Building the central complex of the grasshopper Schistocerca gregaria: temporal topology organizes the neuroarchitecture of the w,x, y,z tracts

The central complex is a brain specific structure involved in multimodal information processing and in coordinating motor behaviour. It possesses a highly organized neuroarchitecture, which is remarkably conserved across insect species. A prominent feature of this neuroarchitecture is the stereotypic projection of axons from clusters of neurons in the pars intercerebralis to the central body via the so-called w, x, y and z tracts. Despite extensive analyses of this neuroarchitecture in adults, little is known about its ontogeny in any insect. In this paper we use the expression pattern of the segment polarity gene engrailed to identify those neuroblasts belonging to the protocerebrum of the early embryonic brain of the grasshopper Schistocerca gregaria. We present a new map for this brain region in which the 95 protocerebral neuroblasts in each hemisphere are organized into seven rows, as they are in the neuromeres of the ventral nerve cord. We then identify a subset of four of these neuroblasts as being the progenitor cells for four clusters of neurons, some of whose axons we show project via discrete tracts (w, x, y, z) into the central complex. These tracts begin to form prior to 39% of embryogenesis. We show further, that the cells from one of these clusters (the Z cluster) are organized according to age, and direct axons topologically according to age into the appropriate z tract. This pattern is repeated in each of the other three clusters, thus establishing a clonally based modular system of fibre tracts consistent with the model proposed for this brain region in the adult.     

An ontogenetic analysis of locustatachykinin-like expression in the central complex of the grasshopper Schistocerca gregaria

We have investigated the ontogenetic basis of locustatachykinin-like expression in a group of cells located in the pars intercerebralis of the grasshopper midbrain. These cells project fibers to the protocerebral bridge and the central body via a characteristic set of fiber bundles called the w, x, y, z tracts. Lineage analyses associate the immunoreactive cells with one of four neuroblasts (termed W, X, Y, Z) in each protocerebral hemisphere of the early embryo. Locustatachykinin is a ubiquitous myotropic peptide among the insects and its expression in the pars intercerebralis begins at approximately 60-65% of embryogenesis. This coincides with the appearance of the columnar neuroarchitecture characteristic of the central body. The number of immunoreactive cells in a given lineage is initially small, increases significantly in later embryogenesis, and attains the adult situation (about 7% of a lineage) in the first larval instar after hatching. Although each neuroblast generates progeny displaying a spectrum of cell body sizes, there is a clear morphological gradient, which reflects birth order within the lineage. Locustatachykinin expressing cells are located stereotypically at or near the tip of their lineage, which an age profile reveals places them amongst the first born progeny of their respective neuroblasts. Although these neuroblasts begin to generate progeny at approximately 25-27% of embryogenesis, their daughter cells remain quiescent with respect to locustatachykinin expression for over 30% of embryogenesis.     

Fascicle switching generates a chiasmal neuroarchitecture in the embryonic central body of the grasshopper Schistocerca gregaria

The central body is a prominent neuropilar structure in the midbrain of the grasshopper and is characterized by a fan-shaped array of fiber columns, which are part of a chiasmal system linking anterior and posterior commissures. These columns are established during embryogenesis and comprise axons from cell clusters in the pars intercerebralis, which project to the central body via the so-called w, x, y, z tracts. Up to mid-embryogenesis the primary axon scaffold in both the brain and ventral nerve cord comprises a simple orthogonal arrangement of commissural and longitudinal fiber pathways. No chiasmata are present and this pattern is maintained during subsequent development of the ventral nerve cord. In the midbrain, individual axons entering the commissural system from each of the w, x, y, z tracts after mid-embryogenesis (55%) are seen to systematically de-fasciculate from an anterior commissure and re-fasciculate with another more posterior commissure en route across the midline, a feature we call "fascicle switching". Since the w, x, y, z tracts are bilaterally symmetrical, fascicle switching generates chiasmata at stereotypic locations across the midbrain. Choice points for leaving and entering fascicles mark the anterior and posterior positions of each future column. As the midbrain neuropil expands, the anterior and posterior groups of commissures condense, so that the chiasmata spanning the widening gap between them become progressively more orthogonally oriented. A columnar neuroarchitecture resembling that of the adult central body is already apparent at 70% of embryogenesis.     

Embryonic development of the pars intercerebralis/central complex of the grasshopper

 We have studied the embryonic development of the pars intercerebralis/central complex in the brain of the grasshopper using immunocytochemical and histochemical techniques. Expression of the cell-surface antigen lachesin reveals that the neuroblasts of the pars intercerebralis first differentiate from the neuroectoderm at around 26% of embryogenesis. Differentiation of medial and lateral neuroblasts occurs first. By the 28% stage a more or less uniform sheet of 20 neuroblasts has formed. As a result of both cell proliferation and cell translocation, the pars intercerebralis proliferative cluster in each hemisphere expands so that at 30% the most medial neuroblasts lie apposed at the midline. We followed the further development of the pars intercerebralis of each brain hemisphere using bromo-deoxy-uridine incorporation and osmium-ethyl-gallate staining. Within the pars intercerebralis itself, the neuroblasts redistribute into discrete subsets. The neuroblasts of each subset generate clusters of progeny which extend in a stereotypic, subset-specific direction in the brain. We have used this feature to identify one subset of four neuroblasts as being the likely progenitor cells for four clusters of embryonic neurons (W, X, Y, Z) which develop at around 55% of embryogenesis. We show that these progeny project axons via four discrete fascicles (w, x, y, z) into the embryonic central complex. At the single cell level, Golgi impregnation reveals that the axons from these neighbouring cell clusters remain discrete, and those from the same cluster tightly fasciculated, as they project into the central complex, consistent with a modular organization for this brain region. Received: 16 June 1997 / Accepted: 25 June 1997     

Embryonic development of the insect central complex: insights from lineages in the grasshopper and Drosophila

The neurons of the insect brain derive from neuroblasts which delaminate from the neuroectoderm at stereotypic locations during early embryogenesis. In both grasshopper and Drosophila, each developing neuroblast acquires an intrinsic capacity for neuronal proliferation in a cell autonomous manner and generates a specific lineage of neural progeny which is nearly invariant and unique. Maps revealing numbers and distributions of brain neuroblasts now exist for various species, and in both grasshopper and Drosophila four putatively homologous neuroblasts have been identified whose progeny direct axons to the protocerebral bridge and then to the central body via an equivalent set of tracts. Lineage analysis in the grasshopper nervous system reveals that the progeny of a neuroblast maintain their topological position within the lineage throughout embryogenesis. We have taken advantage of this to study the pioneering of the so-called w, x, y, z tracts, to show how fascicle switching generates central body neuroarchitecture, and to evaluate the roles of so-called intermediate progenitors as well as programmed cell death in shaping lineage structure. The novel form of neurogenesis involving intermediate progenitors has been demonstrated in grasshopper, Drosophila and mammalian cortical development and may represent a general strategy for increasing brain size and complexity. An analysis of gap junctional communication involving serotonergic cells reveals an intrinsic cellular organization which may relate to the presence of such transient progenitors in central complex lineages.     

Cell death shapes embryonic lineages of the central complex in the grasshopper Schistocerca gregaria

We have investigated cell death in identified lineages of the central complex in the embryonic brain of the grasshopper Schistocerca gregaria. Progeny from these lineages lie in the pars intercerebralis and direct projections to the protocerebral bridge and then the central body via the w, x, y, z tracts. Osmium‐ethyl gallate staining reveals pycnotic cells exclusively in cortical regions, and concentrated specifically within the lineages of the W, X, Y, Z neuroblasts. Minimal cell death occurs in a sporadic, nonpatterned manner, in other protocerebral regions. Immunohistochemistry reveals pycnotic cells express the enzyme cleaved Caspase‐3 in their cytoplasm and are therefore undergoing programmed cell death (apoptosis). The number of pycnotic bodies in lineages of the pars intercerebralis varies with age: small numbers are present in the Y, Z lineages early in embryogenesis (42%), the number peaks at 67–80%, and then declines and disappears late in embryogenesis. Cell death may encompass up to 20% of a lineage at mid‐embryogenesis. Peak cell death occurs shortly after maximum neurogenesis in the Y, Z lineages, and is maintained after neurogenesis has ceased in these lineages. Cell death within a lineage is patterned. Apoptosis is more pronounced among older cells and almost absent among younger cells. This suggests that specific subsets of progeny will be culled from these lineages, and we speculate about the effect of apoptosis on the biochemical profile of such lineages. J. Morphol. 271:949–959, 2010. © 2010 Wiley‐Liss, Inc.     

Patterns of dye coupling involving serotonergic neurons provide insights into the cellular organization of a central complex lineage of the embryonic grasshopper Schistocerca gregaria

All eight neuroblasts from the pars intercerebralis of one protocerebral hemisphere whose progeny contribute fibers to the central complex in the embryonic brain of the grasshopper Schistocerca gregaria generate serotonergic cells at stereotypic locations in their lineages. The pattern of dye coupling involving these neuroblasts and their progeny was investigated during embryogenesis by injecting fluorescent dye intracellularly into the neuroblast and/or its progeny in brain slices. The tissue was then processed for anti-serotonin immunohistochemistry. A representative lineage, that of neuroblast 1-3, was selected for detailed study. Stereotypic patterns of dye coupling were observed between progeny of the lineage throughout embryogenesis. Dye injected into the soma of a serotonergic cell consistently spread to a cluster of between five and eight neighboring non-serotonergic cells, but never to other serotonergic cells. Dye injected into a non-serotonergic cell from such a cluster spread to other non-serotonergic cells of the cluster, and to the immediate serotonergic cell, but never to further serotonergic cells. Serotonergic cells tested from different locations within the lineage repeat this pattern of dye coupling. All dye coupling was blocked on addition of an established gap junctional blocker (n-heptanol) to the bathing medium. The lack of coupling among serotonergic cells in the lineage suggests that each, along with its associated cluster of dye-coupled non-serotonergic cells, represents an independent communicating pathway (labeled line) to the developing central complex neuropil. The serotonergic cell may function as the coordinating element in such a projection system.     

Organization of a midline proliferative cluster in the embryonic brain of the grasshopper

We have studied a group of midline cells in the embryonic brain of the grasshopper by using immunocytochemical and intracellular dye injection techniques. This cluster of midline cells differentiates between the pars intercerebralis lobes of the protocerebrum during early embryogenesis, and is composed of putative midline progenitors as well as neuronal and glial cells. Annulin immunoreactive glial processes surround the borders of the midline cell cluster and also form a network of processes extending from there to the borders of proliferative clusters in the brain hemispheres. Among the cells that derive from the midline cluster are two bilaterally symmetrical pairs of identified primary commissure pioneer neurons. By navigating along the glial bound borders of the midline proliferative cluster, the axons of these pioneers establish an initial axonal bridge across the brain midline. This analysis identifies a glial-bound midline proliferative cluster in the brain and shows that neuronal and glial cells of this cluster are closely associated with neurons pioneering the primary brain commissure. Comparable features of midline cells in the ventral ganglia and similarities to other proliferative clusters in the brain hemispheres are discussed.     

Timelines in the insect brain: fates of identified neural stem cells generating the central complex in the grasshopper Schistocerca gregaria

This study employs labels for cell proliferation and cell death, as well as classical histology to examine the fates of all eight neural stem cells (neuroblasts) whose progeny generate the central complex of the grasshopper brain during embryogenesis. These neuroblasts delaminate from the neuroectoderm between 25 and 30 % of embryogenesis and form a linear array running from ventral (neuroblasts Z, Y, X, and W) to dorsal (neuroblasts 1-2, 1-3, 1-4, and 1-5) along the medial border of each protocerebral hemisphere. Their stereotypic location within the array, characteristic size, and nuclear morphologies, identify these neuroblasts up to about 70 % of embryogenesis after which cell shrinkage and shape changes render progressively more cells histologically unrecognizable. Molecular labels show all neuroblasts in the array are proliferative up to 70 % of embryogenesis, but subsequently first the more ventral cells (72–75 %), and then the dorsal ones (77–80 %), cease proliferation. By contrast, neuroblasts elsewhere in the brain and optic lobe remain proliferative. Apoptosis markers label the more ventral neuroblasts first (70–72 %), then the dorsal cells (77 %), and the absence of any labeling thereafter confirms that central complex neuroblasts have exited the cell cycle via programmed cell death. Our data reveal appearance, proliferation, and cell death proceeding as successive waves from ventral to dorsal along the array of neuroblasts. The resulting timelines offer a temporal blueprint for building the neuroarchitecture of the various modules of the central complex.     

Proliferative cell types in embryonic lineages of the central complex of the grasshopper Schistocerca gregaria

The central complex of the grasshopper Schistocerca gregaria develops to completion during embryogenesis. A major cellular contribution to the central complex is from the w, x, y, z lineages of the pars intercerebralis, each of which comprises over 100 cells, making them by far the largest in the embryonic protocerebrum. Our focus has been to find a cellular mechanism that allows such a large number of cell progeny to be generated within a restricted period of time. Immunohistochemical visualization of the chromosomes of mitotically active cells has revealed an almost identical linear array of proliferative cells present simultaneously in each w, x, y, z lineage at 50% of embryogenesis. This array is maintained relatively unchanged until almost 70% of embryogenesis, after which mitotic activity declines and then ceases. The array is absent from smaller lineages of the protocerebrum not associated with the central complex. The proliferative cells are located apically to the zone of ganglion mother cells and amongst the progeny of the neuroblast. Comparisons of cell morphology, immunoreactivity (horseradish peroxidase, repo, Prospero), location in lineages and spindle orientation have allowed us to distinguish the proliferative cells in an array from neuroblasts, ganglion mother cells, neuronal progeny and glia. Our data are consistent with the proliferative cells being secondary (amplifying) progenitors and originating from a specific subtype of ganglion mother cell. We propose a model of the way that neuroblasts, ganglion mother cells and secondary progenitors together produce the large cell numbers found in central complex lineages.     

Commissure formation in the embryonic insect brain

The primary axon scaffold of the insect brain is established early in embryogenesis and comprises a preoral protocerebral commissure, a postoral tritocerebral commissure and longitudinal fiber pathways linking the two. In both grasshopper and fly its form is approximately orthogonal and is centered around the stomodeum. We show how pioneer fibers from the protocerebrum and tritocerebrum cross the brain midline directly via their respective commissures. The deutocerebrum, however, lacks its own commissure and we describe how deutocerebral pioneers circumnavigate the gut to cross the midline either via the protocerebral commissure or the tritocerebral commissure. In contrast to all other commissures of the central nervous system, the protocerebral commissure persists, albeit in reduced form, in the commissureless mutation in the fly. Besides the com gene, a further, as yet unidentified, mechanism must regulate this commissure. The formation of the tritocerebral commissure involves labial, a member of the Hox gene group. Genetic rescue experiments in labial mutants reveal that the formation of this commissure can be rescued by all other Hox genes except Abdominal-B. However, only in the labial and Deformed null mutants are the commissures associated with the respective expression domains (tritocerebral, mandibular, respectively) absent. This suggests that the molecular mechanisms regulating postoral brain commissure formation are distinct from those in the neuromeres of the ventral nerve cord.     

Postembryonic development of astrocyte-like glia of the central complex in the grasshopper Schistocerca gregaria

Central complex modules in the postembryonic brain of the grasshopper Schistocerca gregaria are enveloped by Repo-positive/glutamine-synthetase-positive astrocyte-like glia. Such cells constitute Rind-Neuropil Interface glia. We have investigated the postembryonic development of these glia and their anatomical relationship to axons originating from the w, x, y, z tract system of the pars intercerebralis. Based on glutamine synthetase immunolabeling, we have identified four morphological types of cells: bipolar type 1 glia delimit the central body but only innervate its neuropil superficially; monopolar type 2 glia have a more columnar morphology and direct numerous gliopodia into the neuropil where they arborize extensively; monopolar type 3 glia are found predominantly in the region between the noduli and the central body and have a dendritic morphology and their gliopodia project deeply into the central body neuropil where they arborize extensively; multipolar type 4 glia link the central body neuropil with neighboring neuropils of the protocerebrum. These glia occupy type-specific distributions around the central body. Their gliopodia develop late in embryogenesis, elongate and generally become denser during subsequent postembryonic development. Gliopodia from putatively type 3 glia within the central body have been shown to lie closely apposed to individual axons of identified columnar fiber bundles from the w, x, y, z tract system of the central complex. This anatomical association might offer a substrate for neuron/glia interactions mediating postembryonic maturation of the central complex.     

Evidence that the primary brain commissure is pioneered by neurons with a peripheral-like ontogeny in the grasshopper Schistocerca gregaria

The commissures represent a major neuroarchitectural feature of the central nervous system of insects and vertebrates alike. The adult brain of the grasshopper comprises 72 such commissures, the first of which is established in the protocerebral midbrain by three sets of pioneer cells at around 30% of embryogenesis. These pioneers have been individually identified via cellular, molecular and intracellular dye injection techniques. Their ontogenies, however, remain unclear. The progenitor cells of the protocerebral midbrain are shown via Annulin immunocytochemistry to be compartmentalized, belonging either to the protocerebral hemispheres or the so-called median domain. Serial reconstructions based on bromodeoxyuridine incorporation confirm that their lineages do not intermingle. Dye injection into progenitor cells and progeny confirms this compartmentalization, and reveals that none of the pioneers are associated with a lineage of cells deriving from a protocerebral neuroblast or midline precursor. Immunocytochemical data as well as dye injection into identified pioneers over several developmental stages indicate that they differentiate directly from epithelial cells, but not from classical progenitor cells. That the commissural pioneers of the protocerebrum represent modified epithelial cells involves a different ontogeny to that described for pioneers in the ventral nerve cord, but parallels that of pioneer neurons of the peripheral nervous system.     

Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

The expression pattern of the engrailed protein was studied in neuroblasts which delaminate at the border of the protocerebrum and antennal lobe of the deutocerebrum in the early embryonic brain of the grasshopper. The antennal lobe is a complex structure comprising both glomerular and non-glomerular components, a cellular organization which distinguishes it from the striate-like neuropil comprising the remainder of the deutocerebrum. Early in embryogenesis engrailed expression in the protocerebrum is restricted to a compact block of neuroblasts located at its interface with the antennal lobe. Subsequently engrailed expression in these cells disappears in a stepwise manner from anterior to posterior so that by 37% of embryogenesis only a single row of three engrailed positive neuroblasts and their progeny remains. Contemporaneously engrailed expression reappears in a group of more anterior progeny deriving from neuroblasts which are no longer immunoreactive. The three remaining engrailed positive neuroblasts then become separated from their non-immunoreactive neighbours by an invagination of the perineurium called the lateral cleft and come to lie completely within the developing antennal lobe. These cells then direct columns of immunoreactive progeny centrifugally towards the centre of the lobe. Such a protocerebral contribution to the antennal lobe suggests that the evolution and ontogeny of this brain region need to be reconsidered.     

Neurons projecting to the retrocerebral complex of the adult blow fly, Protophormia terraenovae

Anatomical study of neurons projecting to the retrocerebral complex of the adult blow fly, Protophormia terraenovae, was done by NiCl2 filling and immunocytochemistry. Retrograde filling through the cardiac-recurrent nerve labeled three groups of neurons in the brain/subesophageal ganglion: (1) paramedial clusters of the pars intercerebralis, (2) neurons in each pars lateralis, and (3) neurons in the subesophageal ganglion. The pars intercerebralis neurons send prominent axons into the median bundle and exit from the brain via the contralateral nervus corporis cardiaci. Based on the projection pattern, two types of the pars lateralis neurons can be distinguished: the most lateral pairs of neurons contralaterally extend through the posterior lateral tract and the remainder ipsilaterally extend through the posterior lateral tract. The neurons in the subesophageal ganglion run through the contralateral nervus corporis cardiaci. The dendritic arborization of the pars intercerebralis and pars lateralis neurons is restricted to the superior protocerebral neuropil and to the anterior neuropil of the subesophageal ganglion where the neurons in the subesophageal ganglion also project. Retrograde filling from the corpus allatum indicated that the pars lateralis neurons and a few pars intercerebralis neurons project to the corpus allatum, but that the neurons in the subesophageal ganglion do not. Orthograde filling from the pars intercerebralis and staining by paraldehyde-thionin/paraldehyde-fuchsin indicated that the pars intercerebralis neurons project primarily to the corpus cardiacum/hypocerebral ganglion complex. Immunostaining with a polyclonal antiserum against diapause hormone, a member of the FXPRLamide family, suggests that some of the subesophageal ganglion neurons contain FXPRLamide-like peptides.     

C. elegans dystroglycan coordinates responsiveness of follower axons to dorsal/ventral and anterior/posterior guidance cues

Neural development in metazoans is characterized by the establishment of initial process tracts by pioneer axons and the subsequent extension of follower axons along these pioneer processes. Mechanisms governing the fidelity of follower extension along pioneered routes are largely unknown. In C. elegans, formation of the right angle‐shaped lumbar commissure connecting the lumbar and preanal ganglia is an example of pioneer/follower dynamics. We find that the dystroglycan ortholog DGN‐1 mediates the fidelity of follower lumbar commissure axon extension along the pioneer axon route. In dgn‐1 mutants, the axon of the pioneer PVQ neuron faithfully establishes the lumbar commissure, but axons of follower lumbar neurons, such as PVC, frequently bypass the lumbar commissure and extend along an oblique trajectory directly toward the preanal ganglion. In contrast, disruption of the UNC‐6/netrin guidance pathway principally perturbs PVQ ventral guidance to pioneer the lumbar commissure. Loss of DGN‐1 in unc‐6 mutants has a quantitatively similar effect on follower axon guidance regardless of PVQ axon route, indicating that DGN‐1 does not mediate follower/pioneer adhesion. Instead, DGN‐1 appears to block premature responsiveness of follower axons to a preanal ganglion‐directed guidance cue, which mediates ventral‐to‐anterior reorientation of lumbar commissure axons. Deletion analysis shows that only the most N‐terminal DGN‐1 domain is required for these activities. These studies suggest that dystroglycan modulation of growth cone responsiveness to conflicting guidance cues is important for restricting follower axon extension to the tracts laid down by pioneers. © 2011 Wiley Periodicals, Inc. Develop Neurobiol, 2012     

Ontogeny of identified cells from the median domain in the embryonic brain of the grasshopper Schistocerca gregaria

In this paper, we propose an ontogeny for previously identified cells from the median domain in the midline of the embryonic brain of the grasshopper Schistocerca gregaria. The so-called lateral cells (LCs) are characteristically located laterally within the median domain at its border with the protocerebral hemispheres. The LC occurs singly and can be identified in the early embryo on the basis of their expression of the cell surface lipocalin Lazarillo. Using immunocytochemical, dye injection, electron microscopical and histological methods, we show that these LC are neurons and derive as postmitotic cells directly from the epithelium of the median domain. Further, they and the other identified cells of the median domain such as the protocerebral commissure pioneers (PCP), co-express the Mes-3 antigen, consistent with a derivation from the mesectodermal germ layer of the embryo. Subsequent to axogenesis, electron microscopy reveals that these Mes-3-expressing LC fasciculate with the co-expressing PCPs within the developing protocerebral commissure. We present a model for the origin of all these cells based on histological data and bromodeoxyuridine incorporation. The model suggests a delamination of cells from the mesectoderm followed by a migration to their ultimate sites within the median domain.     

Neuroanatomy of the retrocerebral complex,in particular the pars intercerebralis and partes laterales in the cockroach Diploptera punctata eschscholtz (Dictyoptera : Blaberidae)

The retrocerebral complex of the cockroach, Diploptera punctata (Dictyoptera Blaberidae) was studied by means of scanning electron microscopy and rapid retrograde axonal diffusion of NiCl,. The corpora cardiaca (CC) are joined to the brain via nervi corporis cardiaci I, II and III, and to each other via a cardiacal-commissure organ. Corpora allata (CA) are joined to the CC via stout nervi corporis allati I (NCA I) and to the postallatal nerves (PAN) via projections of the CC within the CA sheath.Exposure of the retrocerebral complex, after the NCA I were sectioned, to 500 mM NiCl₂ for 0.5 min resulted in the backfilling of neurons with axons in the CA/CC. A total of 60 somata were observed in the pars intercerebralis (PI) and 25 in the pars lateralis (PL) in each protocerebral hemisphere. When the PAN were backfilled, 20 somata in the PI and 5 in each PL were observed. Thus it is possible that a total of 40 somata in the PI and 20 in the PL on each side of the brain send axons to the CA or beyond.     

Neuronal pathways from the dorsal acelli of the house cricket,Acheta domesticus

Each ocellar nerve in the house cricket Acheta domesticus contains giant nerve fibers of 10-15 μ diameter, characterized in Golgi Cox preparations by a single row of short collaterals which runs along nearly the entire length of a fiber. Numerous long collaterals are given off by thin fibers in the ocellar nerve; medium-size fibers give off relatively few collaterals. The lateral ocellar tracts extend posteriorly through the dorsal protocerebrum, crossing the protocerebral bridge dorsally. The smaller median ocellar tract runs more ventrally through the pars intercerebralis; posterior to the bridge its fibers turn out toward the lateral nerves. Golgi and cobalt preparations reveal branching of giant and mediu_-size ocellar fibers posterior to the bridge at two levels, forming bilateral regions of ocellar neuropile. No ocellar processes appear to be given off to the corpora pedunculata, centra! body, nervi corporis cardiaci, antenna! lobes, or circumesophageal connectives; it is uncertain whether ocellar collaterals extend into the protocerebral bridge or optic lobes. Cell bodies of giant and medium-sized fibers are located in the pars intercerebralis.     

Morphology of identified neurosecretory and non-neurosecretory cells in the cockroach pars intercerebralis

Electrophysiologically identified cells of the cockroach pars intercerebralis (Periplaneta americana) were injected with the dye Lucifer Yellow for morphological examination and with horseradish peroxidase for ultrastructural marking. In addition to this, uninjected cells were also studied to elaborate the findings from the injected material. The two electrophysiologically distinct classes of cells (type I and type II) correspond to two distinct morphological and ultrastructural classes. Type I cells are the medial neurosecretory cells of the pars intercerebralis, which project their axons to the retrocerebral neuro-hemal complex. Their cell bodies have a mean diameter of 17 microns, and they contain neurosecretory granules 200 nm in diameter. Arborizations emanate from the axon in the anterior part of the protocerebral neuropil. The type II cell bodies are larger (38 microns in diameter). Their axons project into the contralateral circumesophageal connective. These cells were usually multipolar, having somatic arborizations in the anterior portocerebral neuropil. The cell bodies contain vesicles 40 nm in diameter, numerous trophospongia, and a multi-layered glial envelope.