Department of Biomedical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853-6401, USA
* Author for correspondence (e-mail: dmn2{at}cornell.edu)
Accepted 12 May 2004
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Myogenesis, Quail-chick chimera, Craniofacial muscles, Extra-ocular muscles, Myotome, myf5, myod, paraxis, lbx1, Myosin heavy chain
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Many of the signals and transcription factors necessary for myogenesis in
murine and avian somites have been identified and their roles characterized
experimentally (Borycki and Emerson,
2000; Sabourin and Rudnicki,
2000
; Tajbakhsh and
Buckingham, 2000
). However, few studies have investigated the
interactions controlling myogenesis in the head. The objective of this
research was to characterize the cephalic myogenic environment by assaying its
ability to direct the development of three distinct trunk mesodermal
populations: medial half somite, lateral half somite and segmental plate
(presomitic) mesoderm.
Differentiation and compartmentalization in somites
Trunk myogenesis requires a progressive series of tissue interactions that
begin in the segmental plate region shortly after gastrulation
(Arnold and Braun, 2000;
Buckingham, 2001
;
Stockdale et al., 2000
).
Although most myogenic interactions are similar throughout the trunk,
peripheral signals unique to the occipital, brachial and lumbosacral levels
elicit the formation of migratory laryngoglossal and appendicular myoblasts
from the lateral myotome in chick embryos
(Alvares et al., 2003
;
Brand-Saberi et al., 1996
;
Hayashi and Ozawa, 1995
).
The onset of commitment to the muscle lineage is difficult to ascertain
precisely and shows considerable interspecies variation
(Buckingham et al., 2003).
Classical explant experiments indicate that commitment coincides with the
formation of somites, and autonomous competence to differentiate is acquired
shortly thereafter (Christ and Ordahl,
1995
; Ordahl and Le Douarin,
1992
). This conclusion is consistent with segmental plate cell
culture experiments (Buffinger and
Stockdale, 1994
; Gamel et al.,
1995
; Stern and Hauschka,
1995
). However, PCR assays have found myf5 or
myod transcripts in cranial segmental plate mesodermal cells in the
chick (Kiefer and Hauschka,
2001
), suggestive of an earlier bias towards myogenesis.
Orchestrating myogenic initiation, differentiation, movements,
proliferation and survival among trunk muscle precursors requires a consortium
of extrinsic signals from adjacent tissues
(Alves et al., 2003;
Borycki and Emerson, 2000
;
Christ and Brand-Saberi, 2002
;
Pownall et al., 2002
), as well
as intra-somitic signals. Some signals are distinct for medial and lateral
myogenic zones of the dermamyotome, and maintaining the ratio among signals is
essential for proper spatio-temporal coordination of myogenesis (e.g.
Wagner et al., 2000
). In
addition, the disparate origins of medial and lateral trunk paraxial mesoderm
may predispose these two sets of cells to respond differently to extrinsic
signals (Sporle, 2001
). At
present it is difficult to distinguish these historical biases from those
acquired because of subsequent differences in their position in relation to
neighboring tissues.
In avian embryos, the medial somite compartment gives rise to at least two,
temporally distinct myoblast populations
(Buckingham, 2003;
Denetclaw and Ordahl, 2000
;
Summerbell et al., 2000
).
These form all epaxial muscles and those hypaxial muscles that remain close to
developing vertebrae and proximal ribs
(Burke and Nowicki, 2003
).
Thus, muscles derived from the medial half somite always remain within the
paraxial mesoderm community. By contrast, lateral myotome-derived myogenic
cells at restricted sites along the body axis migrate into neural crest or
lateral mesoderm (Nowicki et al.,
2003
), forming laryngoglossal and appendicular muscles, and some
body wall muscles close to the ventral midline. These cells are
pax3-dependent and during their migrations express the transcription
factors paraxis, lbx1 and six1, and the receptor c-met
(Delfini and Duprez, 2000
;
Gross et al., 2000
;
Laclef et al., 2003
;
Mennerich et al., 1998
;
Uchiyama et al., 2000
;
Williams and Ordahl, 2000
).
Paraxis expression occurs throughout the dermamyotome of all somites,
and is initiated in cells within the cranial border of the segmental plate. By
contrast, expression of lbx1 is restricted to the lateral margins of
somites, and is transient at all axial levels, except those adjacent to the
limb buds and the occipital region (somites 1-5), from which appendicular and
laryngoglossal precursors arise, respectively
(Brohmann et al., 2000
;
Gross et al., 2000
;
Schäfer and Braun,
1999
).
All skeletal myoblasts express myf5 and myod. In both
avian and murine embryos, tongue and laryngeal muscle precursors maintain
myf5 expression throughout their ventral movements; appendicular
myogenic cells do not (Bladt et al.,
1995; Dalrymple et al.,
2000
; Mackenzie et al.,
1998
; Noden et al.,
1999
).
Craniofacial myogenesis
Developing branchial and extra-ocular muscles originate in paraxial
mesoderm that does not undergo epithelialization or form separate segmental
units. Myogenic condensations occur in discrete and separate foci within the
head paraxial mesoderm (Couly et al.,
1992; Hacker and Guthrie,
1998
; Noden,
1983b
; Noden et al.,
1999
). Each primordium initiates the formation of multinucleated
myotubes while simultaneously changing its position and relation to
neighboring tissues. Initially surrounded by nonmyogenic paraxial mesoderm,
head muscle primordia move peripherally to become surrounded by, and later,
infused with, neural crest-derived connective tissues. The directions and
routes of these morphogenetic movements are unique for each extra-ocular and
branchial muscle. These movements separate primordia that are initially
neighboring [e.g. lateral rectus (LR) from dorsal oblique (DO), palpebral
depressor from other proximal first branchial arch (BA1) muscles], or may
bring initially divergent primordia into close proximity [e.g. the dorsal
rectus (DR), medial rectus (MR) and LR all converge near the ciliary
ganglion].
The LR is the first head muscle to exhibit mesenchymal condensation and to
initiate myf5 transcription
(Noden et al., 1999). The LR
primordium moves rostrally towards the ciliary ganglion, then myotubes at the
leading edge turn laterally and expand towards the equatorial zone of the eye
(Wahl et al., 1994
). The
dorsal oblique (DO) primordium arises rostral to the LR, lateral to the
mesencephalon, and then moves rostrally along an arc that parallels the dorsal
margin of the eye. Expression of myf5 and myosin follows shortly
after the LR. The movements of LR and DO primordia begin during day 3 of
incubation (stages 16-18) and are not complete until day 5 (stages 24-25)
(Noden et al., 1999
).
Myoblasts of the first branchial arch (BA1) originate at the same axial level
as the LR, but these precursors are dorsolateral to, and separate from, the LR
progenitors (Couly et al.,
1992
; Noden,
1991b
). They are initially in close contact with the overlying
surface ectoderm, but become separated from this epithelium by migrating
neural crest cells.
Muscle differentiation begins before the onset of these movements. The
chick LR, for example, initiates myf5 expression immediately
ventrolateral to rhombomeres 1 and 2, close to the notochord and trigeminal
ganglion. Myf5 and myod expression are detectable in the LR
at stages 13.5 and 14.5, but myosin heavy chain (MyHC) synthesis does not
occur until stage 21.5 (Noden et al.,
1999). This two-day delay in initiating MyHC production is
characteristic of head muscles, and contrasts with the rapid progression to
MyHC synthesis in most medial myotome-derived cells. Lateral somite-derived
cells that form the hypoglossal cord also have a delayed onset of MyHC
production.
Experimental comparisons of trunk and head mesoderm
Accounts of heterotopic transplants of paraxial mesoderm have been
available since the pioneering work of Adelmann
(Adelmann, 1938), but it was
not until methods of identifying and characterizing the progeny of
transplanted cells became available that the totality of outcomes could be
assessed. In an extensive series of quail-to-chick transplants of whole
somites, or segmental plate mesoderm plus overlying ectoderm, Noden found
trunk mesoderm cells capable of contributing to normal extra-ocular and
branchial arch muscles while also forming large, irregular ectopic muscles
(Noden, 1986
).
More recently, Hacker and Guthrie, and Mootoosamy and Dietrich, implanted
newly formed somites beside the hindbrain and assayed chimeric embryos for
expression patterns of trunk and head muscle markers
(Hacker and Guthrie, 1998;
Mootoosamy and Dietrich,
2002
). Grafted somites formed ectopic condensations that expressed
myf5 and myod plus several somite markers, including
paraxis, pax1 and pax3, but failed to express lbx1,
suggesting that the lateral myotome program is not activated in the head
environment. Both groups also transplanted segmental plate mesoderm, but the
results were inconsistent. Mootoosamy and Dietrich found no differences
between segmental plate and somite grafts
(Mootoosamy and Dietrich,
2002
), whereas Hacker and Guthrie found very limited dispersal or
myogenesis by transplanted segmental plate cells
(Hacker and Guthrie, 1998
).
Thus, the ability of the head environment to promote somitogenesis, muscle
lineage activation and myoblast migration in these somite precursors remains
controversial.
Experiments reported here were designed to resolve inconsistencies in these previous studies, and, in particlur, to examine both early differentiation and later morphogenesis of cell populations derived from medial and lateral half somites, or from presomitic mesoderm (segmental plate) grafted in place of cephalic paraxial mesoderm. We find that grafts of all trunk paraxial mesoderm populations produce both highly mobile and stationary cells, and are able to contribute to both normal and ectopic head muscles. Ectopic muscle condensations typically express trunk muscle markers (e.g. paraxis, lbx1) and differentiate rapidly, mimicking the trunk timetable. Not seen previously is a population of cells that emerge from grafted trunk mesoderm and form scattered individual myocytes and myotubes. In these cells, lineage commitment and differentiation have become uncoupled from the normal process of muscle morphogenesis.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
In situ hybridization
Embryos were fixed at stages 14-22 by immersion in 4% buffered formaldehyde
at 4°C, pH 7.4, then washed, dehydrated and stored in 100% methanol at
20°C. Whole embryo visualization of mRNA was as described
previously (Wilkinson, 1998;
Noden et al., 1999
). In most
cases, the roof of the hind- and midbrain regions was opened longitudinally to
enhance antibody access to deeper tissues. Probes used were
digoxigenin-conjugated myod and myf5
(Kiefer and Hauschka, 2001
),
chick paraxis (Barnes et al.,
1997
), and lbx1. lbx1 was made by RT-PCR of stage 22 limb
tissue using primers of bases 758-777 and 1056-1074 from the murine
lbx1 sequence published by Jagla et al.
(Jagla et al., 1995
), ligated
into Promega pGEM-T Easy vector. After clearing and examination in 100%
glycerol, some embryos were rehydrated, embedded in 4% low-melting point
agarose and sectioned at 60 µm on a vibratome. Embryos were visualized
using darkfield illumination on a Wild M400 Macroscope. Digital images
(QImaging) were processed in Photoshop.
Immunohistochemistry
Embryos were fixed at stages 18-30 in Serra's fixative at 4°C for 2-3
hours, then either embedded in Paraplast and sectioned (6 µm), or
rehydrated and cleared for whole embryo antibody visualization. Primary
antibodies included QCPN (diluted 1:3, Developmental Studies Hybridoma Bank)
to identify all quail cells, F59 (1:10, a gift from Frank Stockdale) to
identify cells producing skeletal (and embryonic cardiac) myosin heavy chain,
and QH1 (1:200, Developmental Studies Hybridoma Bank) to identify quail
endothelial cells. Whole-mount immunohistochemistry for QCPN was performed on
stage 18-24 embryos using methods previously described
(Noden, et al., 1999); many of
these embryos were subsequently paraffin embedded and sectioned. Immunoassays
were performed sequentially or on adjacent sections. Secondary antibodies
including biotinylated anti-mouse (Jackson labs), streptavidin-HRP,
streptavidin-AP, anti-mouse-HRP and anti-mouse-AP (DAKO) were used according
to manufacturer's recommendations. With any sequential analysis,
immunohistochemistry for QCPN was performed first, using the streptavidin-HRP.
Slides were then incubated in protein block (3% BSA in TBS) overnight prior to
application of additional antibodies. DAKO double-stain kit was used for
sequential immunohistochemistry with QCPN, QH1 and F59. Slides were
counterstained using eosin or thionin.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
|
Expression of myogenic transcription factors myf5 and myod in host embryos occurs in both normal head muscle primordia and ectopic condensations. In ectopic quail cells, it is most robust in cell aggregations, but loose mesenchymal tissues surrounding these aggregates are also positive, often presenting a multipunctate appearance in whole embryos.
To determine the timeline of myogenesis in ectopic condensations, embryos were examined for myf5 at stages 13-14, and for MyHC synthesis at stages 19-22, well in advance of myf5 and MyHC expression in normal head muscles. Transplanted trunk mesoderm cells located in ectopic condensations but not those in normal head muscles are already expressing myf5 within 24 hours of transplantation (Fig. 5A-D). This is evident following segmental plate and half somite transplants. MyHC is present in both large and small ectopic, QCPN-positive condensations by stage 20 (Fig. 5E,F), nearly a day before its appearance in normal head muscles. Again, no differences between any of the three classes of heterotopic transplants could be detected. The scattered QCPN-positive, QH1-negative quail mesenchymal cells were not immunopositive for MyHC at these stages (18-22).
|
Differentiation and morphogenesis
To analyze the later distribution and phenotypes of transplant-derived
cells, assays using the preceding antibodies were supplemented by application
of the anti-MyHC antibody F59 to sections of embryos fixed 3.5-6 days after
surgery (stages 23-30). Reconstructions made from these sections show the
spatial distribution of graft-derived cells
(Fig. 6). Endothelial cells
(green dashes in Fig. 6)
derived from grafted trunk mesoderm are consistently present in the proximal
mandibular prominence, throughout the maxillary prominence, beside the
temporal and dorsal quadrants of the eye, and adjacent to the isthmus,
midbrain and diencephalon. Beyond these areas, variable and diminishing
numbers of quail endothelial cells are present in the distal first arch, and
in the frontonasal, telencephalic and myelencephalic regions. All
trunk-into-head categories were qualitatively similar, but orthotopic
transplants generated fewer cells.
|
All heterotopic transplants produce cells that contribute to the lateral rectus muscle (Figs 7, 8). Contributions to the LR and pyramidalis range from total to a localized blaze of myotube nuclei. In chimeric LR muscles, quail and chick cells tend to remain segregated, with the host-derived myotobes usually found primarily at the leading (originally rostral) or trailing margin. This probably reflects slight differences in the site of graft implantation.
|
|
After the initial condensation of myoblasts, there is no movement of myogenic cells between muscle primordia. This is equally true for normal muscles, such as the LR and DR (dorsal rectus), which move into close proximity near the ciliary ganglion (Fig. 8), and for ectopic muscles (Fig. 9E).
|
A large population of scattered mononucleated myocytes and short multinucleated myotubes was present in all trunk-into-head host embryos (Fig. 10). These cells exhibit typical immature myotube morphology and synthesize MyHC proteins, but do not express endothelial antigens or smooth muscle actin (data not shown). Nothing comparable to these cells was present in control embryos.
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The implantation site selected for this study includes the LR and proximal
BA1 primordia, and is adjacent to the DO precursor. Transplanted trunk
mesoderm cells that occupy these exact sites will aggregate and express
muscle-specific transcription factors and MyHC on the normal head timetable.
Moreover, they coordinate their differentiation and morphogenetic movements
with surrounding host-derived cells to form normal extra-ocular and jaw
muscles, thereby exhibiting a community effect
(Buckingham, 2003). These
results indicate that some members of the signal consortium necessary to
direct head myogenesis are highly localized, and are able to initiate and
sustain myogenesis in nearby paraxial mesoderm cells taken from any part of
the body axis. The requirement for early spatial integration may explain why
some previous trunk-into-head grafts failed to populate the LR, whose
primordium is deep within head paraxial mesoderm ventral to rhombomere 2.
Why, then, do many grafted trunk mesoderm cells also initiate myogenesis at
ectopic sites, where they express the transcription factors paraxis,
lbx1 and myf5, and synthesize myosin rapidly, mimicking their
ancestral trunk timetable? Possibly because of a combination of the more
widespread availability of some myogenesis-promoting signals (e.g. sonic
hedgehog) and the reduced levels of inhibitors (e.g. Bmp4)
(Pourquie et al., 1996). In
addition, the transient exposure of some grafted cells to migrating neural
crest cells, which produce myogenesis-modulating signals such as frzb1
(Ladher et al., 2000
), noggin
and gremlin (Tzahor et al.,
2003
), may potentiate their myogenic differentiation.
The presence of myf5- and MyHC-positive cells in somite-like
epithelial condensations is not surprising, as populations within segmental
plate acquire the ability to form epithelial tissues prior to the onset of
somitogenesis (Zheng, 1993).
By contrast, myogenesis in large mesenchymal aggregates adjacent to the
isthmus, and within long, finger-like projections extending from them, is
unexpected, because this is not normally a site of muscle formation. However,
head mesoderm cells in this location will express a lacZ-reporter
construct driven by one of the myf5 enhancer sequences
(Teboul et al., 2002
), and
will also transiently express tbx1, which is found later in some
branchial arch muscles (D.N.N., unpublished)
(Garg et al., 2001
). Thus,
this isthmic environment may be subthreshold for head mesoderm but sufficient
to activate myogenic genes in trunk mesoderm cells.
Previous studies reported a more limited ability of transplanted segmental
plate to contribute to normal head muscle development, either owing to a
failure to populate muscle primordia
(Hacker and Guthrie, 1998), or
to an inability to recognize and respond to head myogenic stimuli
(Mootoosamy and Dietrich,
2002
). Both these studies limited their analyses to younger
stages, so detection was based on the presence of fewer graft-derived cells.
Hacker and Guthrie placed grafts beside rhombomere 4
(Hacker and Guthrie, 1998
),
the normal site of origin of second branchial arch (BA2) muscles. This
presents a special challenge to migrating trunk myoblasts, many of which may
be committed to the slow fiber type lineage
(Noden et al., 1999
). Indeed,
we find that somites grafted into this area fail to participate in the
formation of the large, fast fiber-dominant mandibular depressor muscle in
BA2, but do contribute to smaller muscles (e.g. serpihyoid, stylohyoid), whose
primary myotubes express the slow S3 MyHC isoform (R. Marcucio and D.M.N.,
unpublished). The possibility that the second arch environment may differ in
the spatiotemporal distribution of other myogenic factors, e.g. sonic hedgehog
(Bren-Mattison and Olwin,
2002
) or noggin and gremlin, from migrating crest cells
(Tzahor et al., 2003
) cannot
be excluded.
The identical behavior of medial and lateral half somites, including the
expression of lbx1 by both, suggests that this mediolateral polarity
is lost following transplantation. Indeed, somite rotation experiments have
shown that medial and lateral domains of newly-formed somites are labile
(Aoyama and Asamoto, 1988;
Dockter and Ordahl, 2000
). It
is possible that grafted half somites undergo regulation, restoring the entire
mediolateral complement of myogenic lineages
(Gamel et al., 1995
). However,
this alone would not account for the widespread expression of lbx1.
Because both the LR and DO normally express lbx1 and originate on
either side of the midbrain-hindbrain boundary, it is possible that signals
emanating from this center may participate in lbx1 activation.
Assaying for markers of trunk migratory myoblasts (e.g. pax3, six1,
c-met), and for known activators of lbx1 (e.g. Fgf4)
(Alvares et al., 2003
), might
help resolve this uncertainty.
The differentiation of other lineages
All trunk grafts give rise to multiple cell types. Angioblasts contribute
to the formation of all types of peripheral vessels, and to meningeal and
brain vessels. Half-somite transplants consistently generate larger numbers of
angioblasts than segmental plate grafts, and all generate more than orthotopic
mesoderm grafts. This correlates with differences in the density of these
mesenchymal populations at the time of transplantation
(Feinberg and Noden, 1991).
Previous studies have shown that somites contain angioblasts
(Ambler et al., 2001
;
Noden, 1989
;
Noden, 1990
;
Wilting et al., 1995
) and will
generate a greater number of migratory angioblasts when grafted into the head
than they normally do in situ (Spence and
Poole, 1994
). This exaggerated angiogenic competence may be
correlated to the large number of mesenchymal cells generated by somites in
the head environment. How this epithelial-to-mesenchymal transformation is
linked to the induction of angioblasts is not known.
Results from in vitro analyses of avian segmental plate and epiblast cells
have led to the suggestion that myogenic and angiogenic differentiation may be
non-specific default pathways
(George-Weinstein et al.,
1996; Grim et al.,
1994
). Although the results of our segmental plate transplants are
consistent with this hypothesis, the diversity of lineages that form and the
heterogeneity, especially among myogenic populations, favors an active role
for local myogenic signals.
Muscle compartmentalization
The time at which each individual muscle becomes a distinct and separate
entity is unclear, especially in the limb and head regions where many myogenic
mesenchymal progenitors are contiguous. These transplantation results indicate
that extraocular muscles become closed compartments, analogous to embryonic
cartilage, soon after the initial aggregation stage. Muscle growth and
differentiation continue without recruitment of additional myogenic cells from
surrounding mesenchyme.
Axons, and later, angioblasts and connective tissues, penetrate each muscle
condensation (McClearn and Noden,
1988; Ruberte et al.,
2003
). Also, progenitors of satellite cells or other stem
cell-like populations might subsequently become incorporated in these
embryonic muscles (Asakura et al.,
2002
). Although closed to myogenic cell recruitment, embryonic eye
muscles do interact with adjacent structures. This is evidenced by the ability
of a denervated/hypoinnervated LR primordium to attract inappropriate
oculomotor axons from the nearby ciliary ganglion
(Wahl and Noden, 2001
), as
also occurs in Duane syndrome (Gutowski,
2000
).
Muscle differentiation uncoupled from morphogenesis
A diffuse population of quail mesenchymal cells expressing paraxis
and myf5, but not lbx1, was present surrounding the graft
site 1-2 days after transplantation. However, in contrast to myocytes within
ectopic condensations, these scattered cells did not synthesize MyHC before
stage 24, which corresponds closely with the normal time for appendicular, as
well as head muscle, differentiation. A few scattered myocytes are
occasionally present in normal embryos close to the trailing edge of muscle
primordia, such as the DO, en route to their sites of terminal
differentiation, or at sites where individual muscles separate from a common
precursor aggregate (e.g. MR from VR). Scattered myocytes are also normally
present around subcutaneous muscles such as the cranial cucullaris, which
originates from occipital somites, but not around deeper head and neck
muscles.
Unlike angioblasts, these graft-derived muscle cells do not move
omnidirectionally, but are largely restricted to a supra-orbital band that is
coincident with the normal DO morphogenetic pathway. These dispersed myocytes
and myotubes do not show any preferential orientation, as is normally a
hallmark of head (McClearn and Noden,
1988) and appendicular
(Lance-Jones, 1979
;
Kardon, 1998
) immature
myotubes. Whether this reflects their non-responsiveness to extrinsic signals
or rather is due to the absence of muscle-aligning cues
(Kardon et al., 2004
) in this
part of the periocular mesenchyme is not known.
Normally, myoblast aggregation and fusion, which involve integrin and
cadherin-mediated interactions (Mulieri et
al., 2002; Kang et al., 2003), are prerequisites to sustained
differentiation of embryonic muscle cells. However, among some grafted trunk
cells, the ability to differentiate is not dependent upon maintenance of close
cell-to-cell contacts; this population does not respect the requirement for a
community effect (Buckingham,
2003
; Standley et al.,
2002
). These single cells may be analogous to
lbx1-positive myoblasts that normally leave the lateral myotome and
move into appendicular tissues; assaying for c-met would clarify their
identity. The c-met ligand, HGF, is present in branchial arch 1 and is
associated with several extra-ocular muscles, including the DO
(Caton et al., 2000
) (D.M.N.,
unpublished), but whether it plays a role in myogenesis as well as being
chemoattractive for efferent axons is not known.
The presence of segmental plate-derived myocytes within the trigeminal
ganglion and root, where no known myogenic signals are present, was unexpected
and suggests that these cells may be a unique population of either already
committed or default pathway progenitors. The default pathway hypothesis
suggests that trunk mesoderm cells, some of which transiently express
myf5 but do not normally commit to the myogenic lineage in situ,
maintain this expression in the head environment and progress to myotube
differentiation. This is consistent with our suggestion that some members of
the myogenic signal consortium are present throughout much of the cephalic
paraxial region. Recent studies (Kardon et
al., 2002; Tamaki et al.,
2002
) have defined a unique lineage of bipotential
endothelial-myogenic precursors that move into the limb. Possibly some of the
scattered myocytes/myotubes, particularly those co-localized with angioblasts
in the trigeminal root, represent cells that initially emigrated as
myoangioblasts then diverged to the muscle lineage, albeit in inappropriate
locations. Only by performing clonal analyses on transplanted tissue could the
possibility of bi- (or multi-) potential lineages be confirmed. The signals
directing the divergence of these bipotential cells are not known.
Muscle morphogenesis: active migration versus passive displacement
Embedded in unsegmented cephalic paraxial mesoderm are restricted sites
where progenitors of extra-ocular and branchial muscle primordia arise, and
graft-derived myogenic cells placed outside of these sites do not participate
in head myogenesis. Surrounding these sites are progenitors of angioblasts,
and a variety of hard and soft connective tissues, each of which exhibits a
distinct set of morphogenetic movements. Grafted trunk mesoderm can mimic
angiogenic, but not chondrogenic/osteogenic, morphogenetic events in the head
(Noden, 1986), similar to the
results reported for grafts of somites to different trunk levels
(Alvares et al., 2003
). The
distribution of graft-derived myotubes in normal (dorsal oblique) and ectopic
clusters around the dorsal margin of the eye provides clues as to how head
muscle morphogenesis is orchestrated.
It has been proposed that for each extra-ocular muscle there is a portal at
the neural crest-mesoderm interface that defines subsequent routes of movement
available to aggregated myoblasts (Noden,
1991b). A muscle primordium situated beside such a portal would
migrate as a unit from its site of origin towards and then across the
interface, whereas nearby non-myogenic mesodermal cells are excluded. The flaw
in this model is that it implies an active migration of large aggregates of
differentiating myotubes and myoblasts, a process for which there is no
precedent. Indeed, many events classically described as involving embryonic
cell migration are not accomplished by individual cell motility, but rather by
continually elongating and remodeling cords of cells in which only the leading
edge cells execute displacement behavior. This is equally true for neural
crest cells (Conner et al.,
2003
; Kulesa and Fraser,
2000
; Tosney et al.,
1994
) and some appendicular myoblasts
(Jacob et al., 1979
), and for
some but not all angioblasts (Noden,
1990
).
A second model, proposed by Mootoosamy and Dietrich, is that some head
muscle primordia remain stationary while the neural crest population expands
and surrounds them, pushing non-myogenic mesenchyme aside
(Mootoosamy and Dietrich,
2002). However, mapping studies have shown that extra-ocular and
branchial muscle progenitors do indeed change their positions relative to
fixed objects such as the brain, cranial ganglia and the eye
(Noden, 1983a
;
Wahl et al., 1994
), and that,
certainly for the LR and DO, crest cells do not encroach upon the sites where
myogenesis is initiated.
Recent analyses of the interface between paraxial and lateral mesoderm
populations at limb levels (Nowicki et
al., 2003) offer an alternative model that applies well to the
head. We propose that the neural crest:mesoderm interface is in fact not
crossed by aggregated muscle primordia or by crest-derived connective tissues.
Instead, the interface is a deformable plane. Localized sites of differential
growth or changes in adhesive properties within populations on either side of
the interface cause the formation of finger-like projections that appear to
penetrate the interface but in fact only deform it. This model closely
resembles `convergent extension'
(Wallingford et al., 2002
),
the process by which cell reorganization within embryonic epithelial sheets
occurs. Structures such as the DO do not move across the interface, but rather
are embedded within tips of expanding mesodermal projections. Later, crest
cells encircle these mesodermal projections, isolating extraocular or
branchial muscles from the more proximal parts of the mesodermal
projections.
The supra-orbital mesodermal projection develops in our chimeric embryos,
but is atypically populated by graft-derived myoblasts that remain dispersed
along its length. In most cases, crest cells circumscribe the distal part of
this projection, thereby isolating the DO. However, they are unable to
displace the ectopic population of trunk-derived myocytes and connective
tissues. A similar disruption of normal neural crest movements occurs when
grafted trunk mesoderm populates the maxillary prominence
(Noden, 1986).
The molecular signals and extracellular matrix components guiding the
deformation of this interface are not known. Indirect evidence suggests that
eph-ephrin mediated interactions (De
Bellard et al., 2002; Santiago
and Erickson, 2002
), twist activity
(Soo et al., 2002
) and HGF
expression (Caton et al.,
2000
) (D.M.N., unpublished) may be involved. Also, although we use
the term growth, this process may involve physicochemical interactions
(Newman and Comper, 1990
)
instead of, or in addition to, proliferative pressures. With the availability
of animals in which the neural crest side of the interface can be labeled
genetically, e.g. wnt1-cre mice
(Chai et al., 2000
;
Jiang et al., 2002
), by the
transplantation of labeled cells (e.g. quail-chick chimeras), or by applied
labels, e.g., DiI in zebrafish (Whitlock
et al., 2003
), it should be possible to define the molecular basis
for maintaining and deforming the mesoderm:neural crest interface. Beyond
immediate interest to myogenesis, slight changes in the location of this
interface are likely to underlie evolutionary changes in vertebrate
craniofacial musculoskeletal structure
(Helms and Schneider,
2003
).
![]() |
ACKNOWLEDGMENTS |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Adelmann, H. (1938). An experimental analysis of the developmental properties of the somites of Amblystoma punctatum.Anat. Rec. 70,2 .
Alvares, L. E., Schubert, F. R., Thorpe, C., Mootoosamy, R. C., Cheng, L., Parkyn, G., Lumsden, A. and Dietrich, S. (2003). Intrinsic, Hox-dependent cues determine the fate of skeletal muscle precursors. Dev. Cell 5,379 -390.[Medline]
Alves, H. J., Alvares, L. E., Gabriel, J. E. and Coutinho, L. L. (2003). Influence of the neural tube/notochord complex on MyoD expression and cellular proliferation in chicken embryos. Braz. J. Med. Biol. Res. 36,191 -197.[Medline]
Ambler, C. A., Nowicki, J. L., Burke, A. C. and Bautch, V. L. (2001). Assembly of trunk and limb blood vessels involves extensive migration and vasculogenesis of somite-derived angioblasts. Dev. Biol. 234,352 -364.[CrossRef][Medline]
Aoyama, H. and Asamoto, K. (1988). Determination of somite cells: independence of cell differentiation and morphogenesis. Development 104, 15-28.[Abstract]
Arnold, H. H. and Braun, T. (2000). Genetics of muscle determination and development. Curr. Top. Dev. Biol. 48,129 -164.[Medline]
Asakura, A., Seale, P., Girgis-Gabardo, A. and Rudnicki, M.
A. (2002). Myogenic specification of side population cells in
skeletal muscle. J. Cell Biol.
159,123
-134.
Barnes, G. L., Alexander, P. G., Hsu, C. W., Mariani, B. D. and Tuan, R. S. (1997). Cloning and characterization of chicken Paraxis: a regulator of paraxial mesoderm development and somite formation. Dev. Biol. 189,95 -111.[CrossRef][Medline]
Bladt, F., Riethmacher, D., Isenmann, S., Aguzzi, A. and Birchmeier, C. (1995). Essential role for the c-met receptor in the migration of myogenic precursor cells into the limb bud. Nature 376,768 -771.[CrossRef][Medline]
Borycki, A. G. and Emerson, C. P., Jr (2000). Multiple tissue interactions and signal transduction pathways control somite myogenesis. Curr. Top. Dev. Biol. 48,165 -224.[Medline]
Brand-Saberi, B., Muller, T. S., Wilting, J., Christ, B. and Birchmeier, C. (1996). Scatter factor/hepatocyte growth factor (SF/HGF) induces emigration of myogenic cells at interlimb level in vivo. Dev. Biol. 179,303 -308.[CrossRef][Medline]
Bren-Mattison, Y. and Olwin, B. B. (2002). Sonic hedgehog inhibits the terminal differentiation of limb myoblasts committed to the slow muscle lineage. Dev. Biol. 242,130 -148.[CrossRef][Medline]
Brohmann, H., Jagla, K. and Birchmeier, C.
(2000). The role of Lbx1 in migration of muscle precursor cells.
Development 127,437
-445.
Buckingham, M. (2001). Skeletal muscle formation in vertebrates. Curr. Opin. Genet. Dev. 11,440 -448.[CrossRef][Medline]
Buckingham, M. (2003). How the community effect orchestrates muscle differentiation. Bioessays 25, 13-16.[CrossRef][Medline]
Buckingham, M., Bajard, L., Chang, T., Daubas, P., Hadchouel, J., Meilhac, S., Montarras, D., Rocancourt, D. and Relaix, F. (2003). The formation of skeletal muscle: from somite to limb. J. Anat. 202,59 -68.[CrossRef][Medline]
Buffinger, N. and Stockdale, F. E. (1994).
Myogenic specification in somites: induction by axial structures.
Development 120,1443
-1452.
Burke, A. C. and Nowicki, J. L. (2003). A new view of patterning domains in the vertebrate mesoderm. Dev. Cell 4,159 -165.[Medline]
Caton, A., Hacker, A., Naeem, A., Livet, J., Maina, F., Bladt,
F., Klein, R., Birchmeier, C. and Guthrie, S. (2000). The
branchial arches and HGF are growth-promoting and chemoattractant for cranial
motor axons. Development
127,1751
-1766.
Chai, Y., Jiang, X., Ito, Y., Bringas, P., Jr, Han, J., Rowitch,
D. H., Soriano, P., McMahon, A. P. and Sucov, H. M. (2000).
Fate of the mammalian cranial neural crest during tooth and mandibular
morphogenesis. Development
127,1671
-1679.
Christ, B. and Brand-Saberi, B. (2002). Limb muscle development. Int. J. Dev. Biol. 46,905 -914.[Medline]
Christ, B. and Ordahl, C. P. (1995). Early stages of chick somite development. Anat. Embryol. 191,381 -396.[Medline]
Conner, P. J., Focke, P. J., Noden, D. M. and Epstein, M. L. (2003). Appearance of neurons and glia with respect to the wavefront during colonization of the avian gut by neural crest cells. Dev. Dyn. 226,91 -98.[CrossRef][Medline]
Couly, G. F., Coltey, P. M. and le Douarin, N. M. (1992). The developmental fate of the cephalic mesoderm in quail-chick chimeras. Development 114, 1-15.[Abstract]
Dalrymple, K. R., Prigozy, T. I. and Shuler, C. F. (2000). Embryonic, fetal, and neonatal tongue myoblasts exhibit molecular heterogeneity in vitro. Differentiation 66,218 -226.[CrossRef][Medline]
De Bellard, M. E., Ching, W., Gossler, A. and Bronner-Fraser, M. (2002). Disruption of segmental neural crest migration and ephric expression in delta-1 null mice. Dev. Biol. 249,121 -130.[CrossRef][Medline]
Delfini, M. C. and Duprez, D. (2000). Paraxis is expressed in myoblasts during their migration and proliferation in the chick limb bud. Mech. Dev. 96,247 -251.[CrossRef][Medline]
Denetclaw, W. F. and Ordahl, C. P. (2000). The
growth of the dermomyotome and formation of early myotome lineages in
thoracolumbar somites of chicken embryos. Development
127,893
-905.
Dockter, J. and Ordahl, C. P. (2000).
Dorsoventral axis determination in the somite: a re-examination.
Development 127,2201
-2206.
Feinberg, R. N. and Noden, D. M. (1991). Experimental analysis of blood vessel development in the avian wing bud. Anat. Rec. 231,136 -144.[Medline]
Gamel, A. J., Brand-Saberi, B. and Christ, B. (1995). Halves of epithelial somites and segmental plate show distinct muscle differentiation behavior in vitro compared to entire somites and segmental plate. Dev. Biol. 172,625 -639.[CrossRef][Medline]
Garg, V., Yamagishi, C., Hu, T., Kathiriya, I. S., Yamagishi, H. and Srivastava, D. (2001). Tbx1, a DiGeorge syndrome candidate gene, is regulated by sonic hedgehog during pharyngeal arch development. Dev. Biol. 235, 62-73.[CrossRef][Medline]
George-Weinstein, M., Gerhart, J., Reed, R., Flynn, J., Callihan, B., Mattiacci, M., Miehle, C., Foti, G., Lash, J. W. and Weintraub, H. (1996). Skeletal myogenesis: the preferred pathway of chick embryo epiblast cells in vitro. Dev. Biol. 173,279 -291.[CrossRef][Medline]
Grim, M., Christ, B. and Wachtler, F. (1994). Emergence of myogenic and endothelial cell lineages in avian embryos. Dev. Biol. 163,270 -278.[CrossRef][Medline]
Gross, M. K., Moran-Rivard, L., Velasquez, T., Nakatsu, M. N.,
Jagla, K. and Goulding, M. (2000). Lbx1 is required for
muscle precursor migration along a lateral pathway into the limb.
Development 127,413
-424.
Gutowski, N. (2000). Duane's syndrome. Euro. J. Neurol. 7,145 -149.[CrossRef]
Hacker, A. and Guthrie, S. (1998). A distinct
developmental programme for the cranial paraxial mesoderm in the chick embryo.
Development 125,3461
-3472.
Hayashi, K. and Ozawa, E. (1995). Myogenic cell
migration from somites is induced by tissue contact with medial region of the
presumptive limb mesoderm in chick embryos.
Development 121,661
-669.
Helms, J. A. and Schneider, R. A. (2003). Cranial skeletal biology. Nature 423,326 -331.[CrossRef][Medline]
Huang, R., Zhi, Q., Patel, K., Wilting, J. and Christ, B. (2000). Contribution of single somites to the skeleton and muscles of the occipital and cervical regions in avian embryos. Anat. Embryol. (Berl) 202,375 -383.[CrossRef][Medline]
Jacob, M., Christ, B. and Jacob, H. J. (1979). The migration of myogenic cells from the somites into the leg region of avian embryos. Anat. Embryol. 57,291 -309.
Jagla, K., Dolle, P., Mattei, M. G., Jagla, T., Schuhbaur, B., Dretzen, G., Bellard, F. and Bellard, M. (1995). Mouse Lbx1 and human LBX1 define a novel mammalian homeobox gene family related to the Drosophila lady bird genes. Mech. Dev. 53,345 -356.[CrossRef][Medline]
Jiang, X., Iseki, S., Maxson, R. E., Sucov, H. M. and Morriss-Kay, G. M. (2002). Tissue origins and interactions in the mammalian skull vault. Dev. Biol. 241,106 -116.[CrossRef][Medline]
Kang, J. S., Mulieri, P. J., Hu, Y., Taliana, L. and Krauss, R.
S. (2002). BOC, an Ig superfamily member, associates with CDO
to positively regulate myogenic differentiation. EMBO
J. 21,114
-124.
Kardon, G. (1998). Muscle and tendon
morphogenesis in the avian hind limb. Development
125,4019
-4032.
Kardon, G., Campbell, J. K. and Tabin, C. J. (2002). Local extrinsic signals determine muscle and endothelial cell fate and patterning in the vertebrate limb. Dev. Cell 3,533 -545.[Medline]
Kardon, G., Harfe, B. D. and Tabin, C. J. (2004). A Tcf4-positive mesodermal population provides a prepattern for vertebrate limb muscle patterning. Dev. Cell 5,937 -944.[CrossRef]
Kiefer, J. C. and Hauschka, S. D. (2001). Myf-5 Is transiently expressed in nonmuscle mesoderm and exhibits dynamic regional changes within the presegmented mesoderm and somites I-IV. Dev. Biol. 232,77 -90.[CrossRef][Medline]
Kulesa, P. M. and Fraser, S. E. (2000). In ovo
time-lapse analysis of chick hindbrain neural crest cell migration shows cell
interactions during migration to the branchial arches.
Development 127,1161
-1172.
Laclef, C., Hamard, G., Demignar, J., Souil, E., Houbrun, C. and
Maine, P. (2003). Altered myogesis in six
1-deficient mice. Development
130,2239
-2252.
Ladher, R. K., Church, V. L., Allen, S., Robson, L., Abdelfattah, A., Brown, N. A., Hattersley, G., Rosen, V., Luyten, F. P., Dale, L. et al. (2000). Cloning and expression of the Wnt antagonists Sfrp-2 and Frzb during chick development. Dev. Biol. 218,183 -198.[CrossRef][Medline]
Lance-Jones, C. (1979). The morphogenesis of the thigh of the mouse with special reference to tetrapod muscle homologies. J. Morph. 162,275 -310.[Medline]
Mackenzie, S., Walsh, F. S. and Graham, A. (1998). Migration of hypoglossal myoblast precursors. Dev. Dyn. 213,349 -358.[CrossRef][Medline]
McClearn, D. and Noden, D. M. (1988). Ontogeny of architectural complexity in embryonic quail visceral arch muscles. Amer. J. Anat. 183,277 -293.[Medline]
Mennerich, D., Schafer, K. and Braun, T. (1998). Pax3 is necessary but not sufficient for lbx1 expression in myogenic precursor cells of the limb. Mech. Dev. 73,147 -158.[CrossRef][Medline]
Mootoosamy, R. C. and Dietrich, S. (2002).
Distinct regulatory cascades for head and trunk myogenesis.
Development 129,573
-583.
Mulieri, P. J., Kang, J. S., Sassoon, D. A. and Krauss, R. S. (2002). Expression of the boc gene during murine embryogenesis. Dev. Dyn. 223,379 -388.[CrossRef][Medline]
Newman, S. and Comper, W. (1990). Generic physical mechanisms of morphogenesis and pattern formation. Development 110,1 -18.[Abstract]
Noden, D. M. (1983a). The embryonic origins of avian cephalic and cervical muscles and associated connective tissues. Amer. J. Anat. 168,257 -276.[Medline]
Noden, D. M. (1983b). The role of the neural crest in patterning of avian cranial skeletal, connective, and muscle tissues. Dev. Biol. 96,144 -165.[Medline]
Noden, D. M. (1986). Patterning of avian craniofacial muscles. Dev. Biol. 116,347 -356.[Medline]
Noden, D. M. (1989). Embryonic origins and assembly of blood vessels. Amer. Rev. Respir. Dis. 140,1097 -1103.[Medline]
Noden, D. M. (1990). Origins and assembly of avian embryonic blood vessels. Ann. New York Acad. Sci. 588,236 -249.[Abstract]
Noden, D. M. (1991a). Origins and patterning of avian outflow tract endocardium. Development 111,867 -876.[Abstract]
Noden, D. M. (1991b). Vertebrate craniofacial development: the relation between ontogenetic process and morphological outcome. Brain Behav. Evol. 38,190 -225.[Medline]
Noden, D. M., Marcucio, R., Borycki, A. G. and Emerson, C. P., Jr (1999). Differentiation of avian craniofacial muscles: I. Patterns of early regulatory gene expression and myosin heavy chain synthesis. Dev. Dyn. 216,96 -112.[CrossRef][Medline]
Nowicki, J. L., Takimoto, R. and Burke, A. C. (2003). The lateral somitic frontier: dorso-ventral aspects of anterio-posterior regionalization in avian embryos. Mech. Dev. 120,227 -240.[CrossRef][Medline]
Ordahl, C. P. and le Douarin, N. M. (1992). Two myogenic lineages within the developing somite. Development 114,339 -353.[Abstract]
Pourquie, O., Fan, C. M., Coltey, M., Hirsinger, E., Watanabe, Y., Breant, C., Francis-West, P., Brickell, P., Tessier-Lavigne, M. and le Douarin, N. M. (1996). Lateral and axial signals involved in avian somite patterning: a role for BMP4. Cell 84,461 -471.[Medline]
Pownall, M. E., Gustafsson, M. K. and Emerson, C. P., Jr (2002). Myogenic regulatory factors and the specification of muscle progenitors in vertebrate embryos. Ann. Rev. Cell Dev. Biol. 18,747 -783.[CrossRef][Medline]
Reiss, K. Z. and Noden, D. M. (1989). SEM characterization of a cellular layer separating blood vessels from endoderm in the quail embryo. Anat. Rec. 225,165 -175.[Medline]
Ruberte, J., Carretero, A., Navarro, M., Marcucio, R. S. and Noden, D. M. (2003). Morphogenesis of blood vessels in the head muscles of the avian embryo: spatial, temporal,and VEGF expression analysis. Dev. Dyn. 227,470 -483.[CrossRef][Medline]
Sabourin, L. A. and Rudnicki, M. A. (2000). The molecular regulation of myogenesis. Clin. Genet. 57, 16-25.[CrossRef][Medline]
Santiago, A. and Erickson, C. A. (2002).
Ephrin-B ligands play a dual role in the control of neural crest cell
migration. Development
129,3621
-3632.
Schäfer, K. and Braun, T. (1999). Early specification of limb muscle precursor cells by the homeobox gene lbx1. Nat. Genet. 23,213 -216.[CrossRef][Medline]
Soo, K., O'Rourke, M. P., Khoo, P. L., Steiner, K. A., Wong, N., Behringer, R. R. and Tam, P. P. L. (2002). Twist function is required for the morphogenesis of the cephalic neural tube and the differentiation of the cranial neural crest cells in the mouse embryo. Dev. Biol. 247,251 -270.[CrossRef][Medline]
Spence, S. G. and Poole, T. J. (1994). Developing blood vessels and associated extracellular matrix as substrates for neural crest migration in Japanese quail, Coturnix coturnix japonica. Int. J. Dev. Biol. 38,85 -98.[Medline]
Sporle, R. (2001). Epaxial-adaxial-hypaxial regionalisation of the vertebrate somite: evidence for a somitic organiser and a mirror-image duplication. Dev. Genes Evol. 211,198 -217.[CrossRef][Medline]
Standley, H. J., Zorn, A. M. and Gurdon, J. B. (2002). A dynamic requirement for community interactions during Xenopus myogenesis. Int. J. Dev. Biol. 46,279 -283.[CrossRef][Medline]
Stern, H. M. and Hauschka, S. D. (1995). Neural tube and notochord promote in vitro myogenesis in single somite explants. Dev. Biol. 167,87 -103.[CrossRef][Medline]
Stockdale, F. E., Nikovits, W., Jr and Christ, B. (2000). Molecular and cellular biology of avian somite development. Dev. Dyn. 219,304 -321.[CrossRef][Medline]
Summerbell, D., Ashby, P. R., Coutelle, O., Cox, D., Yee, S. and
Rigby, P. W. (2000). The expression of Myf5 in the developing
mouse embryo is controlled by discrete and dispersed enhancers specific for
particular populations of skeletal muscle precursors.
Development 127,3745
-3757.
Tajbakhsh, S. and Buckingham, M. (2000). The birth of muscle progenitor cells in the mouse: spatiotemporal considerations. Curr. Top. Dev. Biol. 48,225 -268.[Medline]
Tamaki, T., Akatsuka, A., Ando, K., Nakamura, Y., Matsuzawa, H.,
Hotta, T., Roy, R. R. and Edgerton, V. R. (2002).
Identification of myogenic-endothelial progenitor cells in the interstitial
spaces of skeletal muscle. J. Cell Biol.
157,571
-577.
Teboul, L., Hadchouel, J., Daubas, P., Summerbell, D.,
Buckingham, M. and Rigby, P. W. (2002). The early epaxial
enhancer is essential for the initial expression of the skeletal muscle
determination gene Myf5 but not for subsequent, multiple phases of somitic
myogenesis. Development
129,4571
-4580.
Tosney, K. W., Dehnbostel, D. B. and Erickson, C. A. (1994). Neural crest cells prefer the myotome's basal lamina over the sclerotome as a substratum. Dev. Biol. 163,389 -406.[CrossRef][Medline]
Tzahor, E., Kempf, H., Mootoosamy, R. C., Poon, A. C., Abzhanov,
A., Tabin, C. J., Dietrich, S. and Lassar, A. B. (2003).
Antagonists of Wnt and BMP signaling promote the formation of vertebrate head
muscle. Genes Develop.
17,3087
-3099.
Uchiyama, K., Ishikawa, A. and Hanaoka, K. (2000). Expression of lbx1 involved in the hypaxial musculature formation of the mouse embryo. J. Exp. Zool. 286,270 -279.[CrossRef][Medline]
Wagner, J., Schmidt, C., Nikowits, W., Jr and Christ, B. (2000). Compartmentalization of the somite and myogenesis in chick embryos are influenced by wnt expression. Dev. Biol. 228,86 -94.[CrossRef][Medline]
Wahl, C. and Noden, D. M. (2001). Cryptic responses to tissue manipulations in avian embryos. Int. J. Dev. Neurosci. 19,183 -196.[CrossRef][Medline]
Wahl, C. M., Noden, D. M. and Baker, R. (1994). Developmental relations between sixth nerve motor neurons and their targets in the chick embryo. Dev. Dyn. 201,191 -202.[Medline]
Wallingford, J. B., Fraser, S. E. and Harland, R. M. (2002). Convergent extension: the molecular control of polarized cell movement during embryonic development. Dev. Cell 2, 695-706.[Medline]
Whitlock, K. E., Wolf, C. D. and Boyce, M. L. (2003). Gonadotropin-releasing hormone (gnrh) cells arise from cranial neural crest and adenohypophyseal regions of the neural plate in the zebrafish, Danio rerio. Dev. Biol. 257,140 -152.[CrossRef][Medline]
Wilkinson, D. (1998). In Situ Hybridization: A Practical Approach. Oxford, UK: Oxford University Press.
Williams, B. A. and Ordahl, C. P. (2000). Fate
restriction in limb muscle precursor cells precedes high-level expression of
MyoD family member genes. Development
127,2523
-2536.
Wilting, J., Brand-Saberi, B., Huang, R., Zhi, Q., Kontges, G., Ordahl, C. P. and Christ, B. (1995). Angiogenic potential of the avian somite. Dev. Dyn. 202,165 -171.[Medline]
Wong, G. K., Bagnall, K. M. and Berdan, R. C. (1993). The immediate fate of cells in the epithelial somite of the chick embryo. Anat. Embryol. 188,441 -447.[Medline]
Zheng, R.-Z. (1993). Developmental fate of artificially disordered somite cells after heterotopic transplantation. Dev. Repro. Biol. 2,16 -27.