Sensory receptor differentiation and axonal pathfinding in the cercus of the grasshopper embryo

Guidance of peripheral pioneer neurons in the grasshopper: adhesive hierarchy of epithelial and neuronal surfaces

An important question in developmental neurobiology is how a neuron finds its way over long distances to its correct target during embryogenesis. Peripheral pioneer neurons in insect embryos have been used for study because of the relative simplicity of the early embryonic appendages, and the accessibility of the identified neurons whose growth cones traverse this terrain. The data presented suggest an adhesive hierarchy of both epithelial and neuronal surfaces that guides the first growth cones from the appendages of the grasshopper embryo.

Ultrastructure, development, and homology of insect embryonic cuticles

Ultrastructure and deposition of the cuticles secreted by embryos representing eight insect orders were examined by transmission and scanning electron microscopy. Embryos of the apterygote silverfish Thermobia domestica deposit two embryonic cuticles. Deposition of the first (EC1) is initiated at the beginning of appendage development when the intercalary segment and the neural groove are clearly visible. This cuticle lacks surface microsculpture and consists of an outer epicuticle and an underlying fibrous layer, thought to represent procuticle. At the time of dorsal closure, deposition of a second embryonic cuticle (EC2) begins; this bears sensilla and functions in the first instar larva. In representative embryos of seven pterygote orders (Ephemeroptera, Odonata, Plecoptera, Neuroptera, Coleoptera, Lepidoptera, and Mecoptera), three cuticles were found to be secreted. The first cuticle in pterygotes is homologous to EC1 of T. domestica, but consists solely of outer epicuticle. EC2, the "prolarval cuticle," bears a characteristic surface microsculpture in embryos of some species and egg-teeth and other hatching devices, and consists of outer and inner epicuticles and a more or less reduced procuticle. EC2 is reduced in the embryos of derived endopterygotes, where a procuticle is lacking and the inner epicuticle is reduced. After hatching, when EC2 is shed, the first instar larva is covered by a third embryonic cuticle (EC3), whose deposition was initiated while the insect was still within the egg. Presence of only two embryonic cuticles in cyclorrhaphous flies is due to the total loss of prolarval cuticle. Investigated exopterygote and endopterygote insects excluding flies thus deposit three embryonic cuticles, and their juveniles (exopterygote "nymphs"; endopterygote "larvae") seem to hatch at equivalent stages of development. Differences between the modes of cuticulogenesis in silverfish and pterygote embryos suggest that the apterygote first larval instar was embryonized and became a fully embryonic prolarva in pterygotes.

Embryonic development of the sensory innervation of the antenna of the grasshopper Schistocerca gregaria

The establishment of the sensory nervous system of the antenna of the grasshopper Schistocerca gregaria was examined using immunocytochemical methods and in the light of the appendicular and articulated nature of this structure. The former is demonstrated first by the expression pattern of the segment polarity gene engrailed in the head neuromere innervating the antenna, the deutocerebrum. Engrailed expression is present in identified deutocerebral neuroblasts and, as elsewhere in the body, is continuous with cells of the posterior epithelium of the associated appendage, in this case the antenna. Second, early expression of the glial homeobox gene reversed polarity (repo) in the antenna is by a stereotypic pair of cells at the antenna base, a pattern we show is repeated metamerically for each thoracic appendage of the embryo. Subsequently, three regions of Repo expression (A1, A2, A3) are seen within the antenna, and may represent a preliminary form of articulation. Bromodeoxyuridine incorporation reveals that these regions are sites of intense cell differentiation. Neuron-specific horseradish peroxidase and Lazarillo expression confirm that the pioneers of the ventral and dorsal tracts of the antennal sensory nervous system are amongst these differentiating cells. Sets of pioneers appear simultaneously in several bands and project confluent axons towards the antennal base. We conclude that the sensory nervous system of the antenna is not pioneered from the tip of the antenna alone, but in a stepwise manner by cells from several zones. The early sensory nervous systems of antenna, maxilla and leg therefore follow a similar developmental program consistent with their serially homologous nature.

Juvenile hormone acts at embryonic molts and induces the nymphal cuticle in the direct-developing cricket

During embryogenesis of hemimetabolous insects, the sesquiterpenoid hormone, juvenile hormone (JH), appears late in embryogenesis coincident with formation of the first nymphal cuticle. We tested the role of embryonic JH by treating cricket embryos with JH III, or the JH-mimic (JHM) pyriproxifen, during early embryogenesis. We found two discrete windows of JH sensitivity. The first occurs during the formation of the first (E1) embryonic cuticle. Treatment with JHM prior to this molt produced small embryos that failed to complete the movements of katatrepsis. Embryos treated after the E1 molt but before the second embryonic (pronymphal) molt completed katatrepsis but then failed to complete dorsal closure and precociously formed nymphal, rather than pronymphal characters. This second sensitivity window was further assessed by treating embryos with low doses of JH III prior to the pronymphal molt. With low doses, mosaic cuticles were formed, bearing features of both the pronymphal and nymphal stages. The nymphal characters varied in their sensitivity to JH III, due at least in part to differences in the timing of their sensitivity windows. Unexpectedly, many of the JH III-treated embryos with mosaic and precocious nymphal cuticles made a second nymphal cuticle and successfully hatched. JH treatment also affected the growth of the embryos. By focusing on the developing limb, we found that the effect of JH upon growth was asymmetric, with distal segments more affected than proximal ones, but this was not reflected in misexpression of Distal-less or Bric-a-brac, which are involved in proximal-distal patterning of the limb.

Nitric oxide and cGMP influence axonogenesis of antennal pioneer neurons

The grasshopper embryo has been used as a convenient system with which to investigate mechanisms of axonal navigation and pathway formation at the level of individual nerve cells. Here, we focus on the developing antenna of the grasshopper embryo (Schistocerca gregaria) where two siblings of pioneer neurons establish the first two axonal pathways to the CNS. Using immunocytochemistry we detected nitric oxide (NO)-induced synthesis of cGMP in the pioneer neurons of the embryonic antenna. A potential source of NO are NADPH-diaphorase-stained epithelial cells close to the basal lamina. To investigate the role of the NO/cGMP signaling system during pathfinding, we examined the pattern of outgrowing pioneer neurons in embryo culture. Pharmacological inhibition of soluble guanylyl cyclase (sGC) and of NO synthase (NOS) resulted in an abnormal pattern of pathway formation in the antenna. Axonogenesis of both pairs of pioneers was inhibited when specific NOS or sGC inhibitors were added to the culture medium; the observed effects include the loss axon emergence as well as retardation of outgrowth, such that growth cones do not reach the CNS. The addition of membrane-permeant cGMP or a direct activator of the sGC enzyme to the culture medium completely rescued the phenotype resulting from the block of NO/cGMP signaling. These results indicate that NO/cGMP signaling is involved in axonal elongation of pioneer neurons in the antenna of the grasshopper.

Structure and development of larval antennae in embryos ofLytta viridanaLeConte (Coleoptera: Meloidae)

At time of hatch (252–264 h at 25 ± 0.5 °C), each antenna in Lytta viridana has three flagellomeres, three extrinsic muscles, and 25 sensilla of five different types, including a large composite sensillum of 19 sensory units on flagellomere II. Each antenna evaginates from epidermis on either side the stomodaeum beginning at 16% of embryogenesis. At 21%, a cell near its apex divides into two pioneer neurons that move into its lumen and project their axons to the brain by 29%. Sensillar stem cells begin to emerge at 23%, those of the appendix within a large embryonic placode and, from 26 to 48%, divide asymmetrically to generate the neurons and accessory cells of each sensillum. Sensillar axonogenesis begins at 34%, the first axons contact the brain at 35%, and antennal glomeruli begin to form within the deutocerebra at 57%. At 35%, the trichogen cell of each sensillum begins to grow out and larval cuticle is deposited about these, beginning at 57%. Upon withdrawal of trichogen cytoplasm from within the appendix at 81%, the dendrites of each sensory unit grow into it and begin to branch. Functional aspects are addressed and the observations compared with the limited information available on embryos of other insects.

Development of wing sensory axons in the central nervous system of Drosophila during metamorphosis

The development of new, adult-specific axonal pathways in the central nervous system (CNS) of insects during metamorphosis is still largely uncharacterized. Here we used axonal labeling with DiI to describe the timing and pattern of growth of sensory axons originating in the wing of Drosophila as they establish their adult projection pattern in the CNS during pupal life. The wing of Drosophila carries a small number of readily identifiable sensory organs (sensilla) whose neurons are located in the periphery and whose axons travel along specific routes within the adult CNS. The neurons are born and undergo axonogenesis in a characteristic order. The order of axon arrival in the CNS appears to be the same as that of their development in the periphery. Within the CNS, the formation of four prominent axon bundles leading to distant termination sites is followed by the formation of a compact axon termination site near the point of wing nerve entry into the CNS. This sensillum-specific pattern persists into adulthood without discernible modification. We also find a small number of axons filled with DiI prior to the formation of the four permanent bundles. We have only been able to fill them for a few hours in early pupal life and therefore consider them to be transient. The bundles of wing sensory axons travel within tracts that contain other axons as well. Using immunocytochemistry, the tracts start to be histologically identifiable at around 12 h after pupariation (AP), and grow substantially as metamorphosis proceeds. Wing sensory neurons are found in the tracts by 18-20 h AP and the full adult pattern is established by 48 h AP. When sensory axons first enter the CNS, they fan out in the region where their appropriate tracts are located, but they do not wander extensively. They quickly form bundles that become increasingly compact over time. Calculations show that the rate of axon extension within the CNS varies from bundle to bundle and is equal to or greater than that of the same axons growing through wing tissue.

Molecular correlates of neuronal specificity in the developing insect nervous system

The development of the nervous system in insects, as in most other higher animals, is characterized by the high degree of precision and specificity with which synaptic connectivity is established. Multiple molecular mechanisms are involved in this process. In insects a number of experimental methods and model systems can be used to analyze these mechanisms, and the modular organization of the insect nervous system facilitates this analysis considerably. Well characterized molecular elements involved in axogenesis are the cell-cell adhesion molecules that underlie selective fasciculation. These are cell-surface molecules that are expressed in a regional and dynamic manner on developing axon fascicles. Secreted molecules also appear to be involved in directing axonal navigation. Nonneuronal cells, such as glia, provide cellular and noncellular substrates that are important pathway cues for neuronal outgrowth. Once outgrowing processes reach their general target regions they make synapses with the appropriate postsynaptic cells. The molecular mechanisms that allow growth cones to recognize their correct target cells are essential for neuronal specificity and are being analyzed in neuromuscular and brain interneuron systems of insects. Candidate synaptic recognition molecules with remarkable and highly restricted expression patterns in the developing nervous system have recently been discovered.

An axon growth associated antigen is also an early marker of neuronal determination

We have previously described the generation of a monoclonal antibody (DSS-3) that binds to all neurons in cockroach embryos at 50% development and to only a small subset of interneurons in the adult nervous system. This developmental stage-specific antigen was observed to reappear in all axotomized adult neurons that were undergoing axonal regeneration. In the present study the time course of the appearance of this growth-associated antigen during embryonic development was determined. Unexpectedly, the antigen was observed to be present in embryonic neurons long before axon growth. In addition, all cells in the CNS neuronal lineage (neuroblasts, ganglion mother cells, and neurons) bind the antibody as soon as they can be morphologically identified. However, the antigen is also transiently present in all neuroepithelial cells at a stage prior to the morphological differentiation of some of them to neuroblasts. Analogous patterns of DSS-3 binding to cells involved in the development of sensory neurons and leg pioneer neurons are observed. The DSS-3 antigen is therefore a very early marker for the capacity of ectodermal epithelial cells to develop along a neuronal lineage.

Developmental modulation of a glial cell-associated glycoprotein, 5B12, in an insect, Acheta domesticus

The expression of an insect (Acheta domesticus) adult glial cell-specific antigen, 5B12 undergoes major changes during development. The 5B12 antigen is detected as early as 20-25% of embryonic development, when immunoreactivity is distributed throughout the periphery, present at the luminal surface of epithelial cells which compose developing limb buds, sensory appendages, and the body cavity. The antigen is also localized on the cell surface of neural elements within commissural tracts in the embryonic CNS. 5B12 is secreted extracellularly in the periphery, where it is associated with the embryonic basal lamina in developing cercal sensory appendages. Luminal surface expression is transient, and disappears by 95% of embryonic development. As development proceeds, 5B12 distribution becomes more restricted, so that in the adult the antigen is predominantly associated with specific glial elements within the nervous system where it occurs as a specialized component of the extracellular matrix. The 5B12 antigen is also associated with discrete central and peripheral fiber tracts. Antigen 5B12 is present in whole embryos and in the adult CNS as a Mr 185-kDa glycoprotein. Distinct carbohydrate moieties with chondroitin sulfate-like properties are situated on the 5B12 epitope. Thus the glia-associated 5B12 macromolecule has the characteristics of a small proteoglycan. Based upon features of its distribution, pattern of spatiotemporal expression, and biochemical properties, it is speculated that 5B12 participates in events related sequentially to the development and the function of the insect nervous system.

Development of Drosophila larval sensory organs: spatiotemporal pattern of sensory neurones, peripheral axonal pathways and sensilla differentiation

The sensilla of Drosophila larval thoracic and abdominal segments appear in a constant temporal sequence during stage 13/14 (9·5–11·5 h) of embryonic development. Those sensilla innervated by more than one dendrite (basiconical sensilla, chordotonal organs, some of the trichoid sensilla and campaniform sensilla) appear earlier than sensilla innervated by a single dendrite (majority of trichoid sensilla and campaniform sensilla). Furthermore, a dorsoventrally directed gradient underlies the sequence in which sensilla of a given type appear. Sensory axons are emitted in the same sequence. Thus, axons of the polyinnervated sensilla appear first. Together with a distinct set of efferent axons they form ‘pioneer tracts’ of the two fascicles of the segmental nerves. Cytodifferentiation of the sensillum cells resembles the development of larval epidermal cells in many aspects. Thus, the sheath processes formed by sensillum accessory cells and the axons formed by sensory neurones develop from processes transiently formed by all cells. During the phase of cuticle secretion, apical portions of the presumptive accessory cells are modified to form the cuticular apparatus responsible for receiving the sensory stimuli. Finally, two sets of subepidermally located cells which differ with respect to their morphology and, probably, their origin envelop somata and axons of the sensory neurones.

Programmed death of peripheral pioneer neurons in the grasshopper embryo

The Ti1 pioneer neurons arise at the distal tip of the metathoracic leg in the grasshopper embryo, and are the first neurons in the limb bud to extend axons to the central nervous system (C. M. Bate (1976) Nature (London) 260, 54-56; H. Keshishian (1980) Dev. Biol. 80, 388-397). By providing a neural pathway along which growth cones of later arising neurons migrate, these pioneer axons establish the route of one of the major nerve trunks in the leg (Keshishian, 1980; R. K. Ho and C. S. Goodman (1982) Nature (London) 297, 404-406; D. Bentley and H. Keshishian (1982) Science 218, 1082-1088). Here, we demonstrate that at the 55-59% stage of development, the two Ti1 pioneer neurons undergo programmed death. The role which these pioneers serve in establishing a nerve route appears to be their only function, and may be important for the normal development of the peripheral nervous system. The Ti1 pioneers provide an example of a previously hypothesized class (J. W. Truman (1984) Annu. Rev. Neurosci. 7, 171-188) of programmed neuron death: obsolete neurons whose function was developmental rather than behavioral.

Axon guidance in cultured wing discs and disc fragments of Drosophila

The sensory neurons of the Drosophila wing differentiate during the initial stages of metamorphosis, appearing in the imaginal wing disc as it everts and flattens. These identifiable neurons arise in a stereotyped sequence, and lay down a specific pattern of axon bundles which travel proximally to the CNS. In several locations, the early arising "pioneer" neurons send axons in the direction of more proximal pioneer neurons, later joining with these to form continuous peripheral nerves. It is possible that distal neurons can contact more proximal neurons by random filopodial search, and use this information to guide axonal outgrowth. To test this "guidepost" hypothesis, everting wing discs were raised in vitro to allow surgical manipulation. Neural outgrowth was largely normal in vitro, though growth of the wing was stunted. If such discs were cut into proximodistal fragments before or at the time of initial axonogenesis, neural outgrowth remained normal: distal axons still grew in the direction of the now missing proximal neurons. Thus, proximal neurons are not necessary for the correct guidance of distal neurons in the developing wing.

Neuron differentiation and axon growth in the developing wing of Drosophila melanogaster

Sensory neurons in the wing of Drosophila originate locally from epithelial cells and send their axons toward the base of the wing in two major bundles, the L1 and L3 nerves. We have estimated the birth times of a number of identified wing sensory neurons using an X-irradiation technique and have followed the appearance of their somata and axons by means of an immunohistochemical stain. These cells become immunoreactive and begin axon growth in a sequence which mirrors the sequence of their birth times. The earliest ones are born before pupariation and begin axonogenesis within 1 to 2 hr after the onset of metamorphosis; the last are born and differentiate some 12 to 14 hr later. The L1 and L3 nerves are formed in sections, with specific neurons pioneering defined stretches of the pathways during the period between 0 and 4 hr after pupariation (AP), and finally joining together around 12 hr AP. By 16 hr AP the adult complement of neurons is present and the adult peripheral nerve pattern has been established. Pathway establishment appears to be specified by multiple cues. In places where neurons differentiate in close proximity to one another, random filopodial exploration followed by axon growth to a neighboring neuron soma might be the major factor leading to pathway construction. In other locations, filopodial contact between neighboring somata does not appear to occur, and axon pathways joining neural neighbors by the most direct route are not established. We propose that in these cases additional factors, including veins which are already present at the time of axonogenesis, influence the growth of axons through non-neural tissues.