The buccohypophyseal canal is an ancestral vertebrate trait maintained by modulation in sonic

17 Pages·2017·2.26 MB·English

The buccohypophyseal canal is an ancestral vertebrate
trait maintained by modulation in sonic hedgehog
signaling
Roman H. Khonsari, Maisa Seppala, Alan Pradel, Hugo Dutel, Gael Clement,
Oleg Lebedev, Sarah Ghafoor, Michaela Rothova, Abigael Tucker, John G.
Maisey, et al.
To cite this version:
Roman H. Khonsari, Maisa Seppala, Alan Pradel, Hugo Dutel, Gael Clement, et al.. The buccohy-
pophysealcanalisanancestralvertebratetraitmaintainedbymodulationinsonichedgehogsignaling.
BMC Biology, 2013, 11 (1), 15 p. ￿10.1186/1741-7007-11-27￿. ￿hal-01572918￿
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The buccohypophyseal canal is an ancestral
vertebrate trait maintained by modulation in
sonic hedgehog signaling
Khonsari et al.
Khonsarietal.BMCBiology2013,11:27
http://www.biomedcentral.com/1741-7007/11/27
Khonsarietal.BMCBiology2013,11:27
http://www.biomedcentral.com/1741-7007/11/27
RESEARCH ARTICLE Open Access
The buccohypophyseal canal is an ancestral
vertebrate trait maintained by modulation in
sonic hedgehog signaling
Roman H Khonsari1,2*†, Maisa Seppala1,3†, Alan Pradel4, Hugo Dutel5,6, Gaël Clément5, Oleg Lebedev7,
Sarah Ghafoor1, Michaela Rothova1,8, Abigael Tucker1, John G Maisey4, Chen-Ming Fan9, Maiko Kawasaki1,
Atsushi Ohazama1, Paul Tafforeau10, Brunella Franco11, Jill Helms12, Courtney J Haycraft13, Albert David14,
Philippe Janvier5, Martyn T Cobourne1,3† and Paul T Sharpe1†
Abstract
Background: The pituitary gland is formedby the juxtaposition of two tissues: neuroectoderm arising from the
basal diencephalon, and oral epithelium, which invaginates towards the central nervous system from the roof of
themouth.The oral invagination that reachesthebrain from themouth is referred to as Rathke’spouch, withthe
tipformingthe adenohypophysisand thestalk disappearing after the earliest stages of development. In tetrapods,
formation of thecranial base establishesa definitive barrier between the pituitary and oral cavity; however,
numerous extinct and extant vertebratespeciesretain an open buccohypophysealcanal in adulthood, a vestige of
thestalk of Rathke’spouch.Little is currently known about theformation and function of this structure.Here we
have investigated molecular mechanismsdriving the formation of the buccohypophysealcanal and their
evolutionary significance.
Results: We showthatRathke’spouch is located ata boundary region delineated by endoderm, neural
crest-derived oral mesenchymeand theanterior limit of the notochord, using CD1, R26R-Sox17-Cre and R26R-Wnt1-
Cre mouse lines. As revealed by synchrotronX-ray microtomography after iodine staining in mouse embryos, the
pouch has a lobulated three-dimensional structure thatembraces the descending diencephalonduring pituitary
formation. Polarisfl/fl; Wnt1-Cre,Ofd1-/- and Kif3a-/- primary cilia mouse mutants have abnormal sonic hedgehog (Shh)
signaling and allpresent with malformations oftheanterior pituitary gland and midline structures of theanterior
cranial base. Changes in theexpressions ofShh downstream genes are confirmed inGas1-/- mice. Froman
evolutionary perspective, persistence ofthebuccohypophyseal canal is a basal character for all vertebrates and its
maintenance in several groups is related to a specific morphology ofthemidline that can be related to modulation
inShh signaling.
Conclusion: These results provide insightintoa poorlyunderstood ancestral vertebrate structure.It appearsthat
theopeningof the buccohypophysealcanaldependsupon Shhsignaling and that modulation inthis pathway
most probably accounts for itspersistence inphylogeny.
Keywords: Buccohypophyseal canal, Chondrichthyans, Coelacanth, Endoderm, Knock-out mouse, Midline, Neural
crest, Primary cilia, Shh
*Correspondence:[email protected]
†Equalcontributors
1DepartmentofCraniofacialDevelopmentandStemCellResearch,
ComprehensiveBiomedicalResearchCenter,DentalInstitute,King’sCollege
London,London,UK
2ServicedeChirurgieMaxillo-Faciale,CentreHospitalierUniversitaire
Hôtel-Dieu,Nantes,France
Fulllistofauthorinformationisavailableattheendofthearticle
©2013Khonsarietal.;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreative
CommonsAttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginalworkisproperlycited.
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Background important role of Shh in facial midline development
The buccohypophyseal canal (BHC) is a peculiar duct [16,17] and pituitary formation [4], we also investigated
connecting the floor of the diencephalon and the adeno- persistenceoftheBHCinfourShhpathway-relatedtrans-
hypophysis with the roof of the mouth. In all mammals, genic mouse lines: Polarisfl/fl;Wnt1-Cre, Ofd1-/- Kif3a-/-
it closes in the early stages of development (embryonic and Gas1-/-. The expression changes of Shh downstream
day (E) 11.5 in CD1 mice). Its persistence in humans genes were investigated in Gas1-/- mice using in situ
represents an abnormality that occurs in a wide range of hybridization. Finally, we performed a comparative study
disorders of the midline, such as holoprosencephaly [1]. of species that retain an open canal in order to describe
An open canal is in fact very rare in living adult gna- the distribution of BHC persistence amongst vertebrates
thostomes (jawed vertebrates) and entirely lacking in and we have attempted to relate the presence or absence
adult tetrapods: within this group, the BHC only persists of the BHC with modifications in midline anatomy that
in some larval lungfishes [2] and lissamphibians [3]. By couldbeduetomodulationinShhsignaling.
contrast, an open BHC is very common in most fossil
early gnathostomes, in particular Paleozoic fishes [2]. Results
Surprisingly, even though a connection between the Thebuccohypophysealcanalderivesfromanectodermal
mouth and the central nervous system may play some placodelocatedclosetothreedevelopmentalboundaries
physiological role, the mode of formation and func- In the absence of conclusive data, we first aimed to con-
tions of the BHC are poorly understood. firm that RP is located at the anterior limit of the oral
The BHC is a vestige of Rathke’s pouch (RP), which is endoderm in the roof of the oral cavity by using R26R-
formed from the ectodermal diverticulum of the ade- Sox17-Cre mice. Sox17 is a reliable early endoderm mar-
nohypophysis after its separation from the roof of the keressentialforitsdifferentiation[18].
mouth. RP derives from the hypophyseal placode, which Sox17-positive cells extended to the border of the inva-
is initially an epithelial thickening in a small oral region ginatingRPandalsotosomerareendodermcellspopulat-
underlying the diencephalon, where expression of the ing a region corresponding to the former localization of
midline signaling molecule sonic hedgehog (Shh) is the buccopharyngeal membrane. However, there was no
down-regulated. However, Shh target genes are expressed contribution of endoderm to RP (Figure 1a,b). Following
throughout the walls of the RP and are required for its this, we used R26R-Wnt1-Cre reporter mice to confirm
normal outgrowth and differentiation [4]. In mice, the previously published data that RP is bordered anteriorly
pouch starts forming around E9.5 and the transient BHC by neural crest cells (Figure 1c). We have also illustrated
isultimatelyclosedbyE11.5.Inspecieswhereapersistent thepositionofthetipofthenotochordbehindtheposter-
BHC is present at adult stages, and in mutants with an ior wall ofthepouch (Figure1d).Together, thesefindings
open BHC, the pouch does not close and foramina are confirmthepositionofRPattheintersectionofthreeim-
maintained on the intracranial and/or oral sides of the portant boundaries, where molecules involved in midline
basisphenoid bone, sometimes coalescing with foramina formationintersect[16,17,19,20].
oftheinternalcarotidarteries[5]. The three-dimensional (3D) morphology of the devel-
RP is located at the rostral end of the notochord [6] at opingpituitaryglandinwild-typemiceatE11.5islargely
the foremost limit of the gut endoderm [7] and marks unknown and was investigated using synchrotron elec-
the posterior limit of the neural crest-derived territories tron microtomography following contrast enhancement
inthe skull base [8,9]. Itsposition at three important tis- with iodine staining (Figure 2a). This novel method al-
sueboundaries[10]exposesthisregiontonumeroussig- lows a cellular resolution to be reached in virtual sec-
naling pathways involved in cranial base morphogenesis, tions and provides images with histological quality and
such as Shh [11]. Shh is in fact secreted both by the ros- sufficient contrast to allow segmentation and 3D re-
tral tip of the notochord [11,12] and the most rostral construction of small structures such as RP. At E11.5,
zone of the foregut endoderm [13]. While the positions RP is almost completely separated from the oral ecto-
of the RP relative to the rostral end of the notochord derm, forming a heart-shaped pouch and embracing the
and to the posterior border of the neural crest-derived tissue mass originating from the ventral diencephalon
tissues are well documented, there prevails an uncer- (Figure2b).Interestingly,thispouchhasabilobateshape
tainty about its position in relation to the gut endo- alongtheleft-right axis(Figure2c).
derm [14,15].
In this study, we first provide definitive proof of the Persistenceofthebuccohypophysealcanalincilia-related
relative position of endodermal cells to the RP by using mutants:amorphologicalstudy
R26R-Sox17-Cre reporter mice, and thus underline the Shh is a secreted signaling molecule required for pat-
high exposure of this anatomical region to morphogens terning of the craniofacial midline [16,17,19,20] and RP
involved inmidlineformation. Inparticular,knowing the is located in an anatomical region where Shh-related
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performed on these mice have demonstrated abnormal
Shh signaling and phenotypes consistent with both re-
duced and increased Shh signaling activity [19,22,23].
Histological examination of Polarisfl/fl;Wnt1-Cre, Ofd1-/-
andKif3a-/-miceatE18.5revealedvariablemalformations
of the pituitary gland, commonly hindering fusion of the
basisphenoidboneintheregionofRP.Moreprecisely,the
pituitaryglandinPolarisfl/fl;Wnt1-Cremicewasconnected
to the oral cavity. These mutants also had an abnormally
shaped, two-headed gland, reminiscent of the anteropos-
terior pituitary duplications seen in Noggin-/- mutants
[24,25]. Examination of the skeletal preparations showed
widecleftingofthebasisphenoid,dysmorphicbonesasso-
ciated with the anterior cranial base and a generalized
wideningoftheanteriormidlinestructures(Figure3a,b,c).
In both Kif3a-/- and Ofd1-/- mutant mice, a malformed
and displaced pituitary gland was noted, together with
Figure1Rathke’spouchislocatedatatripleboundary. a wide opening in the basisphenoid (Figure 3d,f). In
(a)Whole-mountLacZstainingofanR26R-Sox17-CreE10.5mouse Ofd1-/- mutants this was accompanied by duplications of
embryoshowingtheanteriorlimitoftheendoderm(redarrowhead) the nasal septum (Figure 3e). Importantly, mutations of
andtheposteriorborderofRathke’spouch(bluearrowhead);
theOFD1geneinhumanscanalsoresultindefectsofthe
endobuccalviewoftheoralroof.(b)Anteriorlimitoftheendoderm
craniofacial midline. A mid-sagittal section of a medical
inthemid-sagittalplaneatE10.5(arrowhead),LacZstaining,R26R-
Sox17-Cremouseembryo.(c)Posteriorlimit(arrowhead)ofthe computed tomography scan of a patient with an OFD1
neuralcrest-derivedmesoderminthemid-sagittalplaneatE12.5, mutation (c.412G>A, p.G138S) demonstrated abnormal-
correspondingtothelocationoftheclosingbuccohypophyseal itiesofthesphenoidbonewithdefectiveossificationbelow
canal,inanR26R-Wnt1-Cremouseembryo.(d)Anteriorendofthe
the pituitary gland [26]. Clinical examination of the same
notochord(arrowhead),eosinstaining,E10.5,wild-typemouse
patient revealed mid-facial anomalies, including a midline
embryoant,anterior;post;posterior;RP,Rathke’spouch.
uppercleftlip(Figure3g,h).
The fenestration of the basisphenoid bone and widen-
molecules are produced. We therefore focused our in- ing of the mid-facial structures are therefore common
vestigation on mutant mouse lines with known distur- features in these cilia-related mutants. The formation of
bances inShhsignaling. the pituitary gland is also variably disrupted in these
Primary cilia are implicated in the regulation of Shh mice and can be associated with the partial persistence
function [21]. Thus, to examine the possible role of Shh of the BHC. In comparison to previous studies, these
signaling in closure of the BHC, a phenotypic analysis of midlineanomalies are close tothephenotypesassociated
three cilia-related transgenic mice, Polarisfl/fl;Wnt1-Cre, with modifications in the expression patterns of Shh
Ofd1-/- and Kif3a-/-, was carried out. Previous studies pathway-related genes[19].
Figure2Three-dimensionalmorphologyofRathke’spouch.(a)Rathke’spouchatE11.5inawild-typemouseincoronalsection,visualized
usingsynchrotronX-raymicrotomographyafteriodinestaining.Theoralectodermisstillconnectedtothepouch(redarrow).(b)Three-
dimensionalreconstructionofthesamesample.Anterioraspectclippedinthecoronalplane.Thediencephalon(blue)invaginates(blackarrow)
towardsthepouch(yellow),whichisstillincontinuitywiththeoralectoderm(redarrow).(c)Posteriorviewoftheformingpituitaryatthesame
stage.Thediencephalonevagination(blackarrow)fitsintoaheart-shapedRathke’spouch(yellow)stillconnectedtotheoralectoderm(red).
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Shh signaling in the midline of the frontonasal process
and inthepalatal shelves.Gas1-/- micedisplaymicroform
holoprosencephaly, including mid face hypoplasia, cleft
palateandmaxillaryincisorfusion[28].Humanswithmu-
tations in GAS1 also present with minor forms of holo-
prosencephaly,includingmaxillaryhypoplasia,hypoplastic
nasalseptumandcleftliporpalate[31].
Histological examination of Gas1 mutants demon-
strated a ventrally displaced pituitary gland located im-
mediately adjacent to the oral ectoderm and interfering
with closure of the basisphenoid bone, as seen also in
Polarisfl/fl;Wnt1-Cre, Ofd1-/- and Kif3a-/- mutant mice. A
ventral examination of the skeletal preparations also re-
vealed fenestration of the basisphenoid bone (Figure 4).
In order to understand the early mechanisms underlying
defective pituitary gland development and basisphenoid
clefting in Gas1-/- mutants, we examined the expression
of Shh, Patched-1 (Ptch1), Gli1, Bone morphogenetic
proteins-2, -4 (Bmp2, 4) and Fibroblastgrowthfactors-8,-
10 (Fgf8, 10) at E10.5. The transmembrane-domain pro-
tein Ptch1 acts as the principal receptor for Shh, while
Gli1 is a downstream transcription factor and transcrip-
tional target of Shh. Bmp2 is induced by Shh in RP
where it regulates expression of different ventral tran-
scriptionfactorsinvolvedincellspecificationofthepitu-
itary gland [4]. Early expression of Bmp4 from the
ventral diencephalon is required for initial organ com-
mitmentand itssubsequentexpression inthe infundibu-
lum takes part in the development of the pituitary
Figure3PersistentbuccohypophysealcanalandShhpathway-
relatedabnormalitiesinciliarymutants.(a)AtE18.5,thereisno
midlineforameninthebasisphenoidboneofwild-typemouse
embryos.(b)InPolarisfl/fl;Wnt1-CremutantsatE18.5,alargedefect
inthebasisphenoid(greenarrow)isassociatedwithamidline
maxillarycleft(bluearrowhead).(c)Histologicalstainingshowingan
openbuccohypophysealcanalatE18.5inaPolarisfl/fl;Wnt1-Cre
mutant(redarrow).(d)BasisphenoiddefectinaKif3a-/-mutantat
E18.5(bluearrowheads)withbulgingofthepituitarytowardsthe
oralcavity.(e)DuplicationofthenasalseptuminanOfd1-/-mutant
atE18.5(bluearrowhead).(f)Basisphenoiddefectunderthe
pituitaryatE18.5inanOfd1-/-mutant(bluearrowheads).
(g)IncompletemedianupperlipcleftinapatientwithanOFD1
mutation(blackarrows).(h)Ossificationdefectofthesphenoid
boneinthesamepatient(redarrowhead);standardmedical
computedtomographyscan,mid-sagittalviewat16yearsold. Figure4PersistentbuccohypophysealcanalinGas1-/-mutants.
(a,b)ControlandmutantskeletalpreparationsatP0usingAlcian
blueandAlizarineredshowingaclearlydefinedbonymidline
PersistenceofthebuccohypophysealcanalinShh defect(whitearrow)inthebasisphenoid.(c,d)Controlandmutant
frontalsectionsstainedwithhematoxylin-eosinshowingthebony
pathway-relatedmousemutants:anexpressionpattern
defectinthebasisphenoidboneandthemodificationofthe
study
positionoftheanteriorlobeofthepituitaryglandinmutant
Gas1 is a glycosylphosphatidylinositol-anchored mem- (apg,blackarrow);theanteriorlobeofthepituitaryglandislocated
brane protein that has been shown to act as an agonist directlyundertheoralepitheliumofthepharynx.apg,anteriorlobe
in Shh signaling during development of the early facial ofthepituitarygland;bo:basioccipitalbone;bs,basisphenoidbone;
dc,diencephalon;pl,palatinebone;px,pharynx;pi,posteriorlobeof
midline [27,28] and as a potential antagonist in the early
thepituitarygland;rr,residuallumen.
toothfield[29,30].LossofGas1activityresultsinreduced
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anlage [32]. Fgf8 and Fgf10 are dorsal markers expressed in close proximity to the anterior of RP, which could ex-
by the invaginating diencephalon and havean antagonis- plaintherescuedexpression ofPtch1andGli1inthisre-
tic effect onShh. Fgf8also has theability toinduceinva- gion in comparison to posterior o RP. Bmp2 and Bmp4
gination ofRP[4]. expression remained unchanged in the oral ectoderm
Notably, Shh was similarly expressed in wild-type and and RP in the absence of Gas1 activity. Also, Fgf8 and
Gas1-/- mutant mice by the oral ectoderm posterior to Fgf10wereboth stronglyexpressedinwild-typeandmu-
RP. However, expression of the downstream factors tant mice inthe ventralinvaginationofthediencephalon
Ptch1 and Gli1, although extending throughout the ven- (Figure 5). The fact that both Ptch1 and Gli1 were more
tral and dorsal RP in wild type mice, was limited to the strongly downregulated on the posterior wall of RP
ventral side of the anterior part of the RP in mutants. could also be related to the fact that the pouch is a
Shh is also strongly expressed by the ventral forebrain tissue boundary between anterior neural crest-derived
Figure5ExpressionpatternofShhpathway-relatedgenesinGas1-/-mutants.AtE10.5,theexpressionpatternsofShh,Bmp4,Bmp2,Fgf8
andFgf10aresimilarinwild-typeandGas1-/-mutantmice.TheexpressionterritoryofPtch1andGli1areneverthelessreducedposteriorlyand
apicallywithinRathke’spouch.H&E,hematoxylinandeosin.
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territories and posterior mesoderm-derived regions. In Thebuccohypophysealcanalandthecranialbasein
fact, we show here that a genetic disorder specifically af- chondrichthyans(cartilaginousfish)
fects gene expression at the border of neural crest and In order to investigate the evolutionary origins of BHC
non-neuralcresttissues.Althoughtheroleofthisborder persistence,wefirstexaminedthecranialbaseofseveral
has been emphasized in different morphogenetic pro- key chondrichthyan species. These cartilaginous fish
cesses, such as suture patency [33], differential gene ex- represent a vertebrate group with high variability in the
pression related to this limit had not been reported. The anatomyofthecranialbase,andwenotedaninteresting
fact that this boundary overlaps the endoderm-ectoderm correlation between persistence of the BHC and the
boundary and the anterior limit of the notochord most midlinestructureoftheanteriorcranialbase.
probably contributes to differential gene expression on In many fossil chondrichthyans, such as in Doliodus
anterior and posterior sides of this limit. The altered ex- (Figure 6a,b) [34], Pucapampella [35], Orthacanthus
pression of Shh downstream target genes in Gas1-/- mu- (Figure 6e) [36], Cladodoides [37], Kawichthys [38],
tants indicates that this pathway is involved in abnormal Cobelodus [39] and Egertonodus (Figure 6c,d) [40], the
development of the anterior pituitary gland and subse- palatoquadratesdonotfuseatthemidlineandaresepa-
quently in persistence of foramina in the midline of the ratedbyanethmoidalinternasalseptum.Inthese chon-
basisphenoid bone. drichthyan taxa, there is either a distinct, persistent
BHC separated from the internal carotid canals, or a
Apersistentbuccohypophysealcanalisassociatedwith large hypophyseal fenestra - an opening in the cranial
widermidlinestructuresingnathostomes(jawed base that communicates with the hypophyseal chamber
vertebrates) of the endocranial cavity by which the internal carotid
Persistence of the BHC is considered an abnormal find- arteriesentertheneurocranium.
ing in mice and our results indicate that this is related By contrast, we noticed that living chondrichthyans,
to modifications in the Shh signaling pathway. Interest- such as the neoselachians (modern sharks and rays), and
ingly, the BHC is also present in a large number of holocephalans(modernchimaeroids,ratfish),donotpos-
fossil jawed vertebrate species as a normal anatomical sessanopenBHCintheadult.Inneoselachians,thetwo
feature. palatoquadrates are fused at the symphysis in the mid-
In this section, we have examined the persistence of line, and are thus not separated by an ethmoidal
the BHC in selected chondrichthyans (cartilaginous internasal septum (for example, Isurus; Figure 6f, red
fish) and sarcopterygians (bony lobe-finned fish) and arrow). The condition in holocephalans is different be-
correlate this character with key features of midline causethepalatoquadratesarefusedtotheneurocranium,
anatomy. although they remain separated anteriorly from each
The purpose of this evolutionary investigation was to other by an internasal septum [5,41]. Nevertheless, the
find morphological evidence for the involvement of Shh latter is very narrow and the two nasal capsules are very
inBHCpersistenceinearlyvertebrates. closetoeachotherinthemidline.
Figure6Theclosureofthebuccohypophysealcanalinchondrichthyansisassociatedwithmodificationsoftheanteriormidline.
(a)DetailofthepalateinDoliodusproblematicus,NBMG10127,LowerDevonianofCanada;three-dimensionalreconstructionbasedonX-ray
microtomography,showingpalatoquadrates(inpurple)widelyseparatedbytheethmoidregion(inyellow,arrowhead)(after[34]).(b)Same
specimenwithteeth(red)inplace;notethepresenceofteethontheethmoidkeel.(c)Anteriorviewoftheheadofahybodontshark,
Egertonodusbasanus(Egerton),NHM6356,LowerCretaceous,Wealden,Pevensey,England(after[40]).(d)Samespecimen,withanteriortooth
positionsinred,andanteriorpartsofthepalatoquadratescoloredinyellow;thedistancebetweenthepalatoquadratesisnarrowerthanin
Doliodus(arrowhead).(e)AnteriorviewofanarticulatedPermiansharkbraincaseandjaws,Orthacanthussp.(probablyO.texensis),fromArcher
County,Texas(castofMCZ12872);thepalatoquadratesarestillseparatedbyanarrowbandofethmoidbone(arrowhead).(f)Anteriorviewofan
articulatedextantsharkbraincaseandjaws,Isurusoxyrinchus,showingafusionofthetwopalatoquadrates(arrowhead),onlyseparatedbythe
palatonasalligament.
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Using 3D reconstructions based on synchrotron blue arrow, and [42]). The posterior tilt of the duct is re-
microtomography data (see Additional file 1), we show markable. In fact, in coelacanths, craniofacial development
that in an extant holocephalan embryo, the BHC is partly is most probably marked by a negative allometric growth
obliterated: the cranial base separates the pituitary into betweenthebrainandtheendocranialcavity[43].Inpups
two parts (Figure 7a, blue arrow and Figure 7b, blue out- andjuveniles,theforebrainextendsanteriorlytotheBHC,
line) and the extracranial part aborts during ontogeny whereas in adults the forebrain is located far posterior to
(Figure 7c, the blue arrow indicates the ongoing fusion of these structures (Figure 9a, red arrow). Consequently, a
the cartilage in the region of the vestigial BHC). In brief, posterior tilt of the hypophyseal duct seems to occur dur-
the association of a persistent BHC and wider anterior ingpostnataldevelopment.
midline structures observed within basal chondrichthyans The braincase of basal actinistians is poorly recorded,
compared to living chondrichthyans (Figure 8) resemble but in theDevoniantaxainwhich the braincasehasbeen
ourobservationsinShhpathway-relatedmousemutants. described (for example, Miguashaia, Euporosteus and
Diplocercides), the BHC always remains persistent, with
an oral opening in the parasphenoid [44]. These species
Thebuccohypophysealcanalandthecranialbaseina
display a broad parasphenoid, which reaches far forward,
sarcopterygian(bonylobe-finnedfish),theextant
ventrally to the nasal capsules, and have a wide
coelacanthLatimeriachalumnae
interorbital septum. Furthermore, in these taxa, the brain
Sarcopterygians are a group of osteichthyans currently
ispresumedtohaveextendedanteriorlybeyondtheBHC.
comprising tetrapods, coelacanths and lungfish. Scanning
More derived fossil coelacanths such as the Cretaceous
of the extant coelacanth Latimeria chalumnae revealed
genera Axelrodichthys [45], Macropoma [44] and
thatithasapartlyobliteratedbutgenuineBHC(Figure9a
Megalocoelacanthus [46] display a condition similar to
and Additional file 2). In this species, the parasphenoid is
that of the extant genus Latimeria (Figure 8). Taken
densely covered with denticles on its anterior portion but
together,itappearsthat,inactinistians,theopeningofthe
doesnotshowanyforamenonitsoralside.Togetherwith
BHC into the mouth is a plesiomorphic character associ-
the neurohypophysis, the pars intermedia of the pituitary
ated with a broad-based, platybasic braincase and further
glandislocatedventrallytothetelencephalon,whereasthe
supports the possibleinvolvementof Shhpathway-related
parsdistalisissituateddorsaltotheroofoftheoralcavity
genes in the closure of the BHC during gnathostome
andconnectedtotheparsintermediabyaverylonghypo-
evolution.
physeal duct that extends through the BHC (Figure 9b,
Discussion
A persistent BHC in mice results from the maintenance
of the canal initiated by RP. We have first shown that
RP is located at a triple tissue boundary (Figure 1) and is
thus at the crossroads of several signaling pathways
involvedinskullbaseformation, includingShh.Wehave
also shown that abnormal persistence of the BHC in
mice is associated with defective Shh signaling and is
correlated with anterior midline modifications. In extant
and extinct vertebrate species with a normally persistent
BHC in adults, we have finally found a distinctive
organization of the anterior midline reminiscent of
involvement oftheShhpathway.
Herewediscusstheevolutionaryimportanceofthepos-
ition of the endoderm relative to RP. We then provide an
overview of the distribution of the BHC in all vertebrate
groups,discussthecurrenthypothesisonthefunctionsof
this canal and expose the anatomical relationships be-
Figure7Thebuccohypophysealcanalintheholocephalan
tweentheBHCandthecarotidcanals.
Callorhinchus.(a)SagittalsectionobtainedfromsynchrotronX-ray
microtomographyfromspecimenAMNH257784,showingthe
pituitaryregion(arrowhead).(b)Thepituitaryglandhasrecesseson Endoderm/ectodermlimitandpituitaryformation
bothsidesofthesphenoid(outlinedinblue).(c)Three-dimensional The absence of endoderm in the hypophyseal placode of
reconstructioninventralviewshowingthepharyngealpituitary mice (Figure 1) is of importance from an evolutionary
connectedtotheintracranialpituitarybyalongstalk(arrowhead)
point of view. It is believed that in the common jawless
withzonesofcartilagefusionappearing.
ancestor of all vertebrates, the hypophyseal placode and
Khonsarietal.BMCBiology2013,11:27 Page8of15
http://www.biomedcentral.com/1741-7007/11/27
Figure8Interrelationshipsoffossilandextantvertebrates.Redlinesindicatetheclosureofthebuccohypophysealcanal.Topologybased
ondatafromliterature[2,38,46,57,58].actinopt.,actinopterygians;euchon.,euchondrocephalians;neosel.,neoselacians;tetrapodomor.,
tetrapodomorphs.
Figure9ThebuccohypophysealcanalinanadultspecimenofLatimeriachalumnae.Three-dimensionalreconstructionsbasedon
computedtomographyscansofthespecimenMNHN-CCC22.(a)Lateralsagittalviewshowingthenotochord(yellow),thecranialcavity(purple)
containingaverysmallandposteriorlyplacedbrain(redarrowhead),thebasisphenoid(green)andtheparasphenoid(red).(b)Samestructuresin
closerview;theredarrowheadindicatesthebrain;themuchelongatedpituitaryglandextendsfromthebraintowardstheskullbaseinagroove
formedbythebasisphenoid(bluearrow).BaS,basisphenoid;BC,cranialcavity;NC,notochord;PaS,parasphenoid.