compartmentalisation of the developing trigeminal ganglion into maxillary and mandibular divisions...

J. Anat. (1999) 195, pp. 137–145, with 2 figures Printed in the United Kingdom 137

Compartmentalisation of the developing trigeminal ganglion

into maxillary and mandibular divisions does not depend on

target contact

LISA SCOTT1 AND MARTIN E. ATKINSON2

"School of Nursing, Midwifery & Health Visiting, University of Manchester, and #Department of Biomedical Science,

University of Sheffield, UK

(Accepted 19 April 1999)

During development axons contact their target tissues with phenomenal accuracy but the mechanisms that

control this homing behaviour remain largely elusive. A prerequisite to the study of the factors involved in

hard-wiring the nervous system during neurogenesis is an accurate calendar of developmental events. We

have studied the maxillary and mandibular components of the trigeminal system to determine the stages

during embryogenesis when a gross somatotopic order is first established within the trigeminal ganglion and

the axons projecting to the brainstem. The retrograde transganglionic fluorescent tracers DiO and DiI were

injected into the maxillary and mandibular arches or their derivatives in fixed mouse embryos staged

between 13 and 40 somites (E9–E11). After 1–4 wk, the distribution of the 2 tracers was determined using

confocal laser scanning microscopy. The first maxillary nerve cell bodies and their developing axons were

labelled at the 30 somite stage (E10). This was 2 somite stages earlier than the mesencephalic nucleus and

the ganglion cell bodies of the mandibular nerve. The gross somatotopic division of cells within the

trigeminal ganglion projecting to the maxillary and mandibular targets was established by the 32 somite

stage (E10). This arrangement was evident as 2 groups of cell bodies occupying adjacent but separate

regions of the trigeminal ganglion. The central branches of the maxillary and mandibular cell bodies entered

the metencephalon as 2 distinct bundles at the same stage. The trigeminal motor nucleus was first detected

at the 38 somite stage (E10.5).

Gross somatotopy in the major divisions of the trigeminal ganglion is established before outgrowing

axons have contacted their peripheral target tissue at E10.5. This suggests that target tissues do not induce

somatotopy.

Key words : Neurogenesis ; somatotopy; dye tracing.

Somatotopy is the arrangement of neurons in which

innervation of specific body portions is depicted in

discrete regions of a ganglion, nucleus or higher

centre. It is a characteristic of most sensory and motor

pathways (Nolte, 1993). Somatotopy is evident in the

adult trigeminal system at all levels of the neuraxis

(Bernado et al. 1986) and is especially rigid in the

rodent vibrissal system. The distribution of cortical

barrels within layer IV of the somatosensory cortex is

Correspondence to Dr Martin E. Atkinson, Department of Biomedical Science, University of Sheffield, Alfred Denny Building, Western

Bank, Sheffield S10 2TN, UK.

identical in number and sequence to the arrangement

of maxillary vibrissae follicles (Woolsey & Van der

Loos, 1970). A similar pattern exists within the

brainstem trigeminal sensory nuclear complex (Ma &

Woolsey, 1984; Ma, 1991) and ventrobasal thalamus

(Van Der Loos, 1976). Somatotopy is not restricted to

vibrissal afferents alone since several major branches

of rat maxillary and mandibular nerves have cell

bodies grouped together within separate areas of the

ganglion (Gregg & Dixon, 1973).

Somatotopy in the trigeminal system develops

‘outside-in ’ (Erzurumlu & Killackey, 1983) and there

are critical developmental periods during the for-

mation of the vibrissal sensory neuraxis. Infraorbital

nerve axons from the maxillary area of the rat

trigeminal ganglion innervate the vibrissae and cor-

responding target areas in the brainstem sensory

nuclei. In rats, these projections are spatially ordered

at E12 and E13 respectively, prior to differentiation of

their peripheral and central targets (Erzurumlu &

Jhaveri, 1992). In mice, differentiation of barrelettes

in the brainstem trigeminal complex follows organ-

isation of terminal arbors of proximal afferents (Ma,

1993). Rat thalamic axons enter the cortical plate on

E19 and these projections participate in the matu-

ration of the cortical barrel field on postnatal d 1 and

2 (Erzurumlu & Jhaveri, 1990; Brown et al. 1995). In

newborn mice, the initial thalamocortical afferent

projection is topographic prior to differentiation of

barreloids on postnatal d 3 and of barrels on postnatal

d 5 (Senft & Woolsey, 1991). Killackey et al. (1995)

have reviewed these phenomena.

These studies suggest that the arrangement of the

peripheral neurons of the trigeminal system is a

template for the design of central relay nuclei and

terminal cortical barrel fields. If peripheral infor-

mation is important for the developing arrangement

of central neurons, it follows that peripheral

projections of trigeminal first order neurons must

establish a mature pattern before the pattern emerges

in higher centres.

In rodents, gross topography of all 3 components of

the rat trigeminal ganglion is established at E16

(Rhoades et al. 1990) but distal trigeminal axons have

already innervated peripheral target tissue by this

stage. Thus it is too late to assess any inductive effect

from the target tissue on the initial organisation of

axons. Considering the mouse is slightly precocious in

development in comparison with the rat, one might

expect somatotopy in the mouse to originate a few

somite stages earlier.

If maxillary and mandibular ganglion cell dis-

tribution is heterogeneous prior to pioneer axon

contact with branchial epithelium, then the periphery

may induce somatotopy. If these cell bodies are

grossly somatotopic before axons reach branchial

arch epithelium, then the periphery is permissive and

ganglionic cytoarchitecture may be determined by

other factors. We therefore investigated the timing

and early morphology of gross somatotopy in the

maxillary and mandibular components of the murine

trigeminal ganglion and axons projecting to the

brainstem before establishment of the vibrissal

somatotopic representation.

Pregnant time-mated MF"mice between 9 and 11 d of

gestation were killed by cervical dislocation. The

presence of a vaginal plug was designated as E0. The

uteri were removed and placed in phosphate buffered

saline (pH 7.2). The embryos were dissected from the

uteri and extraembryonic membranes then accurately

aged by external morphological criteria and by somite

counts (Theiler, 1989). The embryos were decapitated

immediately and the heads fixed in a solution of

phosphate buffered 4% paraformaldehyde and 0.2%

glutaraldehyde (pH 7.2) for 1 d at room temperature.

Crystals of the contrasting fluorescent carbocyanine

neuronal tracers DiO [3,3«-dioctadecyloxacarbo-

cyanine percholate ; DiO-C18-(3)] and DiI [1,1«-dioctadecyl-3,3,3«,3«-tetramethylindocarbocyanine

percholate ; DiI-C18-(3)] (Molecular Probes, Oregon,

USA) were dissolved in dimethylformamide (1.5 mg

in 200 µl).

Small quantities of each solution were drawn up by

capillary action into heat stretched glass micropipettes

(25 µm tip diameter), which were used to insert 2 µl of

tracer unilaterally into the right side of each head.

DiO was inserted into the right maxillary arch and DiI

into the right mandibular arch. The exact site of tracer

insertion depended on the embryonic stage. DiO was

inserted into the rostral region of the first branchial

arch in 13 somite specimens (E9) in which the

maxillary arch had not developed but into the

maxillary arch itself after the 30 somite stage. At all

stages, the insertion sites were in the proximal regions

of these anatomical locations to facilitate retrograde

diffusion of tracer in axons that had not reached their

target tissue.

Successful placement of tracer into the correct

target area was monitored by an initial examination

using a Leitz fluorescence microscope equipped with

TRITC filters. When the correct location of the

contrasting tracers was confirmed, the injected heads

were returned to fixative in 2 ml Eppendorf tubes

wrapped in aluminium foil to prevent photobleaching.

The tissues were stored at room temperature in the

dark for between 1 and 4 wk to allow the tracers to be

transported retrogradely.

After 1–4 wk, each head was bisected in the sagittal

plane and the distribution of the contrasting tracers in

the right half observed using fluorescence microscopy.

Specimens with evidence of tracer diffusion received

more detailed examination. Each specimen was

mounted in PBS beneath a coverslip on a well slide,

with the lateral aspect of the bisected head placed

superiorly. Optical sections were obtained at 5–39 µm

138 L. Scott and M. E. Atkinson

intervals to a maximum depth of 429 µm using an

upright Leica TCS 4D confocal laser scanning

microscope set up for observation of DiO and DiI.

DiO was excited at 488 nm with a 515 nm low pass

filter to record emission. DiI was excited at 568 nm

with a 590 nm low pass emission filter (Honig &

Hume, 1989).

Between 8 and 36 full frame images (512 by 512

pixels) were collected for each specimen and stored.

The images were further processed using the manu-

facturer’s software so that serial sections were stacked

to give extended focus views. The colours of the 2

tracers were assigned electronically to give the

characteristic appearance of red-orange DiI and green

DiO. In double-labelled specimens the images were

overlaid to give the appearance of dual fluorescence.

The final images were sized and labelled using the

Micrografx Picture Publisher LE 4.0a programme. A

Lasergraphics slide maker transferred the electronic

images onto Kodak Gold 100 colour print film and

this was developed and printed routinely.

(see Table)

13–29 somites (E9)

Insertion of DiO and DiI into the rostral and caudal

areas respectively of the first branchial arch of 13–29

somite specimens did not result in labelling of any

trigeminal axons or ganglion cell bodies. Only

insertion sites were detectable with some secondary

transcellular labelling in surrounding mesenchyme

(data not shown).

30 somites (E10)

Insertion of DiO into the differentiating maxillary

arch labelled maxillary first order axons (Fig. 1a). As

described by Honig & Hume (1989), DiO fluorescence

is less intense than DiI fluorescence. At this stage, the

DiO traced from the peripheral injection site into the

trigeminal ganglion. Only 1 or 2 pioneering maxillary

axons were detected at this early stage in all specimens

examined. The dye was also transported trans-

ganglionically into the central axons, which contacted

the primitive pons (metencephalon), indicating that at

30 somites the cell bodies in the trigeminal ganglion

had extended peripheral and central axons and the

latter had found their target. The central axons

targeting the brainstem were medial to the peripheral

axons directed towards the maxillary arch. DiI was

visible in the mandibular injection site but axons did

not label, indicating the absence of the mandibular

first order pathway at 30 somites.

31 somites (E10)

Axon labelling persisted as described above except

that 3–6 maxillary first order axons and their cell

bodies were present. The central axons commenced

their descent into the spinal trigeminal tract (Fig. 1b).

Maxillary axons were difficult to detect in all

specimens examined during the early 30–31 somite

stages. This was due to the weak fluorescence of DiO

and the relative scarcity of axons.

32 somites (E10)

At this stage, the maxillary axons continued to trace

as detailed above with further progression along the

trigeminal tract (see description below). Between 5

and 20 nerve cells were present in all specimens

examined.

DiI inserted into the mandibular arch was trans-

ported centrally in developing axons at this stage. The

dye traced 2 separate populations of cell bodies and

their axons. A bipolar group of first order cell bodies

was located in the mandibular component of the

trigeminal ganglion and each cell extended an axon

peripherally to the injection site and another centrally

to the brainstem (Fig. 1c). A unipolar group of

primary cell bodies was located on the brainstem.

Their axons did not form a tract within the brainstem

but each cell relayed an axon directly through the

ganglion to progress laterally to the injection site (Fig.

1c, d). They travelled in company with the axons of

the bipolar group. This brainstem nucleus from which

these unipolar neurons originated was rostromedial

and ventral to the root of the mandibular nerve. The

location was consistent with the mesencephalic nu-

cleus of the trigeminal nerve. Figure 1d is a higher

power image from the same 32 somite specimen

captured in Figure 1c to show detail of the mes-

encephalic nucleus cell bodies on the brainstem.

Comparison of the DiO tracing from the maxillary

arch and the DiI tracing from the mandibular arch

demonstrated that gross somatotopy within the

trigeminal ganglion was established at the 32 somite

stage (Fig. 2a). The 2 groups of bipolar cell bodies

that traced from the maxillary or mandibular injection

sites resided within separate regions of the ganglion.

The maxillary component of the ganglion was

rostromedial and overlapped ventral to the man-

dibular component. Within the mandibular region

there appeared to be 2 subcompartments and the

Somatotopy in the trigeminal nerve 139

Fig. 1. Confocal laser scanning images showing transganglionic tracing of DiO (green) and DiI (red-orange) from the maxillary and

mandibular arches respectively of fixed mouse embryos at E10. Parasagittal views are oriented such that the peripheral injection site is at

the right margin of the field or beyond. The dyes were transported retrogradely via peripheral axons to cell bodies in the ganglion and to

the brainstem at the left margin of the field. (a) At the 30 somite stage maxillary pioneers (Mx) traced from the peripheral injection site (IS)

and contacted the brainstem (Bs). ¬250. (b) At 31 somites the tracing from the maxillary arch continued as in (a) but the central axons


rostral partition was partly ventral to that located

caudally (Fig. 2a). Figure 2b illustrates the lateral

aspect of an E10.5 mouse embryo to demonstrate the

regional anatomy of the trigeminal ganglion. Com-

pare Figure 2a with Figure 2b for orientation.

Both the maxillary and mandibular peripheral exit

points from the ganglion were located corre-

spondingly. The sensory roots from the ganglion

entered the brainstem as 2 distinct bundles. The main

trunk of the central maxillary bundle was rostromedial

and ventral to the mandibular bundle. They appeared

to funnel into the small pontine trigeminal tract as

compacted fascicles and the maxillary pontine tract

was ventral to the mandibular tract. Some axons of

the mandibular bundle formed a tract that extended

rostrally into the region immediately dorsal to the

mesencephalic nucleus (compare Fig. 2a with Fig.

1e, f). Axons descending into the spinal trigeminal

tract emerged lateral to the rostral axons. The

maxillary ascending and descending tracts in the

brainstem replicated the mandibular pattern, but the

ascending tract was not as robust as that of the

mandibular tract. At the brainstem contact site, the 2

bundles interdigitated although there was no actual

mixing or transfer of axons from one bundle to

another. For example in Figure 2c, a distal section

(labelled Mx) of the central maxillary bundle runs

caudal and medial to the rostral fascicle of the

mandibular bundle. A small maxillary fascicle passes

through the mandibular bundle laterally, but the main

maxillary bundle continues obliquely between the

mandibular fork, to cross over the lateral aspect of the

caudal fascicle of the mandibular bundle.

33–35 somites (E10–E10.5)

Axon labelling continued as described above but the

mandibular cytoarchitecture was exceptionally con-

spicuous (Fig. 1e). The mesencephalic nucleus pre-

sented characteristically as a thin column of stacked

cells (Fig. 1 f). Panels e and f of Figure 1 shows images

of the same 34 somite specimen captured in the same

field but the focal plane of f is medial to e thus

confirming the medial relation of the mesencephalic

nucleus to the sensory root of the trigeminal nerve.

commenced their descent into the spinal trigeminal tract (SpV). ¬160. (c) At 32 somites bipolar cell bodies in the mandibular component

of the ganglion (Md arrowed) traced via pioneer axons from the periphery and the corresponding central projections contacted the brainstem

(Bs). Unipolar cell bodies of the mesencephalic nucleus (MesV) on the brainstem were also traced from the mandibular arch. ¬160. (d) Same

specimen as in (c) at higher power to show cell body detail (Cb) in the mesencephalic nucleus. ¬250. (e) At 34 somites the peripheral and

central axons and the cell body detail of the mandibular component (Md arrowed) of the trigeminal ganglion were conspicuous. ¬100. (f)

Same specimen and same field as in (e) but in a more medial focal plane to show the mesencephalic nucleus (MesV) as a characteristically

thin column of stacked cells. ¬100.

36–37 somites (E10.5)

At this stage within the trigeminal ganglion, both

pseudounipolar (maxillary) and bipolar (maxillary

and mandibular) cell body morphologies coexisted

(Fig. 2d). This is likely to represent different de-

velopmental stages of the neurons within the ganglion.

38 somites (E10. 5)

The trigeminal motor nucleus was labelled at this

stage (Fig. 2e). The cell bodies were located dorso-

medial to the site of the trigeminal nerve attachment

to the pons. The axons were unipolar and they

emerged from the brainstem to travel with the central

fascicle of the sensory mandibular division.

39–40 somites (E10 5–E11)

The tracing in the trigeminal system proceeded as

described above, although it became more difficult to

detect as the specimen density increased. The results

are summarised in the Table.

This dye tracing study demonstrated that the maxil-

lary and mandibular components of the developing

trigeminal ganglion and the central branches pro-

jecting to the metencephalon are grossly somatotopic

at the 32 somite stage. The mesencephalic nucleus was

also evident at this stage. The trigeminal motor

nucleus was detected at the 38 somite stage.

There was no axon labelling in 13–29 somite

specimens. This may indicate the absence of maxillary

and mandibular axons during these early stages. It is

possible that the tracer insertion site was not close

enough to emerging distal axons so that the dye could

not trace. Nevertheless, our results are supported by

the whole mount studies of Davies & Lumsden (1984).

They detected just a few peripheral and central axons

emerging from the ganglion at E9.5. Thus our inability

to trace axons from maxillary and mandibular

injection sites before the 30 somite stage is under-

standable.


Table. Summary of results

Age of embryo Retrograde dye tracing

Embryonic day Somite count Mx Md Mes V S TMN

E9 13–29 ® ® ® ® ®E10 30 ® ® ® ®

31 ® ® ® ®32 ®33 ®34 ®

E10.5 35 ®36 ®37 ®38 39

E11 40

Mx, transganglionic tracing of maxillary neurons from maxillary

arch to brainstem; Md, transganglionic tracing of mandibular

neurons from mandibular arch to brainstem; Mes V, mesencephalic

nucleus traced from mandibular arch to brainstem; S, gross

somatotopy in maxillomandibular component of ganglion and

central bundles ; TMN, trigeminal motor nucleus traced from

brainstem to mandibular arch.

At 30 somites, maxillary cell bodies in the trigeminal

ganglion extended peripheral and central axons and

the latter just made contact with the brainstem. This

pathway was more robust at 31 somites. The

mandibular first order pathway traced completely at

32 somites. Again, this reflects the results of Davies &

Lumsden (1984) who detected maxillary and man-

dibular axons emerging from the ganglion during

early E10. The mandibular injection site is further

away from the ganglion than the maxillary injection

site since the mandibular arch protrudes more

ventrally. This may explain why the mandibular

pathway traced 2 somite stages later. It is also possible

that the initial peripheral projections from the

mesencephalic nucleus provide a bridge for the

proximal mandibular nerve to reach the brainstem

long before the mesencephalic nucleus axons have

reached their peripheral targets.

We detected the mesencephalic nucleus and its

peripheral projections at 32 somites. It was labelled

via long axons that would take some time to reach the

distant mandibular arch. This may explain why it did

not appear until early E10. This finding supports that

of Stainier & Gilbert (1990, 1991) who detected

neurons of the mesencephalic nucleus with short

projections at 10 somites (E8.5). They also reported

that mesencephalic axons do not fasciculate, which

was the case in this study. However, we observed

unipolar and not early bipolar and mature pseu-

dounipolar morphologies as described by Stainier &

Gilbert (1990).

Erzurumlu & Killackey (1983) concluded that in

early rat development the maxillary nerve organises

into fasciculi that innervate single rows of vibrissae

follicles and these fasciculi remain discrete throughout

their course. They suggested the ordered growth of

trigeminal axons accounts for the transfer of vibrissal

follicle patterns to the brainstem, and the earliest

axons grow to their target in straight lines. Later

axons follow their predecessors. This causes the

formation of somatotopic projections by the main-

tenance of near-neighbour relationships between

growing axons. However, Davies & Lumsden (1986)

demonstrated that there was little correspondence and

no obvious pattern in fascicular arrangement in the

developing murine maxillary nerve to the emerging

pattern of vibrissal follicles ; fasciculi merge and

branch to form a plexus that is inconsistent in different

embryos. Numerical analysis of plexus axons revealed

they were not simply composed of discrete collections

of associated axons that merge and separate, but that

axonsfreelyexchangebetweenfasciculi.Theyproposed

that selective elimination of erroneous connections

causes the vibrissal-related topography. In our study

the detail observed in mandibular pathways was more

conspicuous than the maxillary pathway. It is possible

that fasciculation of mandibular axons occurs faster

and more efficiently than maxillary axons because the

mandibular innervation is less dense (Davies &

Lumsden, 1984). Thus order in the maxillary nerve

may emerge at a later stage.

We detected maxillary and mandibular ganglion

cell bodies organised discretely within adjacent but

separate compartments of the murine trigeminal

ganglion at 32 somites (E10). This is prior to pioneer

contact with the branchial epithelium at approxi-

mately 37 somites (Stainier & Gilbert, 1990) and

precedes vibrissal organogenesis in the trigeminal

target field of the murine embryo at E13 by 3 d

(Theiler, 1989). Our study and the evidence of

Erzurumlu & Jhaveri (1992) suggest the periphery

does not induce somatotopy in the ganglion. Separate

proximal and distal maxillary and mandibular bundles

were clearly evident indicating that gross somatotopy

in the ganglion reflects the arrangement of nerve

bundles.

Erzurumlu & Killackey (1983) suggested that

proximal axons of trigeminal ganglion cells contact

the brainstem before distal axons of these same cells

contact the periphery. In contrast, Erzurumlu &

Jhaveri (1992) showed that distal and proximal axons

of rat maxillary ganglion cells are spatially ordered on

reaching the undifferentiated vibrissal field at E12 and

the brainstem at E13 respectively. A developmental


order was not detected in this study since axons were

traced from the periphery only and distal axons would

obviously trace first.

Davies & Lumsden (1984) studied the murine

trigeminal ganglion in vitro. They counted neurites

grown in an NGF-free medium enriched with fetal

calf serum and the cells still extended neurites. Stainier

& Gilbert (1991) also found that the maturation of

trigeminal neurons in vitro with horse and calf serum

was target-independent and neurite morphology pro-

ceeded as it did in vivo. Trigeminal ganglia cultured

without a target in a minimal medium (DME-F12)

with no growth factors added still extend neurites

(Scott & Atkinson, unpublished observations). This

implies that axogenesis is independent of target tissues

and supplementary growth factors.

If gross somatotopy in the trigeminal system is

independent of target contact then how does it arise?

Firstly, extracellular matrix molecules may form tracts

in developing head ectomesenchyme and may en-

courage axons to cluster together and even guide

axons home. This mechanism may operate in the

absence of target tissue epithelium. Indeed there is

evidence of a laminin pathway in chick head mes-

enchyme that extends from the trigeminal ganglion to

the mandibular arch (Riggot & Moody, 1987).

Secondly, neural crest cells that migrate to the

branchial region of the head may act as leaders for

ensuing pioneer axons. Presumptive Schwann cells

that can synthesise laminin are followed by sensory

axons in the chick embryo (Moody et al. 1989). This

does not apply to motor axons and may explain why

the trigeminal motor nucleus did not trace until 38

somites. At this time the mandibular first order

pathway was quite robust and could provide a guiding

scaffold for the axons of the trigeminal motor nucleus

to grow along. However, there is evidence that

cutaneous sensory nerve axons grow in accordance

with the innervation pattern of motor axons in their

vicinity (Scott, 1986). Likewise, sensory projections in

the chick hindlimb are altered following early removal

of motor neurons (Landmesser & Honig, 1986). This

sensory-following-motor sequence does not arise with

the murine mandibular nerve and trigeminal motor

nucleus where the sensory pathway precedes the

motor pathway.

Thirdly, trigeminal ganglion cells first extend

pioneers when the distance to travel to the periphery

and brainstem is very small. The ganglion actually

appears to rest on the brainstem in early head

morphogenesis and the branchial region appears

anatomically uncomplicated. Also the regions that

distal maxillary and mandibular fascicles first grow

into may be areas of low physical resistance. It is

unlikely that a fascicle entering the mandibular arch

would then turn back and leave it. Also the distal

maxillary axons span out diffusely to occupy the wide

dorsal aspect of the maxillary arch. These notions of

trigeminal pathway formation are simplistic and

probably do not explain the striking order that

characterises the trigeminal system. They may how-

ever add to already well-documented theories of axon

guidance.

Finally, gross somatotopy in the trigeminal

ganglion might be genetically predetermined. It is now

well established that neural crest tissue migrating

from the closing neural tube follows restricted

migration from specific rhombomeres to populate

specific branchial arches (Lumsden & Keynes, 1989;

Lumsden et al. 1991). Chick crest cells migrating from

the second rhombomere fill the first branchial arch.

The first rhombomere and the caudal mesencephalon

also contribute crest to the first arch, primarily to the

presumptive maxillary arch and the rostral man-

dibular arch. The caudal mandibular arch and the

maxillomandibular components of the trigeminal

ganglion consist of crest cells derived from

rhombomere 1 and 2 (Lumsden et al. 1991). Hox

genes, which confirm identity on the various cell

populations contributing to each branchial arch

(Wilkinson et al. 1989; Hunt et al. 1991a, b) are not

expressed by cranial neural crest contributing to the

maxillary and mandibular arches. Nevertheless, an

intrinsic code confirming a subsegmental identity in

maxillary and mandibular arches is a possibility,

which could be instrumental in positioning cells of the

trigeminal ganglion and determining routes of out-

growth of peripheral and central axons.

In conclusion, this study has indicated that the

peripheral target tissue does not induce gross somato-

topy in the maxillary and mandibular divisions of the

trigeminal ganglion. Therefore, factors other than

target influences may be important in creating

somatotopy in the maxillomandibular components of

the ganglion and their peripheral and subsequently in

their central projections.

The authors thank Dr Adrian Jowett and Mr Mick

Turton for their assistance with the confocal laser

scanning microscopy and production of the final

images. The confocal laser scanning microscope was

purchased with a grant awarded by the Wellcome

Trust. Lisa Scott was supported by an Anatomical

Society of Great Britain and Ireland scholarship.


Fig. 2. Confocal laser scanning images showing transganglionic tracing of DiO (green) and DiI (red-orange) from the maxillary and

mandibular arches respectively of fixed mouse embryos at E10-E10.5. Parasagittal views are oriented such that the peripheral injection site

is at the right margin of the field or beyond and the brainstem is at the left margin. (a) Gross somatotopy was first detected in a double-

labelled specimen at 32 somites. The maxillary (Mx) and mandibular (Md) components of the trigeminal ganglion occupied adjacent but


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compartmentalisation of the developing trigeminal ganglion into maxillary and mandibular divisions...

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