Unité de Biologie moléculaire du Développement, Institut Pasteur, 25, rue du Docteur Roux, 75724 Paris Cédex 15, France
*Author for correspondence (e-mail: jfnicola{at}pasteur.fr)
Accepted 5 October 2001
![]() |
SUMMARY |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Key words: Cell lineage, Mesoderm, Clonal analysis, Dermomyotome, LaacZ, Stem cell, Epaxial, Hypaxial, Somite.
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Two types of muscles are derived from the somites: dorsomedially the epaxial myotome and epaxial muscles (muscles of the back), and ventrolaterally the hypaxial myotome and hypaxial muscles (muscles of the bodywall at trunk level, and of the limbs at limb level). The formation and differentiation of these two muscle types appears to result from different extrinsic signals [extensively reviewed by Hirsinger et al. (Hirsinger et al., 2000)]. For example, formation of the epaxial myotome in the dorsomedial quadrant of the somite is controlled by a number of secreted factors. BMP4, WNT1 and WNT3A are produced by the dorsal neural tube (Hirsinger et al., 1997; Ikeya and Takada, 1998; Marcelle et al., 1997; Tajbakhsh and Sporle, 1998), SHH by the notochord and floor plate (Borycki et al., 1998) and other members of the WNT family are also secreted from the dorsal ectoderm (Cossu et al., 1996; Tajbakhsh and Sporle, 1998). The spatial and temporal coordination of these signals induces expression of the myogenic regulatory factors (MRFs) Myf-5 and MyoD (Ikeya and Takada, 1998; Maroto et al., 1997; Tajbakhsh and Sporle, 1998). The dorsolateral quadrant, on the other hand, responds to signals from the dorsal ectoderm (including members of the WNT family) and from the lateral plate (BMP4) (Dietrich et al., 1998; Hirsinger et al., 1997; Pourquie et al., 1996) by expressing hypaxial markers, including Pax-3, Sim1 and the MRFs (Dietrich et al., 1998; Hirsinger et al., 1997; Pourquie et al., 1996; Tajbakhsh and Sporle, 1998). The situation is somewhat different at the limb level. Cells of the lateral part of the dermomyotome migrate away from the somite to form the limb musculature. These hypaxial cells also receive signals from the lateral plate (Dietrich et al., 1998) but express different hypaxial markers, such as Lbx1 (Dietrich et al., 1998; Mennerich et al., 1998).
Although the differentiation of somite derivatives has been intensively studied at the genetic level, a complete comprehension of muscle development requires that these molecular signals be linked to the successive formation of the involved structures and to the underlying cellular processes that result in their formation. However, cell behaviour during muscle development remains controversial, particularly with respect to the formation of the myotome from the dermomyotome (Cinnamon et al., 2001; Cinnamon et al., 1999; Denetclaw et al., 2001; Denetclaw et al., 1997; Denetclaw and Ordahl, 2000; Kahane et al., 1998a; Kahane et al., 1998b). It has been shown that epaxial and hypaxial myotomes derive from the medial and lateral parts of the dermomyotome, respectively (Denetclaw and Ordahl, 2000; Huang and Christ, 2000; Ordahl and Le Douarin, 1992). However, it is not known how strict this separation is, whether it is progressively established, or if it implicates one or several factors. The exact location of myotomal precursors in the dermomyotome also remains unclear. Some studies in the avian embryo indicate that myocytes of the primary myotome (the initial myocytes that form the most dorsal cellular layers of the myotome) translocate directly from the dorsomedial (DML) and ventrolateral (VLL) lips of the dermomyotome (Denetclaw et al., 2001; Denetclaw et al., 1997; Denetclaw and Ordahl, 2000; Ordahl et al., 2001), whereas other experiments indicate that the myocytes essentially derive from the rostral and caudal lips, and that the myocyte precursors deriving from the DML and VLL first delaminate into a sub-lip domain and then migrate to the rostral and caudal lips before elongating as myocytes (Cinnamon et al., 2001; Cinnamon et al., 1999; Kahane et al., 1998b). To further complicate this issue, it is possible that the spatial origin of myotomal cells changes with maturation of the dermomyotome (Hirsinger et al., 2000).
To investigate the origin of the separation of epaxial and hypaxial myotomes, we used the LaacZ method (Bonnerot and Nicolas, 1993; Eloy-Trinquet et al., 2000; Mathis and Nicolas, 1997; Mathis and Nicolas, 1998) to perform a clonal analysis of myotome formation at the thoracic level of the embryo. This powerful method previously allowed us to propose a model for formation of the myotome from a self-renewing pool of cells in the primitive streak (Eloy-Trinquet et al., 2000; Nicolas et al., 1996). In this model, self-renewing cells (S) in the primitive streak give rise to daughter S cells and to precursor cells (P) that will leave the primitive streak to contribute to a few myotomal segments in the paraxial mesoderm. These P cells subsequently divide into post-mitotic myoblasts. This stem cell system is also at the origin of limb muscles, and persists in the tail bud to form the sacral and caudal segments (Eloy-Trinquet and Nicolas, 2001). Although this model explains the anteroposterior formation of the myotome and probably of all the paraxial mesoderm, it does not address its mediolateral organisation. In the present study, we have analysed the mediolateral contribution of LaacZ/LacZ chimeras in the mouse myotome. A systematic description of the mediolateral position of ß-gal+ cells in the resultant clones led to the following conclusions: (i) the precursors of the myotome are mediolaterally regionalised before somite segmentation, (ii) superimposed on this regionalisation is a clonal separation of epaxial and hypaxial precursors, at the time of, or shortly after, allocation of precursor cells to one segment, and (iii) there is a direct relationship between myotome precursors in the dermomyotome and their daughter cells in the myotome.
![]() |
MATERIALS AND METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Description of the myotome in RNLZ2 embryos
RNLZ2 embryos were observed under a stereomicroscope equipped with a camera (3-CCD, JVC). The length of the myotomal segments and the epaxial and hypaxial domains of the myotome were measured with the LIDA and its accompanying Calibration Server softwares (Leica).
Histological analysis and in situ hybridisation
Histochemically stained RNLZ2 E11.5 embryos were transferred into 1x PBS containing 30% (w/v) sucrose for 48 hours. They were next embedded in Tissue Freezing Medium (Jung). 50-60 µm cryostat transversal sections were obtained at the trunk level. Sections were fixed in 1% paraformaldehyde (PFA) and rinsed twice in 1x PBS, before mounting and observation under microscope. To visualize the pattern of expression of the LaacZ transgene in the
-2 line, in situ hybridization with a LaacZ probe was performed on whole-mount embryos, as described (Mathis and Nicolas, 1998). In addition, an embryo with a long bilateral clone, SC 346, was cut transversally with a razor blade to reveal a segment containing a large number of labelled cells.
Description of mediolateral position of ß-gal+ cells in the LaacZ/LacZ embryos
153 of the clones used in this study were previously described (Nicolas et al., 1996). 162 additional clones were generated for this study and were analysed as previously described (Nicolas et al., 1996). The analysis includes all of the unilateral monosegmented and bisegmented clones that exhibit ß-gal+ cells only in thoracic somites (segments 12-24). The mediolateral position of the ß-gal+ cells for each clone was assessed in each labelled segment using a stereomicroscope equipped with a camera (3-CCD, JVC). The position of each ß-gal+ cell was expressed as a percentage of the length of the myotomal segment, using LIDA and its accompanying Calibration Server softwares (Leica). Position 0% represents the medial end of the segment and position 100%, its lateral end. Some -2 embryos with unilateral monosegmented clones were cut tranversally with a razor blade to better visualize the position of the labelled cells in relation to the morphological indentation of the bodywall. The number and position of the labelled cells detected in these sections were identical to measurements obtained in intact embryos.
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
Mediolateral regionalisation of myotome precursors
We first analysed the 45 unilateral monosegmented clones (Fig. 2). For each clone, the mediolateral position of each ß-gal+ myocyte was determined and represented on a relative scale from 0 (medial) to 100 (lateral) (Fig. 2A). A comparison of the 45 clones reveals two important properties. Firstly, ß-gal+ myocytes are almost systematically intercalated by ß-gal myocytes (Fig. 2A, Fig. 1F-K), which shows that cell intermingling occurs during the formation of the myotome. Secondly, none of the monosegmented clones spread along the entire mediolateral (ML) axis of the myotome (Table 2), the largest clone comprising a maximum 43% of the total segment (Fig. 2A, clone SC349). In other words, the contribution of monosegmented clones is regionalised, since they do not disperse freely throughout the entire myotome.
|
|
|
|
The myotome is not produced from two permanent stem cell systems
In order to understand how the separation between epaxial and hypaxial precursors occurs, we then wanted to describe some aspects of their formation. An attractive model for the production of myocytes from the dermomyotome is one in which the epaxial and hypaxial domains of the myotome are formed from two stem cell systems located in the dorsomedial and lateromedial lips of the dermomyotome, respectively (Denetclaw and Ordahl, 2000). This model would result in myocytes being deposited in a lateral-to-medial direction for the epaxial domain and in the opposite orientation for the hypaxial domain (Fig. 5A). Such a polarised mode of myocyte production from two stem cell systems could result in a clonal separation of the epaxial and hypaxial myotomes. In another version of this model, the stem cell systems remain in the central part of the dermomyotome (Fig. 5B). In this case, stem cell systems deposit cells in the center of the myotome, thus displacing the formerly produced myocytes in a lateral-to-medial direction for the epaxial domain, and in a medial-to-lateral direction for the hypaxial domain.
|
Thus our results refute models of myotome production based on permanent stem cell systems in the dermomyotome. However, they do not exclude more complex situations involving transient stem cells which, after some rounds of asymmetric divisions, become postmitotic and enter the myotome (see Discussion).
A direct relationship between myocytes and their precursors
A striking property of the bisegmented clones represented in Fig. 3D is that they always contribute to regions nearest to the clonal border, such that their participation seems contiguous across the clonal border. More generally, we have never observed a clone that crosses the clonal border, which participates in both extremities of the myotome without also participating in the middle region.
This property of the clones that cross the clonal border allows us to distinguish between two possible relationships between myocytes and their precursors (Fig. 6). With respect to the production of the myotome from a regionalised pool of precursors, this observation suggests a direct topographic relationship between the precursors of these clones and their descendants in the myotome (Fig. 6B). Indeed, in an inverted relationship model (Fig. 6A), clones that cross the clonal border are expected to contribute discontinuously to the two most distal regions of the myotome. The direct topographic relationship between the precursors of the clones which cross the clonal border and their descendants indicates both regional and coherent modes of growth in the central region of the paraxial mesoderm and during the subsequent translocation of the myocytes.
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Formation of the epaxial and the hypaxial myotomes from the dermomyotome
Several models that accomodate the existence of separate precursor pools for the epaxial and hypaxial myotomes could theoretically explain the relationship between myotomal cells at E11.5 and their precursors in the dermomyotome (Fig. 7). There are non-regionalised models based on extensive cell mixing (Fig. 7A), resulting in a high proportion of clones contributing to the whole epaxial or hypaxial domain, or based on two permanent stem cell systems, one in which the stem cell systems are located at the edges of the dermomyotome (Fig. 7B), and another in which the stem cell systems remain in the central part of the dermomyotome (Fig. 7C), resulting in temporally inverted orientations of myocyte production. There are also regionalised models, in which myocyte precursors give rise to descendants in only a fraction of the ML axis of the myotome. In these models, myocyte precursors in the dermomyotome are organised in relation to the future position of their descendants in the myotome, either with a direct topographic relationship (Fig. 7D) or with an inverted one (Fig. 7E). Models based on transient stem cell systems in which the precursors, after some rounds of divisions, become postmitotic and enter the myotome, can be included in this category (Fig. 7F).
|
It is interesting to draw a parallel between this regionalisation of the myotome and recent findings in birds, which indicate that the central part of the dermomyotome contributes to the formation of the medial part of the hypaxial myotome, and the lateralmost dermomyotome contributes to the lateral part of the hypaxial myotome (Olivera-Martinez et al., 2000). Moreover, the expression patterns of some genes, like en1 or sim1 in the E10.5 mouse embryo, suggest the existence of three domains in the dermomyotome and myotome (Spörle et al., 2001; Tajbakhsh and Buckingham, 2000), in which the third, central domain could produce the subjacent myotome. Furthermore, Myf-5 expression has been shown to be regulated independently in different mediolateral subdomains of the myotome by distinct enhancers (Hadchouel et al., 2000). These data indicate that regionalised gene expression is superimposed on cellular regionalisation within the somite.
The direct relationship between myocytes and their precursors in the dermomyotome distinguishes this translocation event from the indirect one that occurs between epiblast and the mesoderm during gastrulation (Keller and Danilchik, 1988; Lawson et al., 1991). We propose that this regionalisation could allow the early establishment of differential signals in relation to the final position of myocytes in the myotome. The direct relationship between precursors in the dermomyotome and myocytes could then allow the latter to remain in the same mediolateral signaling environment after their translocation. For instance, daughters of a precursor cell located near the neural tube will stay nearby, and daughters of a precursor cell close to the lateral plate will maintain this localization.
A clonal separation in the myotome and between its immediate precursors
Circumstantial evidence in birds first suggested a separation between medial and lateral precursors of the paraxial mesoderm (Selleck and Stern, 1991), which was later proposed for the precursors of the epaxial and hypaxial musculature in somites at the limb bud level (Ordahl and Le Douarin, 1992) and at the thoracic level. In contrast, substantial cell mixing occurs between epaxial and hypaxial muscles derived from grafted halves of thoracic somites (Ordahl et al., 2000). More recently, other studies in the dermomyotome have shown distinct localisations of the precursors of the epaxial and hypaxial domains of the myotome, at both limb and trunk levels (Denetclaw et al., 1997; Denetclaw and Ordahl, 2000; Olivera-Martinez et al., 2000; Huang, 2000).
In this study, we have shown that from the time of their formation, the medial and lateral domains of the myotome are clonally distinct and that their immediate precursors in the dermomyotome (and probably in the somites as well) are organised into two strictly distinct pools. This clonal separation is apparently established by the time of budding of the somites from the paraxial mesoderm, because we did not observe any monosegmented clones contributing to both medial and lateral regions of the myotome. Furthermore, four of the eight restricted, bisegmented clones have only one cell in the posterior segment. As weak mixing may occur between cells of adjacent somites, these four clones may have been generated after the time of segmentation. This hypothesis would then significantly reinforce this demonstration of the existence of a clonal separation in the somite. This clonal separation does not, however, occur before the allocation of precursors to a segment, because the precursors of half of the bisegmented clones do not respect this separation. The 8 remaining bisegmented clones restricted to one or the other clonal domains could result either from a possible start of the establishment of the clonal separation in the presomitic mesoderm, or, more probably, from the general regionalisation of the precursors of the myotome established earlier.
We showed that the boundary between the two clonal domains correlates with morphological indentation of the body wall, which marks the limit between epaxial and hypaxial myotomes in E11.5 mouse embryo (Hadchouel et al., 2000; Tajbakhsh and Buckingham, 2000) (S. Tajbakhsh, personal communication). This suggests that epaxial and hypaxial myotomes are clonally distinct and thus represent two cellular compartments in the thoracic segments. This boundary could, in fact, provide a strict separation between two types of muscle formation that require different, and maybe incompatible, regulatory pathways: the epaxial muscles that form in situ, and the hypaxial muscles that are generated from migrating populations of cells. It is not possible, at the present time, to extrapolate our data to somites anterior to the forelimbs and posterior to the hindlimbs. However, if the comparison between mouse and avian embryos is extended further, it can be hypothesised that this clonal boundary also exists at the limb bud level (Denetclaw et al., 1997; Ordahl and Le Douarin, 1992).
If clonal separation is established just after segmentation, at a time when the dermomyotome and sclerotome have not differentiated, it will be of interest to determine whether other somitic structures are also involved. Analyses done in HH15-17 chick embryos show a separate origin of the medial and lateral precursors of the dermomyotome (Denetclaw et al., 1997; Denetclaw and Ordahl, 2000). It has also been suggested that the proximal and distal parts of the skeleton derive from the medial and lateral parts of the sclerotome, respectively (Christ and Wilting, 1992), but this has only been demonstrated for the whole somite and not at the sclerotome level (Olivera-Martinez et al., 2000; Ordahl et al., 2000). The existence of clonal compartments has also been proposed for the dorsal and ventral surface ectoderm at the limb and flank levels, with the separation residing along a line drawn between the bases of the wing and leg buds (Altabef et al., 1997). We suggest that this line may correspond to the morphological indentation that marks the boundary between the epaxial and hypaxial body domains. Although the timing of these latter restrictions is not known, it suggests that the clonal separation observed here for the myotome and its precursors may reflect a more general separation between dorsal and ventral domains, which may involve the whole somite and surface ectoderm. Furthermore, a single mechanism may be involved to establish the clonal separation in these different tissues.
Establishment and maintenance of the clonal boundary
Several elements may be involved in the establishment of this clonal separation. For example, coherent cell growth and behaviour in the epithelial somite and epithelial dermomyotome may be a crucial component of this process. However, if such coherent growth is not oriented differently in the two domains, such a mechanism alone would not be sufficient to prevent cells from crossing the clonal boundary. Thus, the existence of an active frontier seems necessary between the medial and lateral halves of the structures. This frontier could be based on precise cell-cell and/or cell-matrix interactions that would prevent mixing or promote cell-sorting between the different clonal domains, as in the rhombencephalon (Mellitzer et al., 1999; Wizenmann and Lumsden, 1997; Xu and Wilkinson, 1997). Although many adhesion molecules are expressed in the somite, most are not expressed preferentially in one or the other of the clonal domains. A more systematic analysis of gene expression or adhesive properties in the medial and lateral cells of the somite, dermomyotome and ectoderm may reveal important differences and should contribute to our comprehension of the formation of this boundary. At a later stage, during the growth of the dermomyotome, the formation of a quiescent zone between the epaxial and hypaxial precursors (observed in the chick) (Denetclaw and Ordahl, 2000) may serve to maintain and reinforce the initial separation of the two precursor pools.
The maintenance of the clonal boundary during formation of the myotome may also result from passive or active mechanisms. Our observation of intercalating myocytes along the whole mediolateral axis of the myotome, and especially near the clonal boundary, excludes the hypothesis that the clonal boundary in the myotome is maintained simply due to coherent cell behaviour. Thus, other characteristics of the system must be involved. The lag between the start of the epaxial and the hypaxial myotome formation (Denetclaw and Ordahl, 2000) could intervene in the persistence of the clonal boundary. Another attractive possibility is the temporal production of the epaxial and hypaxial myotomes in opposite orientation, from two stem cell pools residing at the edges of the dermomyotome (Denetclaw and Ordahl, 2000). But to be in agreement with a regionalised model of production of the myotome from the dermomyotome, such pools should be dynamic (rapid recruitment and loss of new cells, Fig. 7F). This last model, which is compatible with our results, is both regionalised (Cinnamon et al., 1999; Denetclaw et al., 2001; Kahane et al., 1998b), and temporal (Denetclaw and Ordahl, 2000). Alternatively, the acquisition of different adhesive properties between the epaxial and hypaxial cells that will translocate to the myotome, could also function by preventing cell mixing among the myocytes of the two clonal domains, or by allowing a separation of the epaxial and hypaxial cells through cell sorting. Finally, it is interesting to note that En-1, homologue of engrailed, which is involved in maintenance of the rostrocaudal boundary in the drosophila wing (Blair, 1992), is also expressed at the level of the boundary in the dermomyotome of the mouse embryo (Davis et al., 1991; Spörle et al., 2001), and in the dermomyotome, myotome and surface ectoderm of the chicken embryo at the limb and flank levels (Gardner and Barald, 1992), which suggests that this gene may play a role in the maintenance of the dorsoventral boundary in these tissues.
Formation of the medial and lateral myotomes
We observed more extensive intercalation among myocytes of the monosegmented clones in the lateral domain, compared to the medial domain of the myotome. This may be due to differences in the degree of coherence during the growth of the precursors in the dermomyotome, the degree of intercalation during the translocation of the myocytes, or the subsequent migration of lateral cells to form the bodywall muscles. Moreover, the more rapid growth of the hypaxial part of the dermomyotome and myotome (Denetclaw and Ordahl, 2000), could by itself explain elevated intercalation in the lateral domain.
Despite this difference, the other properties of myotome precursors (such as the number of cells produced, their regionalisation along the ML axis and the existence of intercalation) are remarkably similar in the medial and the lateral domains. This finding suggests that the modes of myocyte production in the two domains are similar, which is also the case in avian embryos (Cinnamon et al., 1999; Denetclaw and Ordahl, 2000). Although this similarity at first seems surprising, because of the known differences between the extrinsic and intrinsic signals in the medial and lateral domains of the dermomyotome [reviewed by Hirsinger et al. (Hirsinger et al., 2000)], it could be that the characteristics involved in production of the medial and lateral myocytes are established independently of these signals. Alternatively, completely different signalling pathways could converge on similar modes of production of differentiated cells.
Intercalation of the myocytes
The existence of a clonal separation during the formation of the myotome and, before this, the regionalised formation of the myotome from the dermomyotome, could not be explained without a certain degree of cell coherence during the formation of the myotome and between myocytes, in addition to that observed in the precursor pool. However, in all the clones analysed, either monosegmented, bisegmented or longer, we observed unlabelled myocytes intercalated with the genealogically related labelled myocytes, indicating that intercalation nevertheless occurs during these processes. Such intercalation has also been suggested to occur in avian embryos, between the myocytes of the primary myotome (Cinnamon et al., 1999; Denetclaw et al., 1997). Because both the primary and the secondary myotomes (already formed at E11.5) can be labelled in our clones, intercalation must also involve myocytes of the secondary myotome. Our observation of intercalation between myocytes in E12.5 mouse embryos (data not shown) is also consistent with this idea. Kalcheim and coworkers have proposed that the myocytes of the primary myotome in quail embryos, when translocating from the dermomyotome, intercalate with older, already translocated, pioneer fibers, suggesting that intercalation occurs between the myocytes produced in successive waves (Cinnamon et al., 1999; Kahane et al., 1998a; Kahane et al., 1998b).
This intercalation of clonally related myocytes could result from different mechanisms. Before the segmentation occurs, the myotome precursors could undergo a coherent intercalation that would respect their mediolateral regionalisation. However, such a mechanism is unlikely for the precursors located in the somite (monosegmented clones), because intercalation does not normally occur in epithelia (Gardner and Lawrence, 1985). Another possibility is that the mediolateral growth of the dermomyotome, together with the formation of the myotome (Denetclaw et al., 1997; Denetclaw and Ordahl, 2000), simply results in a mechanical intercalation of the myocytes, due to a shift in the relative positions of the precursors located in the dermomyotome. Finally, intercalation could occur during translocation of the myocytes into the myotome (Cinnamon et al., 1999). Indeed, intercalation could result from the necessary convergence and extension of myoblasts that translocate from an epithelium (formed by many rostrocaudal layers of cells) to produce fewer layers of unit-length cells, as described during gastrulation in the Xenopus embryo (Keller and Danilchik, 1988; Keller and Tibbetts, 1989).
Whatever the mechanism(s) involved, this intercalation results in the physical separation of myocytes and their precursor cells, and of myocyte daughter cells. Intercalation may serve to disrupt interactions between genealogically related cells and, thereby, permit novel interactions with other types of cells. It is possible that these new interactions are necessary and instrumental in controlling the coordinated growths of the dermomyotome and myotome and/or in further patterning of these structures.
![]() |
ACKNOWLEDGMENTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Altabef, M., Clarke, J. D. and Tickle, C. (1997). Dorso-ventral ectodermal compartments and origin of apical ectodermal ridge in developing chick limb. Development 124, 4547-4556.
Blair, S. S. (1992). Engrailed expression in the anterior lineage compartment of the developing wing blade of Drosophila. Development 115, 21-33.[Abstract]
Bonnerot, C. and Nicolas, J. F. (1993). Clonal analysis in the intact mouse embryo by intragenic homologous recombination. CR Acad. Sci. USA 316, 1207-1217.
Borycki, A. G., Mendham, L. and Emerson, C. P., Jr. (1998). Control of somite patterning by Sonic hedgehog and its downstream signal response genes. Development 125, 777-790.
Christ, B. and Ordahl, C. P. (1995). Early stages of chick somite development. Anat. Embryol. 191, 381-396.[Medline]
Christ, B. and Wilting, J. (1992). From somites to vertebral column. Anat. Anz. 174, 23-32.[Medline]
Cinnamon, Y., Kahane, N., Bachelet, I. and Kalcheim, C. (2001). The sub-lip domain a distinct pathway for myotome precursors that demonstrate rostral-caudal migration. Development 128, 341-351.
Cinnamon, Y., Kahane, N. and Kalcheim, C. (1999). Characterization of the early development of specific hypaxial muscles from the ventrolateral myotome. Development 126, 4305-4315.
Cossu, G., Tajbakhsh, S. and Buckingham, M. (1996). How is myogenesis initiated in the embryo? Trends Genet. 12, 218-223.[Medline]
Davis, C. A., Holmyard, D. P., Millen, K. J. and Joyner, A. L. (1991). Examining pattern formation in mouse, chicken and frog embryos with an En-specific antiserum. Development 111, 287-298.[Abstract]
Denetclaw, W. F., Jr., Berdougo, E., Venters, S. J. and Ordahl, C. P. (2001). Morphogenetic cell movements in the middle region of the dermomyotome dorsomedial lip associated with patterning and growth of the primary epaxial myotome. Development 128, 1745-1755.
Denetclaw, W. F., Jr., Christ, B. and Ordahl, C. P. (1997). Location and growth of epaxial myotome precursor cells. Development 124, 1601-1610.
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.
Dietrich, S., Schubert, F. R., Healy, C., Sharpe, P. T. and Lumsden, A. (1998). Specification of the hypaxial musculature. Development 125, 2235-2249.
Eloy-Trinquet, S., Mathis, L. and Nicolas, J. F. (2000). Retrospective tracing of the developmental lineage of the mouse myotome. Curr. Top. Dev. Biol. 47, 33-80.[Medline]
Eloy-Trinquet, S. and Nicolas, J. F. (2001). A unique system of permanent self renewing cells produces all myotomal segments and the precursors of the limb muscle masses. In The Origin and Fate of Somites, vol. 329 (ed. E. Sanders, J. W. Lash and C. P. Ordahl), pp. 132-140. London (UK): IOS Press.
Gardner, C. A. and Barald, K. F. (1992). Expression patterns of engrailed-like proteins in the chick embryo. Dev. Dyn. 193, 370-388.[Medline]
Gardner, R. L. and Lawrence, P. A. (1985). Single cell marking and cell lineage in animal development. Phil. Trans. R. Soc. Lond. B 312, 1-187.
Hadchouel, J., Tajbakhsh, S., Primig, M., Chang, T. H., Daubas, P., Rocancourt, D. and Buckingham, M. (2000). Modular long-range regulation of Myf5 reveals unexpected heterogeneity between skeletal muscles in the mouse embryo. Development 127, 4455-4467.
Hirsinger, E., Duprez, D., Jouve, C., Malapert, P., Cooke, J. and Pourquie, O. (1997). Noggin acts downstream of Wnt and Sonic Hedgehog to antagonize BMP4 in avian somite patterning. Development 124, 4605-4614.
Hirsinger, E., Jouve, C., Dubrulle, J. and Pourquié, O. (2000). Somite formation and patterning. Int. Rev. Cytol. 198, 1-65.[Medline]
Huang, R. and Christ, B. (2000). Origin of the epaxial and hypaxial myotome in avian embryos. Anat. Embryol. (Berlin) 202, 369-374.[Medline]
Huang, R., Zhi, Q., Schmidt, C., Wilting, J., Brand-Saberi, B. and Christ, B. (2000). Sclerotomal origin of the ribs. Development 127, 527-532.
Ikeya, M. and Takada, S. (1998). Wnt signaling from the dorsal neural tube is required for the formation of the medial dermomyotome. Development 125, 4969-4976.
Kahane, N., Cannamon, Y. and Kalcheim, C. (1998a). The origin and fate of pioneer myotomal cells in the avian embryo. Mech. Dev. 74, 59-73.[Medline]
Kahane, N., Cinnamon, Y. and Kalcheim, C. (1998b). The cellular mechanism by which the dermomyotome contributes to the second wave of myotome development. Development 125, 4259-4271.
Kato, N. and Aoyama, H. (1998). Dermomyotomal origin of the ribs as revealed by extirpation and transplantation experiments in chick and quail embryos. Development 125, 3437-3443.
Keller, R. and Danilchik, M. (1988). Regional expression, pattern and timing of convergence and extension during gastrulation of Xenopus laevis. Development 103, 193-209.[Abstract]
Keller, R. and Tibbetts, P. (1989). Mediolateral cell intercalation in the dorsal, axial mesoderm of Xenopus laevis. Dev. Biol. 131, 539-549.[Medline]
Klarsfeld, A., Bessereau, J.-L., Salmon, A.-M., Triller, A., Babinet, C. and Changeux, J.-P. (1991). An acetylcholine receptor alpha-subunit promoter conferring preferential synaptic expression in muscle of transgenic mice. EMBO J. 10, 625-632.[Abstract]
Lawson, K. A., Meneses, J. J. and Pedersen, R. A. (1991). Clonal analysis of epiblast fate during germ layer formation in the mouse embryo. Development 113, 891-911.[Abstract]
Marcelle, C., Stark, M. R. and Bronner-Fraser, M. (1997). Coordinate actions of BMPs, Wnts, Shh and noggin mediate patterning of the dorsal somite. Development 124, 3955-3963.
Maroto, M., Reshef, R., Munsterberg, A. E., Koester, S., Goulding, M. and Lassar, A. B. (1997). Ectopic Pax-3 activates MyoD and Myf-5 expression in embryonic mesoderm and neural tissue. Cell 89, 139-148.[Medline]
Mathis, L. and Nicolas, J.-F. (1997). Analyse clonale rétrospective chez les vertébrés: méthodes, concepts et résultats. Ann. LInstitut Pasteur / Actualités 8, 3-17.
Mathis, L. and Nicolas, J. F. (1998). Autonomous cell labelling using LaacZ reporter transgenes to produce genetic mosaics during development. In Microinjections and Transgenesis. Strategies and Protocols (ed. A. C. a. A. Garcia), pp. 439-458: Springer-Verlag.
Mathis, L. and Nicolas, J. F. (2000). Different clonal dispersion in the rostral and caudal mouse. Development 127, 1277-1290.
Mellitzer, G., Xu, Q. and Wilkinson, D. G. (1999). Eph receptors and ephrins restrict cell intermingling and communication. Nature 400, 77-81.[Medline]
Mennerich, D., Schafer, K. and Braun, T. (1998). Pax-3 is necessary but not sufficient for lbx1 expression in myogenic precursor cells of the limb. Mech. Dev. 73, 147-158.[Medline]
Nicolas, J. F., Mathis, L. and Bonnerot, C. (1996). Evidence in the mouse for self-renewing stem cells in the formation of a segmented longitudinal structure, the myotome. Development 122, 2933-2946.
Olivera-Martinez, I., Coltey, M., Dhouailly, D. and Pourquié, O. (2000). Mediolateral somitic origin of ribs and dermis determined by quail-chick chimeras. Development 127, 4611-4617.
Ordahl, C. P., Berdougo, E., Venters, S. J. and Denetclaw, W. F., Jr. (2001). The dermomyotome dorsomedial lip drives growth and morphogenesis of both the primary myotome and dermomyotome epithelium. Development 128, 1731-1744.
Ordahl, C. P. and Le Douarin, N. M. (1992). Two myogenic lineages within the developing somite. Development 114, 339-353.[Abstract]
Ordahl, C. P., Williams, B. A. and Denetclaw, W. (2000). Determination and morphogenesis in myogenic progenitor cells: an experimental embryological approach. Curr. Top. Dev. Biol. 48, 319-367.[Medline]
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]
Selleck, M. A. J. and Stern, C. D. (1991). Fate mapping and cell lineage analysis of Hensens node in the chick embryo. Development 112, 615-626.[Abstract]
Spörle, R., Hadchouel, J., Tajbakhsh, S., Schughart, K. and Buckingham, M. (2001). Evidence for subdivisions of epaxial somite derivatives. In The Origin and Fate of Somites, vol. 329 (ed. E. Sanders, J. W. Lash and C. P. Ordahl), pp. 153-165. London (UK): IOS Press.
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]
Tajbakhsh, S. and Sporle, R. (1998). Somite development: constructing the vertebrate body. Cell 92, 9-16.[Medline]
Wachtler, F. and Christ, B. (1992). The basic embryology of skeletal muscle formation in vertebrates: the avian model. Sem. Dev. Biol. 3, 217-227.
Wizenmann, A. and Lumsden, A. (1997). Segregation of rhombomeres by differential chemoaffinity. Mol. Cell. Neurosci. 9, 448-459.
Xu, Q. and Wilkinson, D. G. (1997). Eph-related receptors and their ligands: mediators of contact dependent cell interactions. J. Mol. Med. 75, 576-586.[Medline]