanding of conserved developmental mechanisms in a higher brain center

The insect mushroom bodies are prominent higher order neuropils consisting of thousands of approximately parallel projecting intrinsic neurons arising from the minute basophilic perikarya of globuli cells. Early studies described these structures as centers for intelligence and other higher functions; at present, the mushroom bodies are regarded as important models for the neural basis of learning and memory. The insect mushroom bodies share a similar general morphology, and the same basic sequence of developmental events is observed across a wide range of insect taxa. Globuli cell progenitors arise in the embryo and proliferate throughout the greater part of juvenile development. Discrete morphological and functional subpopulations of globuli cells (or Kenyon cells, as they are called in insects) are sequentially produced at distinct periods of development. Kenyon cell somata are arranged by age around the center of proliferation, as are their processes in the mushroom body neuropil. Other aspects of mushroom body development are more variable from species to species, such as the origin of specific Kenyon cell populations and neuropil substructures, as well as the timing and pace of the general developmental sequence.