The chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites
© Tenin et al; licensee BioMed Central Ltd. 2010
Received: 28 July 2009
Accepted: 25 February 2010
Published: 25 February 2010
Somitogenesis is the earliest sign of segmentation in the developing vertebrate embryo. This process starts very early, soon after gastrulation has initiated and proceeds in an anterior-to-posterior direction during body axis elongation. It is widely accepted that somitogenesis is controlled by a molecular oscillator with the same periodicity as somite formation. This periodic mechanism is repeated a specific number of times until the embryo acquires a defined specie-specific final number of somites at the end of the process of axis elongation. This final number of somites varies widely between vertebrate species. How termination of the process of somitogenesis is determined is still unknown.
Here we show that during development there is an imbalance between the speed of somite formation and growth of the presomitic mesoderm (PSM)/tail bud. This decrease in the PSM size of the chick embryo is not due to an acceleration of the speed of somite formation because it remains constant until the last stages of somitogenesis, when it slows down. When the chick embryo reaches its final number of somites at stage HH 24-25 there is still some remaining unsegmented PSM in which expression of components of the somitogenesis oscillator is no longer dynamic. Finally, we identify a change in expression of retinoic acid regulating factors in the tail bud at late stages of somitogenesis, such that in the chick embryo there is a pronounced onset of Raldh2 expression while in the mouse embryo the expression of the RA inhibitor Cyp26A1 is downregulated.
Our results show that the chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites. In addition, endogenous retinoic acid is probably also involved in the termination of the process of segmentation, and in tail growth in general.
Somitogenesis is the earliest sign of segmentation in the developing vertebrate embryo [1–3]. During this process vertebrate embryos generate transitory structures called somites that later in development give rise to the vertebral column, most of the skeletal musculature and much of the dermis . This process starts very early soon after gastrulation has initiated and proceeds in an anterior-to-posterior direction during body axis elongation. The elongation of the body axis of the vertebrate embryo has been traditionally divided into two phases termed primary and secondary body formation [5, 6]. During the first phase the somites and other types of mesoderm are derived from cells that have traversed the primitive streak (in amniotes) or its equivalent (in anamniote vertebrates). Fate mapping analyses have shown that the primitive streak contains distinct stem cell populations in specific domains along the antero-posterior axis of the streak [7–9]. During secondary body formation the tail bud is the source of somitic mesoderm precursors [10, 11]. At first considered to be a homogeneous blastema of tissue, lineage analysis has since shown that distinct stem cell populations also exist in specific domains within the tail bud, as proposed by Pasteels , and they are capable of contributing to multiple tissue types [5, 13–20].
During axis elongation two parallel bands of paraxial mesoderm tissue known as the unsegmented or presomitic mesoderm (PSM) migrate out from the primitive streak (or tail bud) and come to lie alongside the notochord. Groups of cells at the most rostral end of each PSM bud off with a remarkable periodicity and synchronisation as an epithelial sphere of cells to form the new somite. It is widely accepted that this process is controlled by a molecular oscillator  that drives periodic waves of gene expression caudo-rostrally through the PSM with the same periodicity as somite formation. In fact, in recent years the community has reported the discovery and characterization of a considerable number of these so-called clock genes in a variety of vertebrate species, all of which are components of the Notch, Wnt or FGF pathways [22–24]. This periodic mechanism is repeated a specific number of times until the embryo acquires a defined species-specific final number of somites at the end of the process of axis elongation. This final number of somites varies widely between vertebrate species: 38-39 somites in the human embryo, 51-53 in the chick embryo, 62-65 in the mouse embryo and several hundred somites in snake embryos of different species .
How termination of the process of somitogenesis, and tail bud growth in general, are determined is still unknown. Several mechanisms have been proposed to contribute including an overall control of somite number exerted by the embryo as a whole , a localised cell death overtaking the process [26, 27], inadequacies in the local environment or in the cells themselves  or suppression of the epithelial-mesenchymal transition required for gastrulation-like movements . In this study we analysed the anatomy of the tail bud at the end of the process and went on to investigate this tissue with respect to apoptosis, the somitogenesis oscillator and the regulation by endogenous retinoic acid (RA) of factors associated to the growth and proliferation of the precursor stem cells.
PSM elongation is not happening at the same speed as somite formation
Deceleration of the speed of somite formation
The somitogenesis oscillator is arrested at late stages of development
Furthermore, we found that the expression of c-Tbx6 in the tail bud of different HH stage 26 chick embryos seems to diminish very irregularly until it disappears (n = 10, Figure 3G-J), in contrast to the organized pattern in which Tbx6 expression is normally downregulated in PSM cells as they become integrated into a new somite (Figure 1A-J). We tested if this irregular downregulation of c-Tbx6 could be temporally related to apoptosis, a process reported previously to be affecting the tail bud region [26, 27, 37–39]. To that end we performed TUNEL staining in chick embryos from HH stage 13 to stage 27 (n = 34). This analysis clearly showed the existence of a localized domain of apoptosis in the chick tail bud affecting the whole area until HH stage 20-21 [Additional File 1] and then again at HH stage 25-26 [Additional File 1], which indeed could be a contributing factor to the elimination of the remaining Tbx6 positive PSM tissue that does not segment into somites.
Onset of c-Raldh2 expression and retinoic activity in the chick tail bud
Finally, we also analysed the expression of several relevant genes in a collection of 10.5-13.5 days post-coitum (dpc) mouse embryos to see if this onset of RA in the tail bud is conserved. We observed that m-Wnt3a is expressed in a broad area of the tail bud until E11.5 whereupon its expression is dramatically reduced to the very tip at E12.5 until it finally disappears at E13.5 (n = 14, Figure 4P-S). From E11.5 there is an onset of m-Raldh2 expression in the mouse tail bud (n = 19, Figure 4T-W), although it is noticeably weaker than that observed in chick, as judged by the intensity of the staining compared with the expression detected in the somites. In addition, when we evaluated the endogenous retinoic acid activity in mouse tail bud explants from E12.5 using the F9-1.8 cells we were not able to detect retinoid activity (data not shown). However, in the mouse embryo there is an alternative source of RA. The size of the mouse PSM is quite small [Additional File 2], thus the RA-producing somites are located close to the tail bud and may act as a source of RA, which could therefore be affecting this tissue during somitogenesis. Because of this proximity the tail bud region needs to be constantly protected from this source of RA and if this protection is lost, for example in Cyp26A1-/- embryos, then the tail bud becomes prematurely exposed to RA and the result is the generation of axial truncations [49–51]. Interestingly, we found that m-Cyp26A1 is reduced at E11.5 until it disappears (n = 16, Figure 4X-AA), suggesting that once the protective expression of m-Cyp26A1 is downregulated then RA may also affect the mouse tail bud in the mouse as a result of the onset of m-Raldh2 in the tail bud as well as m-Raldh2 from the somites.
We report here that termination of the process of somitogenesis seems to be the result of a combination of events occurring in the paraxial mesoderm tissue and the precursor population of the tail bud (see schematic representation in Figure 5G).
Speed of somitogenesis and Wnt3a
Our data shows that the termination of somitogenesis is not the consequence of an acceleration of the speed of somite formation. In fact, this speed remains constant during the formation of all somites except for the last few, which require a longer time. For example, our data shows that at HH stage 23 somite formation might require 150 minutes. Why somite formation takes longer at those late stages of development is still unknown. We think it could be a collateral effect related to the diminution of Wnt3a in the tail bud at these stages, since we have previously shown that when Wnt activity is attenuated in vitro the period of c-Lfng oscillations is increased . Interestingly, our data also indicates that there were consistently fewer somites formed in RA-treated embryos as compared to control embryos (Mann-Whitney Rank Sum Test; P = < 0.001 for 1.5 mM RA and P = 0.009 for 10 mM RA; Figure 5K, M), an effect that could be recapitulating in vitro the physiological slowing we observed at late stages of development (Figure 2), and which is coincident with the presence of RA activity in the tail bud and the concomitant downregulation of Wnt3a expression (Figure 4).
The arrest of the somitic oscillator
The examination of the number of somites formed at each specific stage using as reference the expression of Tbx6 in the PSM and Dact2/MyoD in the forming somite shows that chick somitogenesis is completed (51-53 somites) at around HH stage 24-25, rather than at HH stage 22 . Despite the fact that no further somites are formed, some Tbx6-positive PSM tissue remains at HH stage 24-25. This expression domain diminishes progressively until it disappears at HH stage 26-27. These observations indicate that the paraxial mesoderm does not become fully segmented to the tip of the tail, as indeed proposed by Bellairs and colleagues more than two decades ago [26, 56]. Nevertheless, by HH stage 26-27 the last formed somite is positioned adjacent to the tail tip. The gradual shift of position of the last formed somite towards the tail tip may be due to the morphological re-shaping of the tail bud that happens at these stages of development .
There must be a reason why the terminal PSM does not segment. Sanders et al. proposed that this limitation could be imposed by different reasons, including the local environment and the existence of apoptosis . Tbx6 expression within this tissue indicates that it maintains its PSM genetic character even after somitogenesis has concluded. The results clearly indicate that, as judged by the patterns of expression of components of the Notch pathway, the somitogenesis oscillator is arrested at these stages of development. Thus, the expression of Notch components, such as c-Lfng, is initially lost in the caudal PSM and becomes restricted to the rostral half of the PSM (HH stage 24) until it disappears (HH stage 25). This would suggest that the same tissue that displayed the last oscillations in the caudal PSM did so prior to HH stage 24, and as this tissue becomes displaced more rostrally it is also the last tissue to display non cyclic Notch activity implicated in boundary formation and thereby makes the last somitic border. We have recently shown that when all Notch activity is lost then somitogenesis is arrested . The mechanism involved in the loss of Notch activity at these late stages and the subsequent arrest of the somitic oscillator is still unknown, although it is tempting to speculate that the downregulation of Wnt3a observed at HH stages 21-24 could also be implicated in this arrest because the expression of the ligand Delta1 in the caudal PSM has been shown to be dependent on Wnt3a, at least in the mouse embryo [58, 59]. Also consistent with this hypothesis is the fact that when Wnt/FGF activities are temporally blocked after pharmacological treatment the expression of cyclic genes is arrested in the mouse [33, 60]. It would be interesting to test this model during chick development by blocking Wnt/FGF activities long term and then analysing their morphological consequences.
Last remaining PSM and apoptosis
Since the last few Tbx6-positive PSM cells do not contribute to make additional somites the question arises what becomes of them. TUNEL staining at HH stages 25-26 is restricted to the terminal region of the tail [Additional File 1] [37, 38], which suggests that at least part of this remaining unsegmented paraxial mesoderm population could be dying by programmed cell death, as previously suggested . This possibility would be consistent with the irregular pattern of expression of Tbx6 observed at HH stage 26. However, we cannot formally rule out the possibility that part of this population changes fate, loses Tbx6 and is incorporated in other neighbouring structures of the embryo. If current technical limitations can be overcome in the future, it would be very interesting to explore this possibility by following their fate.
Endogenous RA and the tail bud
We describe here an onset of Raldh2 in both the chick and mouse tail bud that at least in the case of the chick embryo is strong enough to generate detectable levels of RA, as scored by lacZ staining with the F9-1.8 RA-responding cell line. The precise temporal correlation between the onset of Raldh2 expression and RA activity in the chick tail bud, and the observed downregulation of Wnt3a suggests these events could be linked, as has been shown to be the case after addition of ectopic RA [45–48]. In addition, as discussed above, we think that the longer period of the somitogenesis oscillator is likely to be an indirect effect of this RA-mediated Wnt3a downregulation.
If endogenous RA is also implicated in the cessation of segmentation in the mouse embryo by downregulation of Wnt3a/Fgf8 then the main source of this RA is probably the somites. In normal conditions the expression of Cyp26A1 in the mouse tail bud, which is regulated by trunk Hox (paralog group 13) and Cdx factors , prevents the diffusion of RA from the somites into the tissue and thereby prevents premature exposure of the progenitor population to RA. However, its removal in the Cyp26A1-/- mouse embryos allows endogenous RA to function unregulated in the mouse tail bud inducing dramatic abnormalities on the development of caudal structures including severe truncations [49–51]. The phenotype of this mouse mutant line is very similar to the sacral agenesis produced after teratological exposure to high levels of ectopic RA [62–65]. Thus, elevated retinoid activity, endogenous or ectopic, leads to a rapid and significant decrease in the expression of caudal markers, such as Wnt3a [45, 46, 48–50]. We think a similar elevated local retinoid activity, although highly regulated, could be affecting the tail bud in normal embryos at late stages of development due to the physiological downregulation of Cyp26A1, and which is concomitant with the onset of Raldh2 expression and the downregulation of Wnt3a and Fgf8.
The signals responsible for the growth of the precursor population seem to be reversible, since transplanting chick and mouse tail bud cells from a late stage embryo to earlier stages results in the grafted tissue contributing to somites along the entire axis, as well as re-colonising the host tail bud after culture [13, 17, 66]. These results indicate that termination of axial elongation is likely to depend upon changes in the tissue environment in the tail bud rather than a loss of competence to self-renew. Further analyses will be required to generate comprehensive answers concerning the nature of the molecular and cellular changes in the tail bud at late stages of somitogenesis. These analyses should provide a better understanding of how the somitogenesis oscillator is arrested at these stages of development, how the growth of the precursor population is affected and how relevant the exposure to endogenous RA is to this process. The degree of conservation of each of these mechanisms in different species, and how they are each regulated will also contribute to explain the generation of the very varied and complex body plans displayed by different vertebrate species.
Our results show that the termination of the process of somitogenesis in the chick embryo is not due to acceleration in the speed of somite formation because it remains constant, except at late stages of development when it slows down. We found that when the chick embryo reaches its final number of somites at stage HH 24-25 there is still some remaining unsegmented presomitic mesoderm, in which the expression of components of the somitogenesis oscillator is no longer dynamic, suggesting that at these stages of development the somitic oscillator is arrested. Finally, endogenous RA emanating from an onset of expression in the tail bud and/or from the somites is probably involved in the control of the process of somitogenesis, and in tail growth in general, by controlling the expression of Wnt3a and Fgf8.
Embryos and in situ hybridisation
White leg horn Gallus gallus eggs were sourced from Henry Stewart & Co (Lincolnshire) and incubated at 38.5°C in a humidified incubator. Embryos were staged according to the Hamburger Hamilton (HH) developmental table  and by somite counting. Wild-type CD1 mouse (Mus musculus) embryos were obtained from timed mated pregnant females between 10.5 and 13.5 dpc (E10.5-E13.5). In situ hybridisation analysis was basically performed as described . Proteinase K treatment for old chick embryos was as follows: HH stage 20, 20.5 min; HH stage 21-22, 22 min; HH stage 23-24, 24.5 min; HH stage 24-25, 26 min, HH stage 26-27, 28 min, HH stage 28, 29 min. Images were captured on a Leica MZ16 APO microscope using a Jenoptik camera. All animals were handled in strict accordance with good animal practice as defined by the Home Office (UK) and local animal welfare bodies, and all animal work was approved by the ethical committee for experiments with animals of the University of Dundee (UK).
Somite formation curve and PSM size measurements
Eggs (n = 105) were incubated at 38.5°C until they reached HH stage 8 (3-5 pairs of somites), then carefully windowed and the shell membrane above the embryo was removed and ink diluted 1:10 with Phosphate-buffered saline (PBS) was injected underneath the embryo to visualize the somites. The initial number of somites was counted. Eggs were then sealed with parafilm and incubated at 38.5°C for specific time periods and the final number of somites was recounted. For the latest stages (HH stage 23-26), somites were visualized and counted after cDact2/cMyoD double in situ hybridisation. The PSM was measured from the posterior boundary of the last formed somite to the end of the neural tube. The size of the PSM and of the last formed somite was measured using Carl Zeiss AxioVision 4 software and a grid of the Neubauer Improved counting chamber as a standard for the measurements.
Apoptosis and manipulation of the Retinoid pathway
For detection of apoptotic cells, the ApopTag Plus Peroxidase In situ kit (Chemicon) was used. For ectopic RA treatment 1.5-10 mM all-trans-RA or Dimethyl sulfoxide (DMSO) soaked beads were implanted in vivo in the tail bud region of HH stage 12-15 embryos, which were then incubated overnight before collecting the embryos for analysis. For RA detection F9 cells transfected with a 1.8 kb promoter sequence of the mouse retinoic acid β2 receptor coupled to the lacZ gene were grown to confluence as described . Chick embryos were dissected in PBS and the tail bud tip or last formed somites were isolated and placed on top of RA-responding cells for 10 minutes to allow their attachment. 0.5 ml media was added and the cultures were incubated overnight at 38.5°C in a humidified incubator with 5% CO2. Alternatively, a solution containing 1-10 nM RA was added in place of the explants as a control for detection of RA activity. LacZ activity was detected by X-Gal staining overnight at 37°C performed according to the protocol for β-Galactosidase reporter gene staining kit (Sigma).
We are grateful to the members of the MM and JKD laboratories and especially to J. Kim Dale for her critical and generous participation in this project. We thank the groups of Paola Bovolenta, Astrid Buchberger, Domingos Henrique, David Ish-Horowicz, Susan Mackem, Gail Martin, Ivor Mason, Virginia Papaioannou and the Hubrecht Institute (Netherlands) for providing reagents. We also thank Marianne Reilly, Marie Pryde and Letitia Gibson for their administrative support and smiles, Stieneke van den Brink for technical advices and Kate Storey for discussions. MM holds an MRC Career Development Award.
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