Postembryonic neurogenesis in the CNS of the tobacco hornworm, Manduca sexta. II. Hormonal control of imaginal nest cell degeneration and differentiation during metamorphosis

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Abstract
The nervous system of an adult moth is comprised of retained larval neurons that are remodeled during metamorphosis and a set of new adult specific neurons. The new neurons arise from a stereotyped array of stem cells (neuroblasts) that divide during larval life to generate nests of up to 100 arrested postmitotic immature neurons, the imaginal nest (IN) cells. At the onset of metamorphosis, some of the IN cells die while the remainder differentiate into mature functional neurons. Metamorphosis in insects is regulated by 2 classes of hormones, the ecdysteroids and the juvenile hormones. The transition from larva to pupa requires the disappearance of juvenile hormones followed by 2 releases of ecdysteroids: a small “commitment peak” and a larger “prepupal peak.” Through a series of endocrine manipulations, we demonstrate that the death and differentiation observed among the abdominal IN cells at metamorphosis are both influenced by these hormonal cues. If the abdomen was isolated from the hormonal sources in the anterior half of the larva before the onset of metamorphosis, death and differentiation of the IN cells were prevented. Infusion of ecdysteroids into such abdomens, to mimic the prepupal peak, resulted in the IN cells showing the same fate as seen in control animals during the early phases of metamorphosis. The response of the IN cells to the small commitment peak of ecdysteroids was heterogeneous. Exposure to this small peak of steroids caused some cells to become committed to resume their development, making them resistant to juvenile hormone application.