Larval and pupal development of the mushroom bodies in the honey bee, Apis mellifera.

The mushroom bodies are paired neuropils in the insect brain that act as multimodal sensory integration centers and are involved in learning and memory. Our studies, by using 5-bromo-2-deoxyuridine incorporation and the Feulgen technique, show that immediately before pupation, the brain of the developing honey bee (Apis mellifera) contains approximately 2,000 neuroblasts devoted to the production of the mushroom body intrinsic neurons (Kenyon cells). These neuroblasts are descended from four clusters of 45 or fewer neuroblasts each already present in the newly hatched larva. Subpopulations of Kenyon cells, distinct in cytoarchitecture, position, and immunohistochemical traits, are born at different, but overlapping, periods during the development of the mushroom bodies, with the final complement of these neurons in place by the mid-pupal stage. The mushroom bodies of the adult honey bee have a concentric arrangement of Kenyon cell types, with the outer layers born first and pushed to the periphery by later born neurons that remain nearer the center of proliferation. This concentricity is further reflected in morphologic and immunohistochemical traits of the adult neurons, and is demonstrated clearly by the pattern of expression of Drosophila myocyte enhancer factor 2 (DMEF2)-like immunoreactivity. This is the first comprehensive study of larval and pupal development of the honey bee mushroom bodies. Similarities to patterns of neurogenesis observed in the mushroom bodies of other insects and in the vertebrate cerebral cortex are discussed.