The effects of mesothoracic leg disc extirpation on the postembryonic development of the nervous system of the blowfly,Sarcophaga bullata

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.

Peripheral organs control central neurogenesis in the leech

Interactions between developing nerve centres and peripheral targets are known to affect neuronal survival and thus regulate the adult number of neurons in many systems. Here we provide evidence that peripheral tissues can also influence cell numbers by stimulating the production of neurons. In the leech Hirudo medicinalis, there is a population of several hundred neurons that is found only in the two segmental ganglia that innervate the genitalia and which seem to be added gradually during post-embryonic maturation. By monitoring 5-bromo-2'-deoxyuridine incorporation immunohistochemically, we have now determined that these neurons are actually born late in embryogenesis, well after all other central neurons are born and after efferent and afferent projections are established between these ganglia and the periphery. Ablation of the male genitalia early in embryogenesis, or evulsion of the nerves that connect them to the ganglia, prevent the birth of these neurons. However, they fail to appear ectopically when male genitalia are transplanted to other segments, despite innervation by local ganglia. We conclude that the generation of the late-appearing neurons depends on a highly localized signal produced by the male genitalia, to which only the ganglia that normally innervate these organs have the capacity to respond.

Development of axon paths of motorneurons after removal of target muscles in a holometabolous insect

The fate of leg motor neurons and the outgrowth of their axons was studied in fleshflies after removal of their target muscles before the onset of axogenesis. When a mesothoracic leg imaginal disc was extirpated in the prepupa the corresponding leg nerve was missing on the operated side of the five-legged adult fly. Horseradish peroxidase and cobalt filling revealed that the target-deprived motorneurons survive and their axons grow out through other nerve roots. The choice of exit of the axons is variable: adjacent pro- or metathoracic leg nerves on the ipsilateral side or nerves supplying thoracic muscles (e.g. indirect flight muscles). In order to see whether factors in the mesothoracic leg disc can attract mesothoracic motorneurons, implantation of supernumerary leg discs was made in prepupae. Motor innervation of the implanted legs was never seen. Our data thus indicate that under the present experimental conditions leg motor neurons are not dependent on their specific targets for survival. They can track along adjacent preexisting axonal paths to innervate other targets. Under normal conditions specific markers may be present in the appropriate targets and the appropriate preexisting pathways (that we disrupt in our experiments) may lead the axons to these markers.

Cholinergic neurochemical development of normal and deafferented antennal lobes during metamorphosis of the moth, Manduca sexta

Sensory neurons with somata in the antennae of the moth, Manduca sexta, make synapses in the antennal lobes of the brain. These lobes develop during metamorphosis of the pupa to the adult while the antennae themselves develop and send presumably cholinergic sensory fibers into the lobes. Levels of acetylcholine, choline acetyltransferase, and acetylcholinesterase rise dramatically in the lobes as sensory axons grow from the antennae to the lobes through the antennal nerves. An [125I]alpha-bungarotoxin-binding activity, which may represent ACh-receptors, develops in the lobes with a time course different from that of the other cholinergic components, rising gradually throughout metamorphosis. This activity is specific to nervous tissue and is blocked by cholinergic agents (carbamylcholine, atropine, curare, and nicotine). Levels of acetylcholine, choline acetyltransferase, and acetylcholinesterase, but not of toxin-binding activity, are greatly reduced in lobes deafferented by amputation of developing antennae.