Neurogenesis in the Insect Central Nervous System and Its Peculiarities in the Brain Mushroom Bodies

Neurogenesis is divided into two main stages: proliferation and differentiation. Neural stem cells were discovered in insects almost a century and a half ago, in 1885, by A.A. Korotneff who called them ganglion cells, while the term neuroblast was introduced by W.M. Wheeler in 1891. Neurogenesis was initially considered a singlestage process, and the smaller daughter cells of the neuroblast were believed to differentiate directly into nerve cells. V. Bauer in 1904 described the ganglion mother cell as a distinct stage and thus established the two-stage nature of neurogenesis. More than a century later, it was found out that two-stage neurogenesis was not the only possible type, and one more, three-stage type was discovered. The latter includes an additional link between the neuroblast and the ganglion mother cell, termed the intermediate neural progenitor. Correspondingly, the traditional neuroblasts are currently termed type I neuroblasts, and those giving rise to intermediate neural progenitors are termed type II neuroblasts. The modern stage of neurogenesis research is characterized by the range of study objects narrowed down practically to a single species, Drosophila melanogaster, and also by application of a great variety of molecular genetic methods capable of revealing the finest mechanisms of neurogenesis. Four different patterns of solitary type I neuroblast activity during insect development have been revealed: (1) only during the embryonic stage; (2) continuously in embryos and larvae; (3) intermittent with a period of quiescence during the late embryonic and the early larval stage; (4) two independent generations of neuroblasts: embryonic and postembryonic. The initial stage of neurogenesis in the mushroom bodies is characterized by longer mitotic activity of neuroblasts and the presence of clusters of neuroblasts of different origins. Some neuroblasts of the mushroom bodies may be type II neuroblasts. A variety of Kenyon cells, the intrinsic cells of the mushroom bodies, is formed at the second stage of neurogenesis. Similar to the descendants of solitary neuroblasts in the cortex, the type of Kenyon cells depends on the time of their formation.