Neurogenesis in adult insect mushroom bodies

The occurrence of neurogenesis in mushroom bodies of adult insects belonging to several orthopteroid and coleopteran families is described. Using injections of 5-bromo, T2'-deoxyuridine, we showed that neuroblasts, which are progenitors of Kenyon cells during preimaginal instars, continue to divide in adult Acheta domesticus. Their progeny constitute a central column in mushroom body cortices of 3-week-old females. Other Gryllidae, Gryllus bimaculatus and Gryllomorpha dalmatina, show the same pattern of neuroblast activity and migration of their progeny. Immunocytochemical staining of glial cells failed to reveal any immunoreactivity, either in proliferating regions or in the resulting cells. In another orthopteran, Locusta migratoria, discrete clusters of cells, located dorsolateral to the Kenyon cells, incorporated 5-bromo, 2'-deoxyuridine, but we could not detect any neuronal progeny migrating to the mushroom body cortices. These cells were strongly labeled with an antiglial antibody, indicating that the replicating cells are glioblasts rather than neuroblasts. In Periplaneta americana (Dictyoptera), cells replicating their DNA were similarly shown to immunoreact with glial antibodies. In contrast, three coleopterans (Tenebrio molitor, Zophobas species, Harmonia axyridis) have two large neuroblasts located in the middle of the mushroom body cortices. These produce cells which migrate within the group of Kenyon cells, their nuclei having the same shape and size as those of surrounding Kenyon cells. In adult insects, neurogenesis in mushroom bodies occurs in Gryllidae and several coleopteran families, but could not be demonstrated in Dictyoptera and Acrididae. Its occurrence and distribution raise the issue of unexpected plasticity in the adult insect brain.