Reorganization of persistent motoneurons in a metamorphosing insect (Tenebrio molitor L., Coleoptera)

Metamorphosis of the central nervous system of Trichogramma telengai (Hymenoptera: Trichogrammatidae)

During metamorphosis, the insect CNS undergoes both structural and allometric changes. Due to their extreme de-embryonization and parasitism, the formation of the CNS in egg parasitoids occurs at the late larval stage. Our study provides the first data on the morphological and volumetric changes of the CNS occurring during the pupal development of the parasitic wasp Trichogramma telengai Sorokina, 1987 (Trichogrammatidae). The prepupal-pupal development includes fusion and concentration of ganglia achieved by the loss of connectives. Volumetric analysis shows that during the pupal development the absolute body volume and CNS volume gradually decrease. The brain and thoracic synganglion slightly increase in volume during the pupal period and extremely decrease from late pupa to adult. The CNS neuropil volume increases from prepupa to adult. The mean cell diameter also decreases during the metamorphosis of the nervous system. The cell body rind volume decreases during pupal development; this decrease correlates with the decrease in the number of cells on the one hand and increase in the neuropilar volume on the other hand.

Developmental origins and architecture of Drosophila leg motoneurons

Motoneurons are key points of convergence within motor networks, acting as the "output channels" that directly control sets of muscles to maintain posture and generate movement. Here we use genetic mosaic techniques to reveal the origins and architecture of the leg motoneurons of Drosophila. We show that a small number of leg motoneurons are born in the embryo but most are generated during larval life. These postembryonic leg motoneurons are produced by five neuroblasts per hemineuromere, and each lineage generates stereotyped lineage-specific projection patterns. Two of these postembryonic neuroblasts generate solely motoneurons that are the bulk of the leg motoneurons. Within the largest lineage, lineage 15, we see distinct birth-order differences in projection patterns. A comparison of the central projections of leg motoneurons and the muscles they innervate reveals a stereotyped architecture and the existence of a myotopic map. Timeline analysis of axonal outgrowth reveals that leg motoneurons reach their sites of terminal arborization in the leg at the time when their dendrites are elaborating their subtype-specific shapes. Our findings provide a comprehensive description of the origin, development, and architecture of leg motoneurons that will aid future studies exploring the link between the assembly and organization of connectivity within the leg motor system of Drosophila.

Embryonic and postembryonic development of serial homologous neurons in the subesophageal ganglion of Tenebrio molitor (Insecta: Coleoptera)

Neuroblast pattern, engrailed expression and proliferation in the subesophageal neuromers of the beetle Tenebrio molitor are characterized throughout embryogenesis. The proliferation of neuroblasts has been studied throughout postembyronic development. Serotonin, crustacean cardioactive peptide and tyrosine hydroxylase-like-immunoreactive neurons are characterized and their neuronal development has been studied. There is an initial posterior-anterior gradient in neuroblast segregation leading to a reduced number of neuroblasts in the frontal subesophageal neuromer. The study of the engrailed expression shows that only the anterior subfraction of the neuromeral neuroblast configuration is reduced, whereas the posterior two rows of engrailed-positive neuroblasts are not affected during the first 40% of embryogenesis. The overall number of proliferations in the first subesophageal neuromer reaches only 30-50% of the value found in each of the other two neuromers. The analysis of serotonin and crustacean cardioactive peptide immunoreactivity allows the identification of serial homologous neurons which persist from the early embryo to the adult stage. In the different gnathal neuromers, these neurons form structurally highly similar projection patterns, but show different extensions of their arborizations, corresponding to the relative size of each neuromer. Structural homologies between subesophageal and thoracic neuromers are discussed.

A motoneuron spared from steroid-activated developmental death by removal of descending neural inputs exhibits stable electrophysiological properties and morphology

Neurons die during the development of nervous systems. The death of specific, identified motoneurons during metamorphosis of the tobacco hornworm, Manduca sexta, provides an accessible model system in which to study the regulation of postembryonic neuronal death. Hormones and descending neural inputs have been shown to influence the survival of abdominal motoneurons during the first few days of adult life in this insect. Motoneurons prevented from undergoing the normal process of developmental degeneration by removal of neural inputs were examined at the physiological and structural levels using several cell imaging techniques. Although these neurons lost their muscle targets and experienced the endocrine cue that normally triggers death, they showed no overt electrophysiological or morphological signs of degeneration. Thus, by appropriate intervention, the MN-12 motoneuron can be spared from developmental neuronal death and remain as a functioning supernumerary element in the mature nervous system.

Postembryonic development of gamma-aminobutyric acid-like immunoreactivity in the brain of the sphinx moth Manduca sexta

We have investigated the distribution of immunocytochemical staining for the neurotransmitter gamma-aminobutyric acid (GABA) in the brain of the sphinx moth Manduca sexta during larval, pupal, and adult development. In the larval brain, about 300 neurons are GABA-immunoreactive. All neuropil areas except the mushroom bodies and central complex show intense immunostaining. Only minor changes in the pattern of immunoreactivity occur during larval development. During metamorphosis, changes in immunostaining occur in two phases. Beginning in wandering fifth-instar larvae (stage W2), immunoreactivity appears in numerous neurons of the central body and optic lobe and becomes more intense during early pupal stages. At the same time, GABA-like immunoreactivity disappears in most neuropil areas of the brain and becomes faint in many immunoreactive somata. Neurons with arborizations in the ventrolateral protocerebrum, however, continue to exhibit intense immunostaining during this period, and strongly immunolabeled fibers connect these areas with the ventral nerve cord. The second phase of transformation begins around pupal stage P5/P6, when faint immunostaining appears in many previously nonimmunoreactive somata and most neuropil areas of the brain. In subsequent stages (P8-P10), this immunoreactivity disappears again in most somata, but in certain cell groups, it becomes more intense and gradually develops to the adult pattern. Most larval GABA-immunoreactive neurons appear to survive through metamorphosis into the adult. Neurons in the midbrain that acquire GABA-like immunoreactivity during metamorphosis usually lie adjacent to larval immunostained neurons, suggesting common lineages. The onsets of the two developmental phases of GABA-like immunoreactivity correlate with sharp rises in hemolymph titers of ecdysteroid hormones, suggesting a role for ecdysteroids in the regulation of GABA synthesis. We hypothesize that the disappearance of GABA in many areas of the brain starting 2 days prior to pupation dramatically alters its functional circuitry and thus may account for profound changes in the behavior of the animal.

Dendritic reorganization of an identified neuron during metamorphosis of the moth Manduca sexta: the influence of interactions with the periphery

During metamorphosis of the moth, Manduca sexta, an identified leg motor neuron, the femoral extensor motor neuron (FeExt MN) undergoes dramatic reorganization. Larval dendrites occupy two distinct regions of neuropil, one in the lateral leg neuropil and a second in dorsomedial neuropil. Adult dendrites occupy a greater volume of lateral leg neuropil but do not extend to the dorsomedial region of the ganglion. The adult dendritic morphology is acquired by extreme dendritic regression followed by extensive dendritic growth. Towards the end of larval life, MN dendrites begin to regress, but the most dramatic loss of dendrites occurs in the 3 days following pupation, such that only a few sparse dendrites are retained in the lateral region of leg neuropil. Extensive dendritic growth occurs over the subsequent days such that the MN acquires an adult-like morphology between 12 and 14 days after pupation. This basic process of dendritic remodeling is not dependent upon the presence of the adult leg, suggesting that neither contact with the new target muscle nor inputs from new leg sensory neurons are necessary for triggering dendritic changes. The final distribution of MN dendrites in the adult, however, is altered when the adult leg is absent, suggesting that cues from the adult leg are involved in directing or shaping the growth of MN dendrites to specific regions of neuropil.