Development 109, 17-28 (1990)Printed in Great Britain © The Company of Biologists Limited 1990

17

Neurogenesis in the insect enteric nervous system: generation of pre-

migratory neurons from an epithelial placode

PHILIP F. COPENHAVER* and PAUL H. TAGHERT

Department of Anatomy and Neurobiology 8108, Washington University School of Medicine, Saint Louis, MO 63110, USA

* Please send correspondence to: Department of Cell Biology and Anatomy L215, Oregon Health Sciences University, Portland, OR 97201,USA

Summary

The enteric plexus (EP) is a major division of the entericnervous system (ENS) in the moth Manduca sexta andcontains a dispersed population of about 360 bipolarneurons, the EP cells. Previously we showed that embry-onic EP cells achieve their mature distributions byextensive migration along the gut surface and thendisplay position-specific phenotypes. We now demon-strate that the entire EP cell population is generatedfrom an ectodermal placode that invaginates from theembryonic foregut. Individual EP cells become post-mitotic just as they leave the epithelium, but theirterminal differentiation is subsequently delayed untilafter their migratory dispersal. Clonal analysis by injec-tion of lineage-tracing dyes has shown that the EP cell

population is derived from a large number of placodalcells, each of which contributes a limited number ofneurons to the ENS. Placodally derived clones produceneurons exclusively, while clones arising from cellsadjacent to the placode are incorporated into the gutepithelium. These results indicate that neurogenesis inthe insect ENS involves a developmental strategy that isdistinct from that seen in the insect CNS and whichresembles the generation of certain cell classes in thevertebrate nervous system.

Key words: enteric nervous system, placode, neurogenesis,Manduca sexta.

Introduction

Fundamental to the formation of the nervous system isthe program of neurogenesis by which cells are gener-ated or recruited into the developmental pathwayleading to neuronal differentiation. Recent advances inthe clonal analysis of the vertebrate nervous systemhave revealed a variety of strategies by which differentcell types are produced. For example, the separation ofneural from non-neural lineages can occur relativelyearly in development, as has been described in thecerebral cortex (Luskin et al. 1988; Price and Thurlow,1988), or may not be apparent until the time of terminalmitosis, as indicated by studies of the developing retina(Turner and Cepko, 1987; Wetts and Fraser, 1988; Holtel al. 1988). Similarly, the commitment of vertebrateneurons to specific phenotypes can vary with respect toother phases of their differentiation. Many of thetransmitter phenotypes in neural crest-derived neuronsare regulated by position-specific cues encountered onlyafter their dispersal (Patterson, 1978; Gershon andRothman, 1984; Adler and Black, 1986), whereas theexpression of peptidergic phenotypes in neurons des-tined to populate the hypothalamus can commence

even before the cells are integrated into the CNS(Schwanzel-Fukuda and Pfaff, 1989; Wray et al. 1989).

In insect systems, two predominant strategies forneurogenesis have been described. Within the develop-ing CNS, the majority of neurons derive from identifi-able stem cells, which produce characteristic sets ofprogeny via stereotyped cell lineages (Bate, 1978;Thomas et al. 1984). Likewise, in the PNS, an invariantlineage is thought to generate the cells that form eachsensory unit, so that the sensory neurons, support cells,and epidermal structural elements appear in an orderlysequence (Lawrence, 1966; Bodmere/a/. 1989). Clonalanalysis of the developing retina in Drosophila hasrevealed a second neurogenic strategy, however, inwhich stereotyped lineage relationships do not play arole in the generation of photoreceptor neurons.Rather, retinal cell fate appears determined by a seriesof inductive interactions that regulate the progressivedifferentiation of identifiable phenotypes (Ready et al.1976; Hafen etal. 1987; Tomlinson et al. 1988). Despitethese initial differences, subsequent developmentalevents throughout the insect nervous system appearsimilar: embryonic neurons in both the central gangliaand the retina typically differentiate soon after their

18 P. F. Copenhaver and P. H. Taghert

terminal mitoses, with little or no dispersal from theirsite of origin. In this regard, neuronal development ininsects appears less complicated than in many ver-tebrate embryos, where extensive phases of cellularmigration often reorganize the spatial relationshipsamong developing neurons and precede the expressionof mature phenotypes (Weston, 1970; Sidman andRakic, 1973; Le Douarin, 1982).

Recently we have characterized the differentiation ofthe insect enteric nervous system (ENS; Copenhaverand Taghert, 1989a,b), a major division of the periph-eral nervous system that supplies innervation to the gutwith only limited input from higher neural centers. Itincludes several small peripheral ganglia, as well as amore dispersed population of cells within the majornerves of the ENS that run superficially along thealimentary tract. In particular, we have focussed on theformation of the major nerve plexus of the ENS, theenteric plexus, during embryonic development in themoth Manduca. This plexus spans much of the foregutand midgut and contains approximately 360 bipolarneurons (the EP cells) that are distributed along theplexus nerves of the gut surface. The EP cells exhibitdiverse morphological and biochemical phenotypes thatare expressed in a position-specific manner (Copen-haver and Taghert, 1989a). In contrast to the pattern ofdifferentiation observed in the insect CNS and retina,the enteric plexus exhibits many of the developmentalfeatures more commonly attributed to the neural crestand placodes of vertebrates: following their initialgeneration, for example, the immature EP cells becomeprogressively dispersed by active migration along diver-gent pathways (Copenhaver and Taghert, 1989b).Unlike many central neurons in insects, the EP cells donot become uniquely identifiable but can be categorizedinto several distinct subtypes within the differentdomains of the plexus. Moreover, the expression ofmany of their mature characteristics is significantlydelayed until after the migratory phase is complete.

To clarify the nature of the developmental programgiving rise to this class of insect neurons, we have nowcharacterized the origins of the enteric plexus and havededuced the mitotic patterns by which the EP cellpopulation is generated. We report that all of the EPcells are derived from a common epithelial placode,arising from a defined region in the dorsal foregutsurface at 30 % of embryogenesis. Within this placode,a large number of mitotically active cells serve asprogenitors for the EP cells, each giving rise to a smallclone of 1-8 cells that assume exclusively neuronalphenotypes. Their progeny become post-mitotic as theyleave the epithelial layer, forming a discrete populationof pre-migratory neurons on foregut surface. We inter-pret these results to indicate that the development ofthe EP neurons of the gut represents a third generalstrategy for neurogenesis in the insects.

Materials and methods

Embryonic staging and immunocytochemistryAnimal rearing and collection were performed as previously

described (Copenhaver and Taghert, 1989a). Synchronousegg batches from a laboratory colony of Manduca sexta werecollected in lh intervals as needed and incubated at 25°C;under these conditions, embryogenesis is complete in about100 h (approximately 1% development per h). Embryonicdevelopment could be monitored by reference to establishedschedules of external and internal markers (Dorn el al. 1987;Copenhaver and Taghert, 1989a). To visualize the developingENS, staged embryos were dissected under physiologicalsaline (modified Weever's: KC1, 34ITIM; NaCl, 7ITIM; MgCl2,8ITIM; CaCl2, 14mM; Dextrose, 173mM; KHCO3, 1.25 ITIM;KH2PO4, 1.25 mM; pH6.2; after Levine and Truman, 1985),restrained in a shallow Sylgard (Dow Corning) chamber, andmonitored using a 50x water immersion lens and Nomarskioptics. Animals prepared in this manner were also fixed for1 h in 2 % paraformaldehyde in phosphate-buffered saline(PBS) and processed for whole-mount immunohistochemistryusing the monoclonal antibody TN-1 (diluted 1:20000 in PBSwith 0.1 % Triton-X 100, 0.1 % NaN3, and 3 % normal horseserum; Copenhaver and Taghert, 19896) and visualized usingthe avidin-biotin-HRP system of Vector Laboratories.Stained preparations were mounted in 90% glycerol with0.1% p-phenylenediamine (Caudy and Bentley, 1986) andphotographed or drawn by camera lucida at 650 x magnifi-cation.

Post-embryonic larvae were anesthetized by immersion inwater and dissected under saline to expose the ENS. After a5min fixation in 2% paraformaldehyde to preserve gutmorphology, the alimentary tract was then further dissectedand flattened onto the surface of a Sylgard-coated culturedish, followed by fixation for 1-2 h. The preparations werethen briefly dehydrated with chilled ethanol (5 min rinses withincremental concentrations from 10-80% before rehy-dration) to enhance tissue permeability, followed by whole-mount immunohistochemical staining as already described.Rabbit antisera were raised against commercially synthesizedPhe-Met-Arg-Phe-NH2 (FMRFamide; Sigma) that had beenconjugated to thyroglobulin or bovine serum albumin. Theywere applied to the tissue at concentrations of 1:3000-1:5000.Immunoreactive staining was blocked by liquid-phase pre-absorption of the antisera with the conjugated antigen. Itshould be noted, however, that the insect nervous systemprobably does not produce the pentapeptide form of FMRF-amide but instead expresses a family of amino-terminallyextended forms of this peptide (Nambu et al. 1988; Schneiderand Taghert, 1988).

Incorporation and detection of BrdUStaged embryos were selected at times throughout the periodof ENS formation (25-85 % of development) and dissected asdescribed above, using a modified insect culture medium:50% Schneider's Drosophila medium, 40% Eagle's basicsalts, 9.9% heat-inactivated fetal calf serum, 0.09% penicil-lin-streptomycin, 0.01 % insulin, supplemented with Man-duca haemolymph (after Chen and Levi-Montalcini, 1969;and Seecof et al. 1971). Animals were then transferred tomedium containing 15-150 j*g ml"1 of 5-bromo-2'deoxy-uridine (BrdU; Sigma) and incubated for 2h at room tem-perature with gentle agitation. Similar results were obtainedwith all concentrations of BrdU, although the highest concen-trations induced abnormal cellular morphologies that becameincreasingly apparent with extended incubation periods.

Following a brief rinse with fresh culture medium, thepreparations were exposed to Carnoy's fixative for 15 min,rinsed in PBS plus 0.3% Triton-X 100, and incubated in asolution of 2N HO for 30min. The tissue was then rinsed toremove the acid and transferred to PBS plus 0.3% Triton,

ENS neurogenesis in the insect embryo 19

0.1% NaN3, and 10% normal horse serum for lh (afterTruman and Bate, 1988). Whole-mount immunohisto-chemistry was subsequently performed as already described,using a monoclonal antibody to BrdU (Becton-Dickinson; atconcentrations of 1:30-1:50). For some preparations, theenteric plexus was subsequently counterstained with themonoclonal antibody TN-1, using the avidin-biotin-HRPprotocol in the presence of NiCl, or using the avidin-biotin-alkaline phosphatase procedure (Vector Laboratories)to distinguish the two immunohistochemical reactions.

Clonal analysis of cell lineagesStaged embryos were isolated in sterile culture medium andindividual cells within the developing ENS were impaled witha glass microelectrode (80-100 megaohms) containing one ofa variety of fluorescent lineage-tracing dyes from MolecularProbes (Gimlich and Braun, 1985): lysinated fluoresceindextran (LFD; 10xl03Mr, lot 7A) and lysinated tetramethyl-rhodamine dextran (LRD; lOxlCP/W,., lot 8A; and40xl03Mr, lot 7A) were employed with similar results.Electrodes were filled at the tip with dye concentrations of100-200mgrnl"1 and backfilled with 1.2M LiCl (after Wettsand Fraser, 1988). Cells were iontophoresed with depolarizingcurrent pulses (100 msec, 2nA) at 9 Hz for 15-30 s. Animalswere then rinsed thoroughly with sterile culture medium andincubated in a humidified chamber at 28CC for 24h. Theywere subsequently dissected and fixed in 2% paraformal-dehyde for lh. Preparations were usually counterstainedimmunohistochemically with TN-1, as described above, usinga fluorescent secondary antibody to complement the ionto-phoresed fluorochrome (e.g. fluorescein-conjugated anti-bodies were used for preparations injected with LRD). Thelabelled descendant cells were then analyzed and photo-graphed in whole-mount using a Zeiss Axioplan epifluor-escence microscope equipped with the appropriate filter setsfor the different fluorochromes.

Results

Developmental origins of the EP cellsThe enteric plexus spans much of the foregut andmidgut and contains approximately 360 bipolar neurons(the EP cells) that are distributed along the plexusnerves of the outer gut surface (Fig. 1). Unlike manycentral neurons in the moth, the EP cells are notuniquely identifiable but can be categorized into severaldistinct subtypes that are interspersed within the plexus(e.g. Fig. IB). For example, one EP cell class expressessubstances that are related to the molluscan peptideFMRFamide (Price and Greenberg, 1977) and inner-vate the midgut musculature (Fig. 1C). Previously weshowed that by 40% of embryogenesis, the entire EPcell population is already present in the form of acoherent, pre-migratory packet of cells on the dorsalforegut surface (Copenhaver and Taghert, 1989b). Wetherefore traced the origins of this cell group through-out the preceding stages of development both by directobservation of staged embryos (viewed under Nomarskioptics) and by immunohistochemical staining with themonoclonal antibody TN-1 (Taghert et al. 1986). TN-1recognizes a membrane-bound epitope expressed bycells of the developing enteric plexus (Copenhaver andTaghert, 1989b) as well as by several other neural and

non-neural tissues in Manduca (Carr and Taghert, 1988,1989), and can be used to differentiate the developingEP cells from the surrounding layers of gut epithelium.

The earliest indication of the EP cell group occurredat 30% of embryogenesis, in the form of a discreteplacode that formed in the protruding dorsal lip of theforegut (Fig. 2A); at this stage, the foregut was anepithelial tube that had invaginated from the anteriorectoderm. As the cells of the placode became increas-ingly columnar, they could be distinguished both mor-phologically and when stained with the TN-1 antibody(Figs2A, 3). The mid-dorsal position of the placodecoincided with the more anterior neurogenic zones ofthe ENS (also TN-1 positive, Fig. 3B), which hadpreviously developed within the foregut epithelium andwhich ultimately gave rise to the anterior entericganglia (Copenhaver and Taghert, in preparation).

Over the next 10 % of embryogenesis, the cells of theplacode participated in two coordinate processes(Figs2A, 3A-F): individual columnar cells roundedout of the epithelial layer and were extruded onto theouter foregut surface, while at the same time, theplacode itself underwent a progressive outpocketingfrom the basal surface of the gut epithelium into thebody cavity - an epithelial transformation defined as aninvagination (Fristrom, 1988). As this sequence con-tinued, the boundaries of the placode became increas-ingly distinct from the adjacent cell layers (Fig. 3B, C).Concentric folding of the epithelium gradually broughtthe ventrolateral margins of the placode into apposition(Fig. 3D). Finally, as the last of the presumptive neur-ons rounded out of the placode (40%), the remainingepithelial layers fused to re-establish the continuity ofthe foregut surface, posterior to the newly formed cellpacket (Fig. 3E, F, arrowheads).

Patterns of DNA replication in the neurogenic placodeTo characterize the mitotic relationships within the gutplacode and during subsequent phases of differen-tiation, we incubated staged embryos for 2h periods(approximately 2% of development) with the thymi-dine analog, 5-Bromo-2'-deoxyuridine (BrdU). BrdUis readily incorporated into the DNA of cells in S-phaseof their mitotic cycle, and when visualized with an anti-BrdU antibody (Gratzner, 1982), it can serve as amarker for the patterns of DNA replication in embry-onic tissue (Truman and Bate, 1988; Bodmer et al.1989). When animals were labelled with BrdU at theonset of placode formation (around 30 % of develop-ment), a uniform pattern of staining was observedthroughout the foregut (Fig. 3G). DNA replication wasdetectable in both placodal and non-placodal domainsof the gut epithelium, labelling a regular but unordereddistribution of cells. Once placode invagination com-menced, however, BrdU incubations revealed a discon-tinuity in the pattern of labelled nuclei: actively repli-cating cells were now seen primarily within thecolumnar portions of the placode, while cells that wererounding out the epithelium were labelled only in-frequently (Fig. 3H). This distinction became increas-ingly obvious in subsequent developmental stages

20 P. F. Copenhaver and P. H. Taghert

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B \

Fig. 1. FMRFamide-immunoreactive EP cells within the enteric plexus. (A) Overview of the mature enteric plexus near theforegut-midgut boundary (black hashmarks). In all figures, anterior is towards the top of the page. The gut of a post-embryonic (3rd instar) larva was cut mid-ventrally and flattened to reveal the eight major longitudinal muscle bands of themidgut along which the FMRFamide-positive EP cells (e.g. black arrowheads) are distributed. (B) Higher magnification ofthe same cells indicated by the arrowheads in A. Small arrows indicate some of the unlabeled EP cells that are interspersedalong the muscle bands. (C) Immunoreactive processes from the EP cells projecting onto the interband midgut musculature,several hundred microns posterior to the EP cell bodies. Also visible in C are immunoreactive gut endocrine cells (g) thatarise within the midgut epithelium and are unrelated to the EP neurons; t, non-immunoreactive (air-filled) trachea. Scale:200 j.im in A, 50/.im in B and C.

(Fig. 3H-K), with synthetically active cells occurringonly in the lateral epithelial folds at the placodemargins.

Concurrent with this gradual redistribution of BrdUlabelling, the total number of stained placodal cells alsochanged in a consistent manner as development pro-ceeded (Fig. 4). During the initial phases of EP cellproliferation (30-34%), there was an increase in thenumber of labelled profiles occurring within the bound-aries of the placode. Subsequently, this number steadilydeclined, as more of the cells rounded out of theepithelium. By 40% of development, the EP cell packetwas completely devoid of labelled nuclei (Figs2B, 4),while much of the underlying foregut epitheliumremained heavily labelled. In contrast, cells of thecentral nervous system and those derived from themore anterior neurogenic zones of the ENS continuedto exhibit cycles of DNA replication (Fig. 3L arrow,

and unpublished observations). The proliferation ofthese anterior zones will be described in a subsequentreport. Incubation with BrdU during later stages ofembryogenesis (up to 90 % of development) failed toreveal any additional labelling of neurons in the entericplexus (not shown), indicating that they did not re-initiate the mitotic cycle throughout their subsequentphases of migration and differentiation. Thus in con-trast to the patterns of neurogenesis seen in the insectCNS, the EP cells are generated via rapid proliferationfrom an epithelial placode, a process that gives rise to acoherent population of post-mitotic, pre-migratoryneurons.

Cell lineage analysis of the neurogenic placodeThe results of the foregoing experiments indicated thatthe generation of the EP cells occurred within a discrete

ENS neurogenesis in the insect embryo 21

TN-1

Fig. 2. Generation of the EP cells by placode invagination. (A) Schematic drawings of the embryonic foregut atprogressively older stages of development (30-40%), stained with the monoclonal antibody TN-1 (anterior is up); theimmunoreactive cells of the neurogenic placode have been highlighted in black. Arrowheads indicate the boundaries of theplacode (also indicated in B and in Fig. 3). The clear tube anterior to the placode on the foregut surface represents thedomain of the ENS produced by the anterior neurogenic zones of the foregut. Cells of this domain ultimately establishcontinuity with the EP cell packet (small arrow). The connection of the foregut with the more posterior midgut anlagen(present throughout this developmental period) has been drawn only in the 40% stage. (B) Patterns of BrdU labelling(taken from camera-lucida drawings) during the same period of embryogenesis; labelled nuclei are indicated in black.Labelled nuclei occur throughout the foregut epithelium during this time (see Fig. 3H, K), but have been included only inthe vicinity of the placode in these drawings. Small arrow indicates a labelled cell derived from the anterior neurogeniczones (same position as in A).

region of the foregut epithelium in a spatially restrictedmanner and occupied a relatively short period ofdevelopment (6-10 h). However, the observed patternsof DNA replication did not provide any informationconcerning the nature of the EP cell precursors, nor ofthe potential lineal relationships among the differentcell types associated with the mature enteric plexus. Tobetter characterize the patterns of division by which theEP neurons are generated, individual cells within theneurogenic placode were filled iontophoretically with afluorescent lineage tracer at stages between 30 and 40 %of development. Differentiation of the enteric plexuswas then allowed to proceed for 24 h in embryo culture,through the phases of placode invagination and EP cellmigration (Copenhaver and Taghert, 1989b). Whileindividual cells within the placode could not beuniquely identified, clonal analysis of lineage relation-

ships in 82 preparations (as revealed by the distributionsof labelled progeny) permitted the following obser-vations.

When individual cells throughout the placode wereinjected during the initial phases of neurogenesis(30-32%), a comparatively large number of labelledprogeny were subsequently found in the developingenteric plexus (Figs5A,B; 6D). Typically, between 4and 8 EP cells were labelled in these experiments,although occasionally only a single fluorescent cellcould be detected (Fig. 5C). When injections wereperformed at later developmental stages (34-36 %), thenumber of labelled progeny diminished in a stage-dependent manner (Figs 5D,E; 6C). Individual pro-genitors at these stages gave rise to a maximum of 4 EPcells and more often resulted in only 1 or 2 labelledprogeny. The number of labelled progeny also varied

22 P. F. Copenhaver and P. H. Taghert

30 32 34

Fig. 3. Photomicrographs of the invaginating EP cell placode at progressive stages of development. TN-1 stainedpreparations (A-F) and BrdU-labelled preparations (G-L) at comparable ages. Arrowheads indicate the boundaries of theinvaginating placode (compare with Fig. 2A). Small arrows indicate cells derived from the anterior neurogenic zones of theforegut (non-EP cells). Stars in H and I indicate gut epithelium that is adjacent to the neurogenic domains of the foregut andwhich is heavily stained with BrdU labelling throughout this period of development; in I and L, much of this region has beenstripped to better delineate the unstained EP cells. Several preparations were also counterstained lightly with TN-1 toenhance the visibility of the EP cells (e.g. I, L). Scale=50/

36

ENS neurogenesis in the insect embryo

40

23

CNS neuroblasts within the brain during similar periodsof development consistently yielded a large number oflabelled neurons following a 24 h period in culture(Fig. 6F). In a small minority of preparations (

24 P. F. Copenhaver and P. H. Taghert

Stage Injected Examples of Labelled Progeny

32°/c A.

36°/c D

40°/c

Fig. 5. Patterns of labelled progeny in the enteric plexus following fluorescent dye injections of individual placode cells. Thestage at which single placode cells were injected with LRD is indicated on the left (in percent of development). Injected cellsare illustrated schematically in black (arrows). Representative examples of labelled progeny after 24 h in culture (drawn bycamera lucida) are shown to the right of each injection stage: A, B, and C were injected between 30 and 32%; D, E, and Fwere injected between 33 and 36%; and G, H, and I were injected between 37 and 40%. The examples shown were chosento illustrate the range of progeny numbers following single cell injections at the different developmental stages (see Fig. 6).The irregular boundaries of the developing enteric plexus in these preparations (outlined in black on the gut surface) are dueto minor perturbations in the overall structure of the foregut-midgut boundary that arise in culture. Scale=200 jtm.

differentiation within three or four percent of develop-ment and with little or no displacement from their initiallocation. Similarly in the PNS, embryonic sensoryneurons arise from stereotyped precursors (Lawrence,1966; Technau and Campos-Ortega, 1986; Hartenstein,1988), each of which produces one or more neuronswhich commence differentiation soon after birth (Ghy-sen et al. 1986). A modification of this sequence hasbeen seen in neurons born post-embryonically in theinsect CNS: these arise from identifiable neuroblasts

but do not complete their differentiation until the timeof metamorphosis (Booker and Truman, 1987).

In contrast, the EP cells of the ENS are generated asa coherent group from a proliferative epithelium(Fig. 8), becoming post-mitotic in the course of theiremergence from the columnar layer. Rather than beinggenerated via a sequence of asymmetric divisions fromneuroblast-like precursors, the EP cells are produced assmall clones from an ensemble of columnar epithelialcells that undergo a limited number of divisions, im-

Fig. 6. Photomicrographs of labelled progeny after 24h in culture, following fluorescent dye injections of single precursorcells. Preparations shown in panels A-E were injected with lysinated tetramethylrhodamine dextran (LRD); F was injectedwith lysinated fluorescein dextran (LFD). B-E were counterstained immunohistochemically with the monoclonal antibodyTN-1 (followed with a fiuorescein-conjugated secondary antibody) to reveal the extent of the developing enteric plexus.Panel A shows an example of a single placode cell injected at 32 % of development and fixed immediately; all other embryoswere cultured for 24 h following the initial injection of the lineage tracer. Labelled progeny are indicated by the whitearrowheads. (B) A single EP cell labelled in the enteric plexus after a placode cell was injected at 38% of development. Theposterior process of this bipolar cell can be seen projecting along the migratory pathway of the developing enteric plexus.(C) Two labelled EP neurons after injection of a placode cell at 36%. (D) Four labelled EP neurons after injection of aplacode cell at 32% (not all of the cells are in the same focal plane). In this preparation, the cells had undergone extensivemigration along the gut surface prior to fixation. (E) Example of the progeny of an epithelial cell filled at 38% within thelateral folds of the invaginating placode. The cell produced 25-30 cells that became incorporated into the epithelial wall ofthe developing gut (the fluorescein-counterstained neurons of the enteric plexus can be seen anterior to the labelled gutcells). (F) Labelled progeny of a brain neuroblast injected at 40% of development. Scale=50^m.

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ENS neurogenesis in the insect embryo

CNS ENS

25

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30 32 34 36 38 40

Developmental Stage at Injection (in percent)

Fig. 7. Numerical distribution of the progeny of dye-injected placode cells after 24 h in culture. The stage atwhich the initial injection was performed is indicated on thehorizontal axis (in percent of development). Each filledcircle represents the result from a single preparation. Onlyanimals in which a single placode cell was unambiguouslyfilled (confirmed visually; see Fig. 6A) were included in thisanalysis. The open squares represent the mean number oflabelled progeny for each developmental stage.

mediately followed by their delamination from theepithelial layer. The simplest interpretation of our datawould be that an initial population of ~45 progenitorcells arises within the placode, each of which undergoes3 symmetrical divisions to produce 8 of the 360 EP cellsfound in the mature plexus. This interpretation issupported by our observations that progressively fewerEP cells were produced by individual placode cells asdevelopment proceed (Figs 5, 6), and that the clonesizes from single neurogenic cells were clustered around1, 2, 4, and 8 (Fig. 7). However, more complex vari-ations in the size of the 'founder' population within theplacode and the numbers of their progeny are equallypossible. At present, we do not have an unambiguousmarker to distinguish neuronal and epithelial precur-sors within the placode, prior to the differentiation ofthe cells they produce.

Once generated from the placode, the EP cellsacquire neuronal phenotypes exclusively, while the glialcells that subsequently ensheath them arise from adistinct proliferative zone (Copenhaver and Taghert, inpreparation). However, as shown previously (Copen-haver and Taghert, 19896), the EP cells delay theexpression of their mature phenotypes for up to 35 % ofsubsequent development (Fig. 8). During this time,they become dispersed by a phase of extensive mi-gration into divergent environments of the entericplexus (Copenhaver and Taghert, 1989a,b), and manyof the biochemical and morphological characteristicsthat distinguish the different EP cell types (includingthe FMRFamide-related phenotype, Fig. 1) are ulti-mately expressed in a position-specific manner.Together, these observations suggest that neurogenesisin this system involves the recruitment of placode cellsvia a binary decision between ectodermal fates (Harrisand McKeown, 1986), a choice that affects both theduration of their mitotically active phase and thephenotypes that their progeny will ultimately express.

In several regards, the developmental events giving

Fig. 8. Comparative models for neurogenesis in differentdomains of the embryonic insect nervous system. In theCNS, the proliferation of neurons commences with thesegregation of precursor cells from the neurectoderm, whichare mitotically active and which assume unique identitiesthat correspond to their subsequent divisional patterns andprogeny types. The emerged neuroblasts (large ovals)typically undergo a stereotyped sequence of asymmetricdivisions; these give rise to as many as 50 ganglion mothercells (small ovals), each of which divides once more andproduce pairs of neurons of predictable phenotypes (filledcircles). The differentiation of central neurons typicallyproceeds soon after their terminal mitosis and with little orno migration. In the ENS, by contrast, the proliferation ofthe EP cells occurs within an epithelial placode, and theneurons (open circles) become post-mitotic as soon as theyemerge from the columnar cell layer. Their differentiation isthen delayed for up to 35 % of subsequent embryogenesis(filled circles), until after their dispersal via an extendedphase of migration into divergent regions of the entericplexus.

rise to the insect enteric plexus resemble patterns ofneurogenesis previously described in a variety of ver-tebrate embryos. For example, the majority of neuronsin chick cranial sensory ganglia are derived fromdiscrete epithelial placodes (Hamburger, 1961;D'Amico-Martel, 1982). These placodal cells undergo amesenchymal transformation as they leave the epi-thelium, migrate from their site of origin, and thencoalesce into peripheral ganglia before completing theirdifferentiation (D'Amico-Martel and Noden, 1983; LeDouarin and Smith, 1988). Another example of thisdevelopmental sequence has recently been described inthe mouse, where neurons derived from the olfactoryplacode also become post-mitotic upon their delami-nation and then migrate into the developing forebrainand hypothalamus (Schwanzel-Fukuda and Pfaff, 1989;Wray et al. 1989). As with the EP cells of Manduca, at

26 P. F. Copenhaver and P. H. Taghert

least some of these cells also become peptidergic,producing substances related to luteinizing hormone-releasing hormone; although in this case, the peptider-gic phenotype is expressed even before the onset ofmigration. Thus similar types of cellular processes mayregulate the recruitment of epithelial cells towards aneuronal phenotype during ENS formation in Manducaas occurs during the epithelial-to-neuronal transform-ation in these more complex systems.

While morphogenesis from an epithelial placode hasnot previously been described during the embryonicformation of the insect nervous system, it is a commonfeature in the differentiation of many other embryonictissues and organs in invertebrates (Fristrom, 1988).Most secretory glands in insects (including mandibular,salivary, and silk glands), portions of the neuroendo-crine system, and cells forming the respiratory trachealtree originate from invaginated placodes of epithelium(Anderson, 1972; Campos-Ortega and Hartenstein,1985), as do the imaginal disc cells that subsequentlydifferentiate into adult structures at metamorphosis(Poulson, 1965; Fristrom and Rickoll, 1982; Nardi,1981). Neurogenic placodes have also been described inthe mollusc Aplysia, in which each of the CNS gangliaaccumulates cells from an adjacent region of proliferat-ive epithelium during both embryonic and post-embry-onic development (McAllister et al. 1983; Jacob, 1984).Thus as with cell migration (McAllister et al. 1983), thismode of neuronal proliferation may prove to be a moreprevalent feature of invertebrate development thanpreviously appreciated.

Mitotic patterns and cell lineage in the neurogenicplacodeThe combined use of BrdU labelling and intracellularlineage tracing techniques has given us insight into thepatterns of mitosis by which the enteric plexus neuronsare generated: cells within a discrete region of epi-thelium undergo a limited number of divisions prior totheir extrusion from the epithelial layer. An importantissue is the validity of the BrdU protocol for theconstruction of mitotic maps, as this technique will alsolabel cells that are undergoing DNA replication but notpreparing to divide (an increase in ploidy). However, ifthe EP cells became polyploid in the course of theirdifferentiation, we would expect continued or enhancedlabelling in the emerged cell population, rather than thecomplete loss of staining that was observed. Usingsectioned material (not shown), we also observed adistribution of mitotic profiles that was consistent withthe patterns of DNA replication described in this paper.Moreover, in contrast to most other ectodermallyderived cell types, insect neurons classically have notbeen found to become polyploid in the course of theirdifferentiation (Anderson, 1972) - a finding that hasbeen augmented by other studies of DNA replication inthe insect nervous system (Truman and Bate, 1988;Bodmer et al. 1989). Lastly, our investigation of celllineage relationships within the placode confirmed theoverall patterns of replication revealed by the BrdUtechnique. Numerical analyses have shown that the vast

majority of EP cells in the mature plexus can beaccounted for at this time (Copenhaver and Taghert,19896), indicating that the combined processes of plac-ode invagination and epithelial delamination representsthe primary neurogenic sequence by which this class ofenteric neurons is generated.

Clonal analysis of individual placode cells allowed usto determine the patterns of division that gave rise tothe EP cell population: a large proportion of mitoticallyactive epithelial cells underwent a limited number ofdivisions and contributed progeny to the enteric plexus,in contrast to the neuroblast-derived lineages found inthe insect CNS. However, several issues concerningspecific mitotic relationships within this neurogenicplacode remain unexplored. For example, placode cellswere found to contribute progeny either to the entericplexus or to the gut epithelium but not both. The degreeto which mitotic ancestry governs the commitment tothese alternative developmental pathways remains ob-scure. Also uncertain is whether all of the neurogeniccells give rise to equal numbers of EP cells, or whetherother factors regulate progeny number - such as theinterval between a placode cell's recruitment into theneurogenic pathway and the completion of placodeinvagination (when EP cell generation is complete).Conversely, the process of placode formation andinvagination may ultimately regulate the number of EPcells that are produced. As noted above, the columnarcells that become neurogenic contribute a relativelysmall number of neurons to the EP cell packet (Fig. 7),while epithelial cells adjacent to the invaginated plac-ode continue to proliferate for many more rounds ofdivision (Fig. 6E). Possibly, the placodal environmentsupplies local mechanical or chemical constraints on themitotic activity of the neurogenic cells, thereby limitingthe number of divisions that they undergo.

Previous work has shown that as a population, the EPcells emerge from the epithelium in an undifferentiatedform but subsequently exhibit a predictable array ofmorphological and biochemical phenotypes (Copen-haver and Taghert, 1989a,6). Although the expressionof many of these characteristics is delayed until after EPcell migration is complete (Copenhaver and Taghert,19896), the developmental potential of individual neur-ons may nevertheless be constrained by lineage-derivedfactors. For example, individual placode cells mightgive rise to a particular class of EP cell or to an orderlyprogression of cellular subtypes. The combined appli-cation of our lineage-tracing protocol with an extendedanalysis of neuronal differentiation will be required toaddress this issue.

Placode formation, cell proliferation and epithelialmorphogenesisPlacode formation is a consistent feature during themorphogenesis of both invertebrates and vertebrateembryos (Wessells et al. 1971; Ettensohn, 1985), but themechanisms that regulate the diversity of epithelialrearrangements originating from these structures arepoorly understood (for review, see Fristrom, 1988). Forexample, the initial specification of placode size and

ENS neurogenesis in the insect embryo 27

position must clearly depend on antecedent events inepithelial development. In this regard, it is noteworthythat the EP placode of Manduca is generated at a sitethat, in part, is already neuralized. Prior to the forma-tion of the placode, three small neurogenic zones formwithin the foregut epithelium and eventually give rise tothe anterior regions of the ENS (Copenhaver andTaghert, in preparation). The most posterior of thesezones originates from the same epithelial region thatlater is encompassed by the EP placode, although thezone has ceased to elaborate cells by the onset ofplacode formation. The generation of the placode thusrepresents a second phase of a more complex neuro-genic sequence within this ectodermal region, whichwill be described in a subsequent report.

A second major issue concerning the mechanisms ofepithelial transformation is the role of restricted pat-terns of mitosis during the morphogenetic process. In atleast some systems, extensive cellular rearrangementscan continue even when cell division is completelyblocked, such as in amphibian gastrulation (Cooke,1978) and imaginal disc morphogenesis in Drosophila(Siegel and Fristrom, 1978). Alternatively, differentialpatterns of mitosis can play a morphogenetic role whenthe cell divisions are localized to particular domains ofan epithelial layer (Fristrom, 1988). Such a localizationwas observed during ENS formation in Manduca: thedistribution of synthetically active nuclei becameincreasingly confined to the outer limits of the placode,where the most pronounced folding of the epithelialsheet occurred during the invagination process (Fig. 3).Recently, we have also found that placode invaginationwill proceed to completion even when the stomodealrudiment is cultured in isolation from the rest of theembryo (unpublished observations). Thus the gener-ation and elaboration of the EP placode is autonomousto a relatively small region of the foregut, a region thatcan now be made accessible to experimental manipu-lation in vitro.

Taken as a whole, the events that give rise to theenteric plexus suggest a coherent program of neurogen-esis: generation of an undifferentiated cell populationfrom a proliferative zone of epithelium, migratorydispersal along divergent pathways, and the delayedexpression of neuronal phenotypes in a position-specificmanner. As such, this neurogenic sequence wouldrepresent one of several developmental programs avail-able for the construction of an embryonic nervoussystem, distinct from the more stringent programsinvolving stereotyped lineages that direct CNS develop-ment in insects and other invertebrates. Perhaps thesealternative strategies for neurogenesis permit varyingdegrees of plasticity in different domains of the nervoussystem, corresponding to the physiological require-ments of different organ systems. For example, incontrast to many cells of the insect CNS, the EP cellsare not uniquely identifiable but comprise distinctsubtypes that differentiate in particular domains of theenteric plexus following migration. While the functionalroles of the insect ENS are poorly characterized, it willbe useful to examine the physiological significance of

EP cell distributions with respect to the neurogenicstrategy that governs their development.

We wish to thank Scott Fraser for helpful advice concerningthe injection of fluorescent dyes for clonal analysis. We alsothank Alan L. Pearlman and Joshua R. Sanes for reviewing anearlier draft of this manuscript. This research was supportedby an NIH postdoctoral fellowship (No. F32NS07957) toP.F.C. and an NIH grant (No. NS-21749) to P.H.T.

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(Accepted 6 February 1990)