大鼠脊神经节内交感神经支配的荧光组织化学与HRP示踪观察

本研究应用乙醛酸诱发儿茶酚胺（ＣＡ）荧光技术观察大鼠肾上腺素（ＮＡ）能神经在脊神经节内的分布;并应用ＨＲＰ顺、逆行追踪技术对脊神经节内ＮＡ能神经纤维的起源及其与脊神经节神经元的关系进行了探讨。荧光组织化学观察发现、有些神经节神经元胞体周围分布有带膨体的ＮＡ能神经末梢;有的紧密围绕脊神经节细胞——卫星细胞复合体。颈上交感神经节内注射霍乱毒素Ｂ亚单位结合ＨＲＰ（ＣＢ┐ＨＲＰ）,在同侧Ｃ３～６节段脊神经节内可见标记的点状纤维末梢紧邻于节细胞旁。Ｔ１１～Ｌ２节段脊神经节内注射ＨＲＰ后,在同侧椎旁交感链（Ｔ９～Ｌ１）内可见标记的交感节后神经元胞体。上述实验结果表明,交感节后神经元发出节后纤维可直接到达脊神经节内,与节细胞发生接触。本研究提示、交感神经在脊神经节水平可能参与躯体初级传入信息的调制     

Blastomere ablation and the developmental origin of identified monoamine-containing neurons in the leech

Ablation of different identifiable blastomeres of the early embryo of the leech Helobdella triserialis was found to lead to the absence of different sets of segmentally iterated monoamine-containing neurons in subsequent development. Thus the ablation of one of the paired N ectoteloblasts leads to the absence of one member of each of the three bilateral pairs of serotonin-containing neurons (one of which is the Retzius cell) from each segmental ganglion. The ablation of one of the paired OP blastomeres (precursors of the paired O and P ectoteloblasts) leads to the absence of one member of each of the two bilateral pairs of lateral dopamine-containing neurons that lie in the body wall of each segment. And the ablation of one of the paired Q ectoteloblast leads to the absence of one member of the bilateral pair of medial dopamine-containing neurons that lie in the body wall of each segment. These results suggest that each of these sets of monoamine-containing neurons is derived from a particular blastomere. Upon ablation of that blastomere the set does not develop from any other source.     

Initiation of swimming activity by trigger neurons in the leech subesophageal ganglion

In papers I and II of this series, we described two pairs of interneurons, Tr1 and Tr2, in the leech subesophageal ganglion which can trigger swimming activity in the isolated central nervous system (CNS). In this paper, we describe sensory inputs to these trigger neurons from previously identified mechanosensory neurons. We found that: Weak mechanical stimulation (stroking) of a body wall flap attached to a segmental ganglion in an otherwise isolated CNS excites the contralateral Tr1 slightly. Strong mechanical stimulation (pinching) of a mid-body wall flap evokes a burst of impulses in the contralateral Tr1. For both means of stimulation the effects on the ipsilateral Tr1 and on the Tr2 cell pair were much weaker. Stroking a body wall flap attached to the head ganglion (supra- and subesophageal ganglia) in an otherwise isolated CNS excites both Tr1s and both Tr2s, although the effect is weaker for the Tr2s. Pinching strongly excites both trigger neurons bilaterally. Pressure and nociceptive mechanosensory neurons (P and N cells) in the subesophageal ganglion and the first segmental ganglion appear to make direct excitatory synapses with the contralateral Tr1 and Tr2. Mechanosensory interactions with the ipsilateral trigger neurons appear to be indirect. Functional inactivation of Tr1 by hyperpolarization does not prevent swim initiation either by weak mechanical stimulation of a body-wall flap or by intracellular stimulation of P cells.2+ We conclude that the trigger neurons, Tr1 and Tr2, provide an excitatory pathway by which mechanosensory stimulation can initiate leech swimming activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

GAS CHROMATOGRAPHIC–MASS SPECTROMETRIC ANALYSIS OF BIOGENIC AMINES IN IDENTIFIED NEURONS AND TISSUES OF HIRUDO MEDICINALIS

—Previous workers have reported that the colossal cells of Retzius in the segmental ganglia of the medicinal leech contain about 2.3 pmol of 5-hydroxytryptamme (5-HT)/cell body. We verify the identify of 5-HT in the Retzius cells by gas chromatography-mass spectrometry and derive concentrations of 1.3–4.1 pmol/neuron by analyses of eight individually dissected Retzius cell bodies. The Retzius cell bodies contain about 30% of the 5-HT in each ganglion. An average of 25 pmol 5-HT/mg tissue, a concentration about 500 times lower than that in the Retzius cell, was found in the fibrous, pigmented tissue surrounding the leech nervous system. We could not detect γ-aminobutyric acid, octopamine, dopamine or norepinephrine in the Retzius cells, in the pigmented tissue, or, with the possible exception of dopamine (±0.4 pmol/ganglion), in whole ganglia. Furthermore, we could not detect 5-HT in pooled samples of 100 non-chromaffin control neurons.     

Formation and specification of neurons during the development of the leech central nervous system

In the leech embryo, neurogenesis takes place within the context of a stereotyped cell lineage. The prospective germ layers are formed during the early cleavage divisions by the reorganization and segregation of circumscribed domains within the cytoplasm of the fertilized egg. The majority of central neurons arise from the ectoderm, and central neuroblasts are distributed throughout both the length and width of each ectodermal hemisegment. Much of the segmental ganglion arises from medial neuroblasts, but there are also lateral ectodermal neuroblasts and mesodermal neuroblasts that migrate into the nascent ganglion from peripheral sites of origin. Some of these migratory cells are committed to neurogenesis prior to reaching their central destination. In addition, the leech embryo exhibits a secondary phase of neurogenesis that is restricted to the two sex segment ganglia. Secondary neurogenesis requires that a mitogenic or trophic signal be conveyed from the peripherally located male sex organ to a particular set of centrally located neuroblasts, apparently via already differentiated central neurons that innervate the sex organ. The differential specification of neuronal phenotypes within the leech central nervous system occurs in multiple steps. Some aspects of a neuron's identity are already specified at the time of its terminal cell division and would seem to involve the lineal inheritance of developmental commitments made by one of the neuron's progenitors. This lineage-based identity can then be modified by interactions between the postmitotic neuron and other neurons or non-neuronal target cells encountered during its terminal differentiation. © 1995 John Wiley & Sons, Inc.     

Convergence of mechanosensory inputs onto neuromodulatory serotonergic neurons in the leech

By the frequency-dependent release of serotonin, Retzius neurons in the leech modulate diverse behavioral responses of the animal. However, little is known about how their firing pattern is produced. Here we have analyzed the effects of mechanical stimulation of the skin and intracellular stimulation of mechanosensory neurons on the electrical activity of Retzius neurons. We recorded the electrical activity of neurons in ganglia attached to their corresponding skin segment by segmental nerve roots, or in isolated ganglia. Mechanosensory stimulation of the skin induced excitatory synaptic potentials (EPSPs) and action potentials in both Retzius neurons in a ganglion. The frequency and duration of responses depended on the strength and duration of the skin stimulation. Retzius cells responded after T and P cells, but before N cells, and their sustained responses correlated with the activity of P cells. Trains of five impulses at 10 Hz in every individual T, P, or N cell in isolated ganglia produced EPSPs and action potentials in Retzius neurons. Responses to T cell stimulation appeared after the first impulse. In contrast, the responses to P or N cell stimulation appeared after two or more presynaptic impulses and facilitated afterward. The polysynaptic nature of all the synaptic inputs was shown by blocking them with a high calcium/magnesium external solution. The rise time distribution of EPSPs produced by the different mechanosensory neurons suggested that several interneurons participate in this pathway. Our results suggest that sensory stimulation provides a mechanism for regulating serotonin-mediated modulation in the leech.     

Developmental interdeterminacy in embryos of the leech Helobdella triserialis

In embryonic development of the leech Helobdella triserialis, each of the four paired positionally identifiable, ectodermal teloblasts (N, O, P, and Q) generates a bandlet of blast cell progeny that merges with ipsilateral bandlets into a germinal band. Left and right germinal bands coalesce into the germinal plate which gives rise to the segmental tissues of the leech and wherein the progeny of each teloblast generate a characteristic pattern of epidermal and neuronal cells. Experiments reported here show that the positionally identified O teloblast sometimes generates the P pattern and vice versa. The reversal of these teloblasts' generative identities was shown to correspond to the formation of chiasmata by their blast cell bandlets, so that the positions of their bandlets in the germinal band are reversed as well. Thus it is the position of the bandlet in the germinal band, rather than the position of the parent teloblast, which correlates with the fate of o and p blast cells. Moreover, two types of ablation experiments have shown that, in the absence of generative P teloblast progeny, those cells which would normally generate the O pattern take on a new fate and give rise to the P pattern in the nervous system, both at the gross pattern level in the segmental ganglia, and at the level of identified neurons in the peripheral nervous system. If related, these phenomena suggest that the O and P teloblasts, which derive from the symmetric cleavage of the OP proteloblasts, have a common developmental pluripotency. And in that case, the fates of their progeny are determined hierarchically on the basis of relative position in the nascent germinal band, with P-type fate being preferred.     

Horseradish peroxidase localization of sympathetic postganglionic and parasympathetic preganglionic neurons innervating the monkey heart

The localization of the sympathetic postganglionic and parasympathetic preganglionic neurons innervating the monkey heart were investigated through retrograde axonal transport with horseradish peroxidase (HRP). HRP (4 mg or 30 mg) was injected into the subepicardial and myocardial layers in four different cardiac regions. The animals were euthanized 84-96 hours later and fixed by paraformaldehyde perfusion via the left ventricle. The brain stem and the paravertebral sympathetic ganglia from the superior cervical, middle cervical, and stellate ganglia down to the T9 ganglia were removed and processed for HRP identification. Following injection of HRP into the apex of the heart, the sinoatrial nodal region, or the right ventricle, HRP-labeled sympathetic neurons were found exclusively in the right superior cervical ganglion (64.8%) or in the left superior cervical ganglion (35%). Fewer labeled cells were found in the right stellate ganglia. After HRP injection into the left ventricle, labeled sympathetic cells were found chiefly in the left superior cervical ganglion (51%) or in the right superior cervical ganglion (38.6%); a few labeled cells were seen in the stellate ganglion bilaterally and in the left middle cervical ganglion. Also, in response to administration of HRP into the anterior part of the apex, anterior middle part of the right ventricle, posterior upper part of the left ventricle, or sinoatrial nodal region, HRP-labeled parasympathetic neurons were found in the nucleus ambiguus on both the right (74.8%) and left (25.2%) sides. No HRP-labeled cells were found in the dorsal motor nucleus of the vagus on either side.     

Pressure sensation by an anterior pagoda neuron population distributed in multiple ganglia in the leech, Whitmania pigra

Sensory processing of pressure signals in the central nervous system of the leech, Whitmania pigra, was studied through the interaction between pressure sensory neurons and anterior pagoda neurons. The responses of anterior pagoda neurons to one pulse or a train of pulses in pressure sensory neurons were characterized by the latency and amplitude of excitatory postsynaptic potentials. Here we show that each pressure sensory neuron is able to activate all the anterior pagoda neurons throughout the leech central nervous system. The response patterns of all anterior pagoda neurons were appropriate to the pressure location: in the longitudinal direction the anterior pagoda neuron further away from the pressure sensory neuron had a smaller response with longer latency; inside each ganglion, the anterior pagoda neuron on the contralateral side had a larger response with shorter latency than that on the ipsilateral side. All anterior pagoda neurons excited by pressure sensory neurons comprised a parallel system in which each anterior pagoda neuron was independent from the others. The location information of pressure stimuli was represented through the response of all 40 anterior pagoda neurons covering the whole leech body with a specific pattern of latency and amplitude.     

Control of leech swimming activity by the cephalic ganglia

We investigated the role played by the cephalic nervous system in the control of swimming activity in the leech, Hirudo medicinalis, by comparing swimming activity in isolated leech nerve cords that included the head ganglia (supra- and subesophageal ganglia) with swimming activity in nerve cords from which these ganglia were removed. We found that the presence of these cephalic ganglia had an inhibitory influence on the reliability with which stimulation of peripheral (DP) nerves and intracellular stimulation of swim-initiating neurons initiated and maintained swimming activity. In addition, swimming activity recorded from both oscillator and motor neurons in preparations that included head ganglia frequently exhibited irregular bursting patterns consisting of missed, weak, or sustained bursts. Removal of the two head ganglia as well as the first segmental ganglion eliminated this irregular activity pattern. We also identified a pair of rhythmically active interneurons, SRN1, in the subesophageal ganglion that, when depolarized, could reset the swimming rhythm. Thus the cephalic ganglia and first segmental ganglion of the leech nerve cord are capable of exerting a tonic inhibitory influence as well as a modulatory effect on swimming activity in the segmental nerve cord.     

Similarities and differences in the structure of segmentally homologous neurons that control the hearts in the leech,Hirudo medicinalis

Summary The neural circuit that controls the hearts in the leech comprises an ensemble of synaptically interconnected cardiac motor neurons (HE cells) and cardiac interneurons (HN cells). Both the HE cells and the HN cells constitute segmentally homologous sets. We have investigated the structure of these neurons by iontophoretic injection of Lucifer Yellow dye.Bilateral pairs of HE cells have been identified in segmental ganglia 3–19 of the nerve cord. Their structure was found to be nearly identical from ganglion to ganglion and from animal to animal.Bilateral pairs of HN cells have been identified in segmental ganglia 1–7 of the nerve cord. Their dendritic structure was found to vary from ganglion to ganglion. These segmental differences among HN cells were observed consistently from animal to animal. Some of the segmental differences in HN cell structure correlate with previously described physiological differences.     

Fluorescence marking of neuropile glial cells in the central nervous system of the leech Hirudo medicinalis

Summary Neuropile glial (NG) cells in the central nervous system of the medicinal leech, Hirudo medicinalis L., were studied by histological and intracellular electrophysiological methods. Potential profiles of single leech ganglia were mapped by advancing an electrolyte-filled microelectrode into the ganglion as far as the NG cell. A small negative potential usually appeared during or immediately after penetration of the ganglion sheath. Most of the ganglia in the chain (ganglia 1–4 and 7–21) have Retzius-cell-bodies of normal size; in these, the potential associated with the ganglion sheath was followed by a jump to a more negative potential. Superimposed action potentials were associated with entry of the electrode into a Retzius cell. When the electrode tip passed out of the cell into the center of the ganglion, another potential change was observed, namely that to the membrane potential of the anterior NG cell. This membrane potential averaged -60.2 mV and ranged from -50 to -73 mV. In ganglia 5 and 6 the Retzius-cell-bodies are particularly small, and no changes of potential associated with these cells were observed; the first potential to appear after the electrode passed through the sheath of the ganglion was the membrane potential of the NG cell. Potential profiles like those of ganglia 5 and 6 are recorded in the posterior parts of all ganglia.Potential profiles of single leech ganglia were also recorded with microelectrodes filled with the fluorescent dye Procion Yellow M4-RAN. When the presumed membrane potential of an NG cell appeared, the dye was injected into the ganglion. Subsequent histological examination with the fluorescence microscope revealed that all of the dye was contained in NG cells.Supported by a Fellowship (Heisenberg-Stipendium, Schl 169/5) and grants (Schl 169/2, 4) to W.R.S. from the Deutsche ForschungsgemeinschaftThe authors thank Gisela Geiger for excellent assistance during this work     

Gangliogenesis in leech: morphogenetic processes leading to segmentation in the central nervous system

 Using intracellular lineage tracers to study the main neurogenic lineage (N lineage) of the glossiphoniid leech embryo, we have characterized events leading from continuous columns of segmental founder cells (nf and ns primary blast cells) to discrete, segmentally iterated ganglia. The separation between prospective ganglia was first evident as a fissure between the posterior boundary of nf- and the anterior boundary of ns-derived progeny. We also identified the sublineages of nf-derived cells that contribute parallel stripes of cells to each segment. These stripes of cells project ventrolaterally from the dorsolateral margin of each nascent ganglion to the ventral body wall. The position and orientation of the stripes suggests that they play a role in forming the posterior segmental nerve; they are not coincident with the ganglionic boundary, and they form well after the separation of ganglionic primordia. Previous work has shown that cells in the anterior stripe express the leech engrailed-class gene. Thus, in contrast to the role of cells expressing engrailed in Drosophila, the stripes of N-derived cells expressing an engrailed-class gene in leech do not seem to play a direct role in segmentation or segment polarity. Received: 10 October 1997 / Accepted: 12 December 1997     

Innervation of the knee joint in the rabbit and the Formosan rock-monkey (Macaca cyclopis): a retrograde HRP study.

The number and segmental distribution of cell bodies of sensory afferents and sympathetic efferent innervating to the knee joint of the rabbit and the Formosan rock-monkey were investigated using retrograde transport with horseradish peroxidase (HRP). After injecting HRP into the articular knee joint capsule of the rabbits, labeled neurons were found in the ipsilateral L4-S2 dorsal root ganglia (DRG). However, following injection of HRP into the articular cavity of the knee joint in the rabbit and the monkey, labeled neurons were found in both the ipsilateral DRG (L5-S2 and L4-S1 of the rabbit and monkey, respectively) and in the ipsilateral sympathetic ganglia (SG) (L4-S3 (rabbit) and L3-S1 (monkey)). The majority of labeled neurons within the DRG and the SG were composed of medium and large neurons in the monkey and the rabbit, respectively. The present findings suggest that the sensory projections from and sympathetic projection to the knee joint in rabbits and monkeys are similar, but that both projections of monkeys were "shifted" one segment cranially compared to the rabbit on both projections.     

Embryonic origins of cells in the leech Helobdella triserialis

To ascertain the embryonic origins of the cells in various tissues of the leech Helobdella triserialis, horseradish peroxidase (HRP) was injected as a cell lineage tracer into all identified blastomeres of the early embryo in turn, except for a few of the micromeres, and the resulting distribution of HRP-labeled cells was then examined in the late embryo. In this way it was found that in every body segment a topographically characteristic set of neurons in the ganglion and body wall and a characteristic territory of the epidermis is derived from each of the four paired ectodermal teloblasts N, O/P, O/P, and Q, whereas the muscles, nephridia, and connective tissue, as well as a few presumptive neurons in each segmental ganglion, are derived from the paired mesodermal teloblast, M. Each topographically characteristic, segmentally iterated set of neurons descended from a given teloblast is designated as a kinship group. However, the prostomial (nonsegmental) epidermis and the neurons of the supraesophageal ganglion were found to be derived from the a, b, c, and d micromere quartet to which the A, B, C, and D blastomeres give rise at the dorsal pole of the embryo. The superficial epithelium of the provisional integument, which covers the surface of the embryo midway through development and is sloughed off at the time of body closure, was found to be derived from the a, b, c, and d micromere quartet, as well as from other micromeres produced in the course of teloblast formation. The contractile fibers of the provisional integument were found to be derived from the paired M teloblast. These results demonstrate that development of the leech embryo proceeds according to a highly stereotyped pattern, in the sense that a particular identifiable blastomere of the early embryo regularly gives rise to a particular set of cells of the adult (or provisional embryonic) tissues.     

Glutamate-like immunoreactivity in the leech central nervous system

Summary Using a monoclonal antibody for glutamate the distribution was determined of glutamate-like immunoreactive neurons in the leech central nervous system (CNS). Glutamate-like immunoreactive neurons (GINs) were found to be localized to the anterior portion of the leech CNS: in the first segmental ganglion and in the subesophageal ganglion. Exactly five pairs of GINs consistently reacted with the glutamate antibody. Two medial pairs of GINs were located in the subesophageal ganglion and shared several morphological characteristics with two medial pairs of GINs in the first segmental ganglion. An additional lateral pair of GINs was also located in segmental ganglion 1. A pair of glutamate-like immunoreactive neurons, which are potential homologs of the lateral pair of GINs in segmental ganglion 1, were occasionally observed in more posterior segmental ganglia along with a selective group of neuronal processes. Thus only a small, localized population of neurons in the leech CNS appears to use glutamate as their neurotransmitter.     

Glutamate-like immunoreactivity in the leech central nervous system.

Using a monoclonal antibody for glutamate the distribution was determined of glutamate-like immunoreactive neurons in the leech central nervous system (CNS). Glutamate-like immunoreactive neurons (GINs) were found to be localized to the anterior portion of the leech CNS: in the first segmental ganglion and in the subesophageal ganglion. Exactly five pairs of GINs consistently reacted with the glutamate antibody. Two medial pairs of GINs were located in the subesophageal ganglion and shared several morphological characteristics with two medial pairs of GINs in the first segmental ganglion. An additional lateral pair of GINs was also located in segmental ganglion 1. A pair of glutamate-like immunoreactive neurons, which are potential homologs of the lateral pair of GINs in segmental ganglion 1, were occasionally observed in more posterior segmental ganglia along with a selective group of neuronal processes. Thus only a small, localized population of neurons in the leech CNS appears to use glutamate as their neurotransmitter.     

Physiological and anatomical properties of Leydig cells in the segmental nervous system of the leech

Summary Each of the 21 segmental ganglia in the American leechMacrobdella decora contains a pair of Leydig cells (ca. 45 m) each of which is located in a posteriolateral glial packet. Leydig cells exhibit spontaneous action potentials (1–10/s) whose duration and undershoot depend upon membrane polarization. The two Leydig cells within each ganglion are bidirectionally-coupled (V ₂/V ₁0.3). Pairs of ipsilateral Leydig cells within adjacent ganglia are mutually excitatory such that an impulse in one generates an impulse in the other. The interganglionic latency for any cell pair is constant regardless of the direction of impulse conduction and is unchanged by 20 mM Mg²⁺ saline. These data indicate that the interactions are not mediated by chemical synapses. Additionally, the results of collision experiments lead us to infer that ipsilateral Leydig cell pairs utilize common axonal pathways for interganglionic interactions. If Leydig cells are driven by current injection to fire impulses at frequencies of six to ten per s, cells in adjacent ganglia exhibit impulse failures. The combination of spontaneous activity, intraganglionic coupling and interganglionic interactions results in the generation of constant, low frequency impulse activity and can cause impulse reverberations.The branching pattern of Leydig cells filled with HRP is consistent with their functional properties and connectivity. Each cell sends axons to both adjacent ganglia through the ipsilateral connectives and projects to the periphery only by the lateral roots of these adjacent ganglia. This unusual morphology was verified Lucifer Yellow CH.In addition to intraganglionic dye-coupling, dye coupling was occasionally evident between ipsilateral cells in adjacent ganglia. Electron microscopy of Leydig cells depicts abundant 100 nm granules in both their somata and neuropilar processes. Although this fine structure suggests a neurosecretory role, we were unable to discern a peripheral function for these neurons.Abbreviation H R P horseradish peroxidase     

<Emphasis Type="Italic">Hau-Pax6A</Emphasis> expression in the central nervous system of the leech embryo

The leech Helobdella sp. (Austin) has two genes of the Pax6 subfamily, one of which is characterized in detail. Hau-Pax6A was expressed during embryonic development in a pattern similar to other bilaterian animals. RNA was detected in cellular precursors of the central nervous system (CNS) and in peripheral cells including a population associated with the developing eye. The CNS of the mature leech is a ventral nerve cord composed of segmental ganglia, and embryonic Hau-Pax6A expression was primarily localized to the N teloblast lineage that generates the majority of ganglionic neurons. Expression began when the ganglion primordia were four cells in length and was initially restricted to a single cell, n_s.a, whose descendants will form the ganglion’s anterior edge. At later stages, the Hau-Pax6A expression pattern expanded to include additional CNS precursors, including some descendants of the O teloblast. Expression persisted through the early stages of ganglion morphogenesis but disappeared from the segmented body trunk at the time of neuronal differentiation. The timing and iterated pattern of Hau-Pax6A expression in the leech embryo suggests that this gene may play a role in the segmental patterning of CNS morphogenesis.     

Migration of neurons between ganglia in the metamorphosing insect nervous system

Migration of neurons over long distances occurs during the development of the adult central nervous system of the sphinx moth Manduca sexta, and the turnip moth Agrotis segetum. From each of the suboesophageal and three thoracic ganglia, bilaterally-paired clusters of immature neurons and associated glial cells migrate posteriorly along the interganglionic connectives, to enter the next posterior ganglion. The first sign of migration is observed at the onset of metamorphosis, when posterio-lateral cell clusters gradually separate from the cortex of neuronal cell bodies and enter the connectives. Cell clusters migrate posteriorly along the connective to reach the next ganglion over the first three days (approximately 15%) of pupal development. During migration, each cell cluster is completely enveloped by a single giant glial cell spanning the entire length of the connective between two adjacent ganglia. Intracellular cobalt staining reveals that each migrating neuron has an ovoid cell body and an extremely long leading process which extends as far as the next posterior ganglion; this is not a common morphology for migrating neurons that have been described in vertebrates. Once the cells arrive at the anterior cortex of the next ganglion, they rapidly intermingle with the surrounding neurons and so we were unable to determine the fate of the migrating neurons at their final location.