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The chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites.

Tenin G, Wright D, Ferjentsik Z, Bone R, McGrew MJ, Maroto M - BMC Dev. Biol. (2010)

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Cell and Developmental Biology, College of Life Sciences, University of Dundee, Dow Street, Dundee, UK.

ABSTRACT

Background: 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.

Results: 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.

Conclusions: 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.

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Regression of the PSM domain in the chick tail bud. Lateral view of whole-mount chick tail buds at (A, F, K) HH stage 20, (B, G, L) stage 22, (C, H, M) stage 24, (D, I, N) stage 26 and (E, J, O) stage 27 analysed by in situ hybridisation with (A-J) c-Tbx6 and (K-O) c-Dact2/c-MyoD showing the position of the PSM or the first somite, respectively. (F-J) Sections of tail buds stained with the c-Tbx6 probe; the borders of the PSM domain (p) and the last somite (s) are demarcated. (P) Bar chart showing the size of the PSM (green bars) and the size of the last somite (red bars) at HH stages 10-25. (Q) Bar chart showing the PSM/last somite ratio during the same window of development calculated separately for each embryo and then the average ratio was calculated for each stage. Error bars represent standard deviation.
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Figure 1: Regression of the PSM domain in the chick tail bud. Lateral view of whole-mount chick tail buds at (A, F, K) HH stage 20, (B, G, L) stage 22, (C, H, M) stage 24, (D, I, N) stage 26 and (E, J, O) stage 27 analysed by in situ hybridisation with (A-J) c-Tbx6 and (K-O) c-Dact2/c-MyoD showing the position of the PSM or the first somite, respectively. (F-J) Sections of tail buds stained with the c-Tbx6 probe; the borders of the PSM domain (p) and the last somite (s) are demarcated. (P) Bar chart showing the size of the PSM (green bars) and the size of the last somite (red bars) at HH stages 10-25. (Q) Bar chart showing the PSM/last somite ratio during the same window of development calculated separately for each embryo and then the average ratio was calculated for each stage. Error bars represent standard deviation.

Mentions: To investigate the cessation of the process of somitogenesis in the chick tail bud it is important first to know if indeed, as previously proposed, the somites extend to the tip of the tail at HH stage 22 [30] or whether this is not the case and there remains some unsegmented PSM [26,28]. The somites extending close to the tip would indicate there is an imbalance between the speed of axis elongation and the recruitment of new cells to become part of the PSM tissue, while if the paraxial mesoderm remains unsegmented this would indicate that the somitogenesis oscillator stops producing somites at a specific stage of development. Thus, we collected chick embryos ranging in development from HH stage 20 to 27 and stained them by in situ hybridisation either with c-Tbx6, a probe specific for the non-segmented PSM (n = 14, Figure 1A-J), or with c-Dact2/c-MyoD a combination of probes to specifically label the most recently formed somites (n = 21, Figure 1K-O). As development proceeds we observed that the domain of c-Tbx6 expression in the non-segmented mesoderm becomes progressively smaller in the tail bud until it disappears at HH stage 26-27. Concurrently, we detected the domain of c-Dact2/c-MyoD expression in the last formed somites gradually moving closer to the end of the tail bud until it almost reached the tip at stage 26-27 HH. We concluded from these results that in fact at the end of somitogenesis the somites nearly reach the tip of the tail bud. We then measured the length of the PSM, the diameter of the last formed somite and the ratio PSM length versus last somite diameter in a large number of embryos ranging from HH stage 10-24 (n = 105). These graphical representations show that until HH stage 14 both parameters increase slightly and then decrease as development proceeds further (Figure 1P, Q; [31]) indicating that from HH stage 14 the elongation of the chick PSM tissue is not happening at the same speed as somite formation. We reasoned that the observed imbalance could be produced by an acceleration in the speed of somite formation or the result of a progressive reduction in the growth/number of precursor stem cells due to changes in their proliferation status and/or apoptosis. We investigated if either of these mechanisms indeed contributes to the cessation of this process.


The chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites.

Tenin G, Wright D, Ferjentsik Z, Bone R, McGrew MJ, Maroto M - BMC Dev. Biol. (2010)

Regression of the PSM domain in the chick tail bud. Lateral view of whole-mount chick tail buds at (A, F, K) HH stage 20, (B, G, L) stage 22, (C, H, M) stage 24, (D, I, N) stage 26 and (E, J, O) stage 27 analysed by in situ hybridisation with (A-J) c-Tbx6 and (K-O) c-Dact2/c-MyoD showing the position of the PSM or the first somite, respectively. (F-J) Sections of tail buds stained with the c-Tbx6 probe; the borders of the PSM domain (p) and the last somite (s) are demarcated. (P) Bar chart showing the size of the PSM (green bars) and the size of the last somite (red bars) at HH stages 10-25. (Q) Bar chart showing the PSM/last somite ratio during the same window of development calculated separately for each embryo and then the average ratio was calculated for each stage. Error bars represent standard deviation.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2836991&req=5

Figure 1: Regression of the PSM domain in the chick tail bud. Lateral view of whole-mount chick tail buds at (A, F, K) HH stage 20, (B, G, L) stage 22, (C, H, M) stage 24, (D, I, N) stage 26 and (E, J, O) stage 27 analysed by in situ hybridisation with (A-J) c-Tbx6 and (K-O) c-Dact2/c-MyoD showing the position of the PSM or the first somite, respectively. (F-J) Sections of tail buds stained with the c-Tbx6 probe; the borders of the PSM domain (p) and the last somite (s) are demarcated. (P) Bar chart showing the size of the PSM (green bars) and the size of the last somite (red bars) at HH stages 10-25. (Q) Bar chart showing the PSM/last somite ratio during the same window of development calculated separately for each embryo and then the average ratio was calculated for each stage. Error bars represent standard deviation.
Mentions: To investigate the cessation of the process of somitogenesis in the chick tail bud it is important first to know if indeed, as previously proposed, the somites extend to the tip of the tail at HH stage 22 [30] or whether this is not the case and there remains some unsegmented PSM [26,28]. The somites extending close to the tip would indicate there is an imbalance between the speed of axis elongation and the recruitment of new cells to become part of the PSM tissue, while if the paraxial mesoderm remains unsegmented this would indicate that the somitogenesis oscillator stops producing somites at a specific stage of development. Thus, we collected chick embryos ranging in development from HH stage 20 to 27 and stained them by in situ hybridisation either with c-Tbx6, a probe specific for the non-segmented PSM (n = 14, Figure 1A-J), or with c-Dact2/c-MyoD a combination of probes to specifically label the most recently formed somites (n = 21, Figure 1K-O). As development proceeds we observed that the domain of c-Tbx6 expression in the non-segmented mesoderm becomes progressively smaller in the tail bud until it disappears at HH stage 26-27. Concurrently, we detected the domain of c-Dact2/c-MyoD expression in the last formed somites gradually moving closer to the end of the tail bud until it almost reached the tip at stage 26-27 HH. We concluded from these results that in fact at the end of somitogenesis the somites nearly reach the tip of the tail bud. We then measured the length of the PSM, the diameter of the last formed somite and the ratio PSM length versus last somite diameter in a large number of embryos ranging from HH stage 10-24 (n = 105). These graphical representations show that until HH stage 14 both parameters increase slightly and then decrease as development proceeds further (Figure 1P, Q; [31]) indicating that from HH stage 14 the elongation of the chick PSM tissue is not happening at the same speed as somite formation. We reasoned that the observed imbalance could be produced by an acceleration in the speed of somite formation or the result of a progressive reduction in the growth/number of precursor stem cells due to changes in their proliferation status and/or apoptosis. We investigated if either of these mechanisms indeed contributes to the cessation of this process.

Bottom Line: 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.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Cell and Developmental Biology, College of Life Sciences, University of Dundee, Dow Street, Dundee, UK.

ABSTRACT

Background: 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.

Results: 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.

Conclusions: 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.

Show MeSH
Related in: MedlinePlus