Anatomy of the Mushroom Bodies in the Honey Bee Brain: the Neuronal Connections of the Alpha-lobe

Neural connections between the mushroom body (MB) and other protocerebral areas of the honeybee's brain were studied with the help of cobalt chloride and Golgi staining methods. Focal injections of cobalt ions into the alpha-lobe neuropil of the MB reveal seven clusters of somata located in the protocerebrum and deutocerebrum of each brain hemisphere. These neurons connect the mushroom body neuropil with protocerebral areas and number approximately 400. They contact the layered organization of the alpha-lobe at different locations. Some project not only into the alpha-lobe, but also into the beta-lobe and pedunculus neuropils. Fifteen cell types which form intraprotocerebral circuits are morphologically described. They can be divided into three categories: 1) unilateral neurons, with projection fields restricted to the ipsilateral protocerebrum; these neurons connect the alpha-lobe with areas in the protocerebral lobe and ramify with densely layered arborisations arranged perpendicularly to the longitudinal axis of the alpha-lobe; 2) recurrent neurons, which interconnect subcompartments of the MB, forming loops at different levels of the neuropil; their arborisations are mainly restricted to the alpha-lobe, beta-lobe, pedunculus, and calyces of the ipsilateral MB; they also ramify sparsely around the neuropil of the alpha-lobe; and 3) bilateral neurons, which either interconnect both alpha-lobes or connect the ipsilateral alpha-lobe and protocerebral lobe with the dorsolateral protocerebral lobe of the contralateral hemisphere. The connections of different compartments of the MB with other parts of the protocerebrum as revealed in this study are discussed in the context of hypotheses about the functional role of MBs in the honeybee brain.