- Email: [email protected]
Embryonic Axes: The Long and Short of It in the Mouse
Current Biology, Vol. 14, R239–R241, March 23, 2004, ©2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cub.2004.02.056
Dispatch
Embryonic Axes: The Long and Short of It in the Mouse Patrick P.L. Tam
Contrary to conventional wisdom, recent studies indicate that, during mouse development, the embryo’s anterior-posterior axis shifts from the shorter transverse axis to the orthogonal longer one as the shape of the pre-gastrula-stage embryo is remodelled.
Conventional wisdom about the determination of the embryonic axes in the mouse is long on speculation and short on understanding of the cellular and molecular mechanisms. Two meticulous analyses of the orientation of the embryonic axis during the remodelling of the pre-gastrula mouse embryo, published recently in Current Biology [1,2], have now provided a more complete description of the intricate developmental processes at work. The intriguing findings show that, while the prospective anterior-posterior embryonic axis is consistently aligned with the transverse axis of the uterus, it is not always oriented along the long axis of the pre-gastrula embryo, as would be predicted from the anatomy of the gastrulating embryo. Rather, the anterior-posterior axis is first oriented along the shorter transverse axis of the embryo and then it switches to alignment with the longer axis following the remodelling of the pre-gastrula embryo. A major milestone in embryonic development is the generation of the architectural asymmetry that delineates the orientation and polarity of the three axes — anterior-posterior, dorso-ventral and left-right — of the basic body plan. The mouse embryo goes through the first four days of development like other eutherian pre-implantation embryos, dividing up the fertilized egg into multiple blastomeres and putting the descendants of the blastomeres into a blastocyst. The blastocyst is a vesicular structure containing an outer epithelial layer of trophectoderm enclosing a fluid-filled cavity. A cluster of cells, the inner cell mass which gives rise to the embryo proper, is attached to the internal wall on one side of the vesicle. This lopsided location of the inner cell mass gives the mouse embryo a distinct asymmetry. The side of blastocyst on which the inner cell mass is localised becomes the embryonic pole and the diametrically opposite side is the abembryonic pole; these poles define an embryonic-abembryonic axis (Figure 1A). Superimposed on this visible morphological asymmetry is a more subtle breach of radial symmetry. When viewed from the embryonic pole, the inner cell mass does not form a perfectly round shape, but acquires an oblong one with a short and a long Embryology Unit, Children’s Medical Research Institute, University of Sydney, Locked Bag 23, Wentworthville, NSW 2145, Australia. Email: [email protected]
diameter; the latter defines an axis of bilateral symmetry that is orthogonal to the embryonic-abembryonic axis (Figure 1A). The longer axis, however, is unique in that one end of it is marked by the presence of the second polar body and/or the tilting of the inner cell mass away from the embryonic-abembryonic axis (Figure 1A) [3–6]. The second polar body, which is extruded by the oocyte as it completes meiosis shortly before fertilization, is a landmark of the animal pole of the zygote; the opposite side, by default, is the vegetal pole. The animal-vegetal axis, in conjunction with the point of sperm entry, not only defines the orientation of the first cleavage of the zygote but also seems to influence the differential allocation of the progeny of the first two blastomeres to cells in the embryonic versus abembryonic compartment of the blastocyst [3,7–9]. These observations raise the intriguing possibility that the pertinent morphogenetic program for defining cell fates and patterning the mouse embryo is installed very early in development. Following implantation in the uterus at about 4 days post coitum, the mouse blastocyst does not grow into a discoid embryo like those of other eutherian species. Instead, the embryo adopts a cylindrical configuration, with a column of extraembryonic ectoderm derived from the growth of the trophectoderm into the cavity of A Blastocyst
B Pre-gastrula Embryonic proximal mesometrial Ectoplacental cone
Embryonic Trophectoderm
Extraembryonic ectoderm Short axis
Long axis
Inner cell mass Epiblast
Abembryonic
Visceral endoderm Abembryonic distal antimesometrial Second polar body
Current Biology
Figure 1. The anatomy of (A) the blastocyst and (B) the pregastrula-stage mouse embryo. On the left is a lateral view of the embryo, and on the right a transverse section of the embryo along the dashed line. (A) In the blastocyst, the embryonic-abembryonic axis (doubleheaded arrow) is defined by the position of the inner cell mass, and the axis of bilateral symmetry (dashed line) by the shape and the tilting (curved arrow) of the inner cell mass and the position of the second polar body. (B) In the pre-gastrula, the embryonic-abembryonic axis lies in the same plane as the proximal-distal axis of the cylindrical embryo. The proximal pole is on the mesometrial side of the uterus — the mesometrium is the mesentery-like tissue connecting the uterus to the body wall — and the distal pole points to the antimesometrial side — away from the mesometrium — of the uterus. A transverse section of the embryo at the level of the epiblast reveals an oblong shape with a short and a long axis.
Dispatch R240
A
C Mesometrium Longitudinal Transverse
D
Top view I
B
etrium
Vertical
Mesom
II II
I
III
Transverse
Current Biology
Figure 2. Orientation of the embryonic axes with the uterine axes in the mouse. (A) Anti-parallel orientation of the embryonic-abembryonic axis (arrows) of the implanting blastocysts with the longitudinal axis of the uterus [5]. (B) Re-alignment (curved arrow) of the embryonic-abembryonic axis (straight arrows) from the longitudinal to the vertical axis of the uterus [5,6]. (C) Rotation (curved arrows) of the anterior-posterior axis from the proximal-distal to the transverse axis [15]. (D) The anteriorposterior axis remains aligned with the transverse axis of the uterus while the short axis is converted into the long axis — shown in successive stages I–III — during the remodelling of the pre-gastrula embryo [1,2]. Dashed lines link the crosssection to the lateral view of the embryo. Red and blue arrows indicate the opposite polarity of the anterior-posterior axis of embryos in the same uterus.
the conceptus, carrying at its tip the inner cell mass which is transformed into an epithelial cup known as the epiblast (Figure 1B). The column and the cup are enveloped by a layer of visceral endoderm. This cylinder, however, is not perfectly symmetrical in shape: it is curved longitudinally, and the ectoplacental cone (derived from the trophectoderm) that attaches the proximal end of the cylinder to the uterine wall is frequently lopsided, which seems to reflect the original tilting of the inner cell mass relative to the embryonicabembryonic axis of the blastocyst (Figure 1A,B) [4,5]. The cylinder is also flattened on the sides such that a cross-section of it appears ellipsoidal, and the embryo has a long and a short transverse axis (Figure 1B). In embryos that have commenced gastrulation, the primitive streak — a localised region where the epithelial-mesenchyme transition has taken place — is invariably found on the posterior side of the long transverse axis. In the light of these observations, it is tempting to propose that the bilateral symmetry defined by the animal-vegetal axis of the blastocyst is the forerunner of the bilateral axis of symmetry of the embryo, and that the long axis of the pre-gastrula heralds the orientation and polarity of the prospective anterior-posterior axis of the embryo [6,10].
The validity of this notion of axis determination has been challenged by findings on the development of the anterior-posterior axis in the pre-gastrula-stage embryo. In an embryo close to initiating gastrulation — about 6 days post coitum — although the orientation of the anterior-posterior axis can be predicted based on the long axis defined by asymmetry of the cylinder, it was found that the polarity of the axis may be completely in reverse of the expected direction [3]. Interstingly, the emerging anterior-posterior axis tends to be positioned to the right hand side of the predicted axis, with a hint of clockwise rotation of the anterior pole in the transverse plane of the embryo [3]. More unexpectedly, when the expression pattern of genes characteristic of the anterior and posterior tissues in the body axis was studied in even younger embryos — about 5 to 5.5 days post coitum — the prospective anterior-posterior axis of the embryo was seen to align initially with the embryonic-abembryonic axis of the conceptus (equivalent to the proximo-distal axis of the cylindrical pre-gastrula, Figure 1B) and later to re-align with the transverse axis of the embryo, at about 5.75 post coitum [11,12]. This rotation of the anterior-posterior axis is accompanied by movement of visceral endoderm cells from the distal region to the prospective anterior side of the pre-gastrula [11,13]. This re-alignment of the prospective anterior-posterior axis, and the discordance of anterior-posterior axis from the predicted axis based on morphological asymmetry, strongly suggest that axis determination is likely to be a dynamic process coupled to morphogenetic activity in the post-implantation embryo [10,14,15]. A critical and yet unresolved issue is whether the ‘anterior’ movement of the visceral endoderm cells follows a longitude pre-determined by any axis of bilateral symmetry of the pre-gastrula. The two new studies [1,2] on the regionalization of the expression domains of genes that characterize the anterior visceral endoderm — Cerl, Hex and Gsc — and the posterior epiblast — Nodal, Fgf8, Evx1 and T — in the mouse pre-gastrula have produced some very surprising results. Contrary to what would be predicted by conventional wisdom, the two groups found that anterior visceral endoderm genes are expressed in the visceral endoderm at one pole of the short transverse axis and not the long one. The new work also shows that posterior molecular markers are expressed in the epiblast at the other end of the short axis, suggesting that the prospective anterior-posterior axis aligns with the short axis and that the anterior visceral endoderm has moved from the distal region of embryo along the longitude in the meridional plane of the short axis. As the pre-gastrula embryo grows, its cross-sectional shape changes. Concomitantly, there is a shift of the anterior-posterior gene expression domains first to sectors between the long and short axes and eventually consolidating to opposite ends of the long axis. These findings show unequivocally that the orientation of the anterior-posterior axis is not consistently related to any plane of symmetry or morphological landmark until gastrulation commences. The shift
Current Biology R241
was shown not caused by displacement of cells from the short to the long axis, but more likely reflects the conversion of the short axis of the pre-gastrula into the long axis of the gastrula by tissue remodelling. A possibility which has not been excluded is that the apparent rotation of the anterior-posterior axis in the transverse plane is brought about by changes in gene activity in the visceral endoderm and the epiblast, which may be elicited in response to mechanical stress and deformation [16] generated by growth of the embryo in a confined space. It may be interesting to test whether constraining the space for growth by increasing the number of implanting embryos, either artificially or by inducing embryonic twinning, has any effect on the alignment of the anterior-posterior axis. The spatial relationship of the axes of the mouse conceptus to that of the uterus changes in a dynamic manner. The tubular uterus has three anatomical axes: longitudinal (oviduct to cervix), vertical (anti-mesometrial to mesometrial) and transverse (cross-sectional) (Figure 2). The blastocyst usually implants on the antimesometrial side of the uterus with the embryonicabembryonic axis parallel to the longitudinal axis of the uterus [5] (Figure 2A). Presumably as a result of the growth of the tissues of the uterine crypts, the implanted blastocyst rotates to re-align the embryonic-abembryonic axis with the vertical axis so that the distal end of the cylindrical pre-gastrula points in the anti-mesometrial direction [5,6] (Figure 2B). The prospective anterior-posterior axis is then re-aligned from the proximal-distal orientation to the short axis of the pre-gastrula (Figure 2C) [15]. Mesnard et al. [1] have shown that the short axis of the embryo is aligned with the transverse axis of the uterus (Figure 2D), suggesting the orientation of the anterior-posterior axis becomes fixed after it is re-aligned from the proximo-distal to the transverse plane, and that its orientation relative to the uterus remains unchanged despite the remodelling of the embryo. It is likely that the rotation of the embryonic-abembryonic axis of the implanting blastocyst (Figure 2B) requires interactions with the uterine environment. Consistent with this notion, attachment to the culture substrate appears to be a pre-requisite for post-blastocyst development of embryo in vitro [17,18]. In contrast, the rotation of the anterior-posterior axis from the proximo-distal to the transverse plane and the remodelling of tissues can be accomplished in embryos developing free from any uterine influence [1,2], suggesting that these two processes may be controlled autonomously by the embryo. Among embryos in the same uterine tube, the polarity of the anterior-posterior axis relative to the transverse axis of the uterus can be randomly antiparallel to each other (Figure 2D). It is not known if the polarity of the anterior-posterior axis is determined by the positioning of the blastocyst relative to the uterine axes as it implants [5] or by signalling activity that controls the direction of rotation of the anterior-posterior axis in the pre-gastrula embryo. Our attention should now be focussed on resolving when and how the development of the anterior-posterior axis is controlled by the interaction with the uterine environment
and the morphogenetic activities that are intrinsic to the embryo, respectively. References 1. Mesnard, D., Filipe, M., Belo, J.A., and Zernicka-Goetz, M. (2004). Emergence of the anterior-posterior axis after implantation relates to the re-orienting symmetry of the mouse embryo rather than the uterine axis. Curr. Biol. 14, 184-196. 2. Perea-Gomez, A., Camus, A., Moreau, A., Grieve,K., Moneron, G., Dubois, A., Cibert, C., and Collignon, J. (2004). Initiation of gastrulation in the mouse embryo is preceded by an apparent shift in the orientation of the anterior-posterior axis. Curr. Biol. 14, 197-207. 3. Gardner, R.L., Meredith, M.R., and Altman, D.G. (1992). Is the anterior-posterior axis of the fetus specified before implantation in the mouse? J. Exp. Zool. 264, 437-443. 4. Gardner, R.L. (1997). The early blastocyst is bilaterally symmetrical and its axis of symmetry is aligned with the animal-vegetal axis of the zygote in the mouse. Development 124, 289-301. 5. Smith, L.J. (1980). Embryonic axis orientation in the mouse and its correlation with blastocyst relationships to the uterus. Part 1. Relationships between 82 hours and 4 1/4 days. J. Embryol. Exp. Morphol. 55, 257-277. 6. Smith, L.J. (1985). Embryonic axis orientation in the mouse and its correlation with blastocyst relationships to the uterus. II. Relationships from 4 1/4 to 9 1/2 days. J. Embryol. Exp. Morphol. 89, 15-35. 7. Zernicka-Goetz, M. (2002). Patterning of the embryo: the first spatial decisions in the life of a mouse. Development 129, 815-829. 8. Piotrowska, K., and Zernicka-Goetz, M. (2002). Early patterning of the mouse embryo-contributions of sperm and egg. Development 129, 5591-5598. 9. Plusa, B., Grabarek, J.B., Piotrowska, K., Glover, D.M., ZernickaGoetz, M. (2002). Site of the previous meiotic division defines cleavage orientation in the mouse embryo. Nat. Cell Biol. 4, 811-815 10. Tam, P.P.L., Gad, J.M., Kinder, S.J., Tsang, T.E., Behringer, R.R. (2001). Morphogenetic tissue movement and the establishment of body plan during development from blastocyst to gastrula in the mouse. Bioessays 23, 508-517. 11. Thomas, P., and Beddington R. (1996). Anterior primitive endoderm may be responsible for patterning the anterior neural plate in the mouse embryo. Curr. Biol. 6, 1487-1496. 12. Thomas, P.Q., Brown, A., and Beddington, R.S.P. (1998). Hex: a homeobox gene revealing peri-implantation asymmetry in the mouse embryo and an early transient marker of endothelial cell precursors. Development 125, 85-94. 13. Rivera-Perez, J.A., Mager, J., and Magnuson, T. (2003). Dynamic morphogenetic events characterize the mouse visceral endoderm. Dev Biol. 261, 470-487. 14. Gardner, R.L. (2001). The initial phase of embryonic patterning in mammals. Int. Rev. Cytol. 203, 233-290. 15. Beddington, R.S.P, and Robertson, E.J. (1999). Axis development and early asymmetry in mammals. Cell 96, 195-209. 16. Farge, E. (2003). Mechanical induction of Twist in the Drosophila foregut/stomodeal primordium Curr. Biol. 13,1365-1377. 17. Wiley, L.M., and Pedersen, R.A. (1977). Morphology of mouse egg cylinder development in vitro: a light and electron microscopic study. J. Exp. Zool. 200, 389-402. 18. Hsu, Y.C., Baskar, J., Stevens, L.C., and Rash, J.E. (1974). Development in vitro of mouse embryos from the two-cell egg stage to the early somite stage. J. Embryol. Exp. Morphol. 31, 235-245.
Dispatch
Embryonic Axes: The Long and Short of It in the Mouse Patrick P.L. Tam
Contrary to conventional wisdom, recent studies indicate that, during mouse development, the embryo’s anterior-posterior axis shifts from the shorter transverse axis to the orthogonal longer one as the shape of the pre-gastrula-stage embryo is remodelled.
Conventional wisdom about the determination of the embryonic axes in the mouse is long on speculation and short on understanding of the cellular and molecular mechanisms. Two meticulous analyses of the orientation of the embryonic axis during the remodelling of the pre-gastrula mouse embryo, published recently in Current Biology [1,2], have now provided a more complete description of the intricate developmental processes at work. The intriguing findings show that, while the prospective anterior-posterior embryonic axis is consistently aligned with the transverse axis of the uterus, it is not always oriented along the long axis of the pre-gastrula embryo, as would be predicted from the anatomy of the gastrulating embryo. Rather, the anterior-posterior axis is first oriented along the shorter transverse axis of the embryo and then it switches to alignment with the longer axis following the remodelling of the pre-gastrula embryo. A major milestone in embryonic development is the generation of the architectural asymmetry that delineates the orientation and polarity of the three axes — anterior-posterior, dorso-ventral and left-right — of the basic body plan. The mouse embryo goes through the first four days of development like other eutherian pre-implantation embryos, dividing up the fertilized egg into multiple blastomeres and putting the descendants of the blastomeres into a blastocyst. The blastocyst is a vesicular structure containing an outer epithelial layer of trophectoderm enclosing a fluid-filled cavity. A cluster of cells, the inner cell mass which gives rise to the embryo proper, is attached to the internal wall on one side of the vesicle. This lopsided location of the inner cell mass gives the mouse embryo a distinct asymmetry. The side of blastocyst on which the inner cell mass is localised becomes the embryonic pole and the diametrically opposite side is the abembryonic pole; these poles define an embryonic-abembryonic axis (Figure 1A). Superimposed on this visible morphological asymmetry is a more subtle breach of radial symmetry. When viewed from the embryonic pole, the inner cell mass does not form a perfectly round shape, but acquires an oblong one with a short and a long Embryology Unit, Children’s Medical Research Institute, University of Sydney, Locked Bag 23, Wentworthville, NSW 2145, Australia. Email: [email protected]
diameter; the latter defines an axis of bilateral symmetry that is orthogonal to the embryonic-abembryonic axis (Figure 1A). The longer axis, however, is unique in that one end of it is marked by the presence of the second polar body and/or the tilting of the inner cell mass away from the embryonic-abembryonic axis (Figure 1A) [3–6]. The second polar body, which is extruded by the oocyte as it completes meiosis shortly before fertilization, is a landmark of the animal pole of the zygote; the opposite side, by default, is the vegetal pole. The animal-vegetal axis, in conjunction with the point of sperm entry, not only defines the orientation of the first cleavage of the zygote but also seems to influence the differential allocation of the progeny of the first two blastomeres to cells in the embryonic versus abembryonic compartment of the blastocyst [3,7–9]. These observations raise the intriguing possibility that the pertinent morphogenetic program for defining cell fates and patterning the mouse embryo is installed very early in development. Following implantation in the uterus at about 4 days post coitum, the mouse blastocyst does not grow into a discoid embryo like those of other eutherian species. Instead, the embryo adopts a cylindrical configuration, with a column of extraembryonic ectoderm derived from the growth of the trophectoderm into the cavity of A Blastocyst
B Pre-gastrula Embryonic proximal mesometrial Ectoplacental cone
Embryonic Trophectoderm
Extraembryonic ectoderm Short axis
Long axis
Inner cell mass Epiblast
Abembryonic
Visceral endoderm Abembryonic distal antimesometrial Second polar body
Current Biology
Figure 1. The anatomy of (A) the blastocyst and (B) the pregastrula-stage mouse embryo. On the left is a lateral view of the embryo, and on the right a transverse section of the embryo along the dashed line. (A) In the blastocyst, the embryonic-abembryonic axis (doubleheaded arrow) is defined by the position of the inner cell mass, and the axis of bilateral symmetry (dashed line) by the shape and the tilting (curved arrow) of the inner cell mass and the position of the second polar body. (B) In the pre-gastrula, the embryonic-abembryonic axis lies in the same plane as the proximal-distal axis of the cylindrical embryo. The proximal pole is on the mesometrial side of the uterus — the mesometrium is the mesentery-like tissue connecting the uterus to the body wall — and the distal pole points to the antimesometrial side — away from the mesometrium — of the uterus. A transverse section of the embryo at the level of the epiblast reveals an oblong shape with a short and a long axis.
Dispatch R240
A
C Mesometrium Longitudinal Transverse
D
Top view I
B
etrium
Vertical
Mesom
II II
I
III
Transverse
Current Biology
Figure 2. Orientation of the embryonic axes with the uterine axes in the mouse. (A) Anti-parallel orientation of the embryonic-abembryonic axis (arrows) of the implanting blastocysts with the longitudinal axis of the uterus [5]. (B) Re-alignment (curved arrow) of the embryonic-abembryonic axis (straight arrows) from the longitudinal to the vertical axis of the uterus [5,6]. (C) Rotation (curved arrows) of the anterior-posterior axis from the proximal-distal to the transverse axis [15]. (D) The anteriorposterior axis remains aligned with the transverse axis of the uterus while the short axis is converted into the long axis — shown in successive stages I–III — during the remodelling of the pre-gastrula embryo [1,2]. Dashed lines link the crosssection to the lateral view of the embryo. Red and blue arrows indicate the opposite polarity of the anterior-posterior axis of embryos in the same uterus.
the conceptus, carrying at its tip the inner cell mass which is transformed into an epithelial cup known as the epiblast (Figure 1B). The column and the cup are enveloped by a layer of visceral endoderm. This cylinder, however, is not perfectly symmetrical in shape: it is curved longitudinally, and the ectoplacental cone (derived from the trophectoderm) that attaches the proximal end of the cylinder to the uterine wall is frequently lopsided, which seems to reflect the original tilting of the inner cell mass relative to the embryonicabembryonic axis of the blastocyst (Figure 1A,B) [4,5]. The cylinder is also flattened on the sides such that a cross-section of it appears ellipsoidal, and the embryo has a long and a short transverse axis (Figure 1B). In embryos that have commenced gastrulation, the primitive streak — a localised region where the epithelial-mesenchyme transition has taken place — is invariably found on the posterior side of the long transverse axis. In the light of these observations, it is tempting to propose that the bilateral symmetry defined by the animal-vegetal axis of the blastocyst is the forerunner of the bilateral axis of symmetry of the embryo, and that the long axis of the pre-gastrula heralds the orientation and polarity of the prospective anterior-posterior axis of the embryo [6,10].
The validity of this notion of axis determination has been challenged by findings on the development of the anterior-posterior axis in the pre-gastrula-stage embryo. In an embryo close to initiating gastrulation — about 6 days post coitum — although the orientation of the anterior-posterior axis can be predicted based on the long axis defined by asymmetry of the cylinder, it was found that the polarity of the axis may be completely in reverse of the expected direction [3]. Interstingly, the emerging anterior-posterior axis tends to be positioned to the right hand side of the predicted axis, with a hint of clockwise rotation of the anterior pole in the transverse plane of the embryo [3]. More unexpectedly, when the expression pattern of genes characteristic of the anterior and posterior tissues in the body axis was studied in even younger embryos — about 5 to 5.5 days post coitum — the prospective anterior-posterior axis of the embryo was seen to align initially with the embryonic-abembryonic axis of the conceptus (equivalent to the proximo-distal axis of the cylindrical pre-gastrula, Figure 1B) and later to re-align with the transverse axis of the embryo, at about 5.75 post coitum [11,12]. This rotation of the anterior-posterior axis is accompanied by movement of visceral endoderm cells from the distal region to the prospective anterior side of the pre-gastrula [11,13]. This re-alignment of the prospective anterior-posterior axis, and the discordance of anterior-posterior axis from the predicted axis based on morphological asymmetry, strongly suggest that axis determination is likely to be a dynamic process coupled to morphogenetic activity in the post-implantation embryo [10,14,15]. A critical and yet unresolved issue is whether the ‘anterior’ movement of the visceral endoderm cells follows a longitude pre-determined by any axis of bilateral symmetry of the pre-gastrula. The two new studies [1,2] on the regionalization of the expression domains of genes that characterize the anterior visceral endoderm — Cerl, Hex and Gsc — and the posterior epiblast — Nodal, Fgf8, Evx1 and T — in the mouse pre-gastrula have produced some very surprising results. Contrary to what would be predicted by conventional wisdom, the two groups found that anterior visceral endoderm genes are expressed in the visceral endoderm at one pole of the short transverse axis and not the long one. The new work also shows that posterior molecular markers are expressed in the epiblast at the other end of the short axis, suggesting that the prospective anterior-posterior axis aligns with the short axis and that the anterior visceral endoderm has moved from the distal region of embryo along the longitude in the meridional plane of the short axis. As the pre-gastrula embryo grows, its cross-sectional shape changes. Concomitantly, there is a shift of the anterior-posterior gene expression domains first to sectors between the long and short axes and eventually consolidating to opposite ends of the long axis. These findings show unequivocally that the orientation of the anterior-posterior axis is not consistently related to any plane of symmetry or morphological landmark until gastrulation commences. The shift
Current Biology R241
was shown not caused by displacement of cells from the short to the long axis, but more likely reflects the conversion of the short axis of the pre-gastrula into the long axis of the gastrula by tissue remodelling. A possibility which has not been excluded is that the apparent rotation of the anterior-posterior axis in the transverse plane is brought about by changes in gene activity in the visceral endoderm and the epiblast, which may be elicited in response to mechanical stress and deformation [16] generated by growth of the embryo in a confined space. It may be interesting to test whether constraining the space for growth by increasing the number of implanting embryos, either artificially or by inducing embryonic twinning, has any effect on the alignment of the anterior-posterior axis. The spatial relationship of the axes of the mouse conceptus to that of the uterus changes in a dynamic manner. The tubular uterus has three anatomical axes: longitudinal (oviduct to cervix), vertical (anti-mesometrial to mesometrial) and transverse (cross-sectional) (Figure 2). The blastocyst usually implants on the antimesometrial side of the uterus with the embryonicabembryonic axis parallel to the longitudinal axis of the uterus [5] (Figure 2A). Presumably as a result of the growth of the tissues of the uterine crypts, the implanted blastocyst rotates to re-align the embryonic-abembryonic axis with the vertical axis so that the distal end of the cylindrical pre-gastrula points in the anti-mesometrial direction [5,6] (Figure 2B). The prospective anterior-posterior axis is then re-aligned from the proximal-distal orientation to the short axis of the pre-gastrula (Figure 2C) [15]. Mesnard et al. [1] have shown that the short axis of the embryo is aligned with the transverse axis of the uterus (Figure 2D), suggesting the orientation of the anterior-posterior axis becomes fixed after it is re-aligned from the proximo-distal to the transverse plane, and that its orientation relative to the uterus remains unchanged despite the remodelling of the embryo. It is likely that the rotation of the embryonic-abembryonic axis of the implanting blastocyst (Figure 2B) requires interactions with the uterine environment. Consistent with this notion, attachment to the culture substrate appears to be a pre-requisite for post-blastocyst development of embryo in vitro [17,18]. In contrast, the rotation of the anterior-posterior axis from the proximo-distal to the transverse plane and the remodelling of tissues can be accomplished in embryos developing free from any uterine influence [1,2], suggesting that these two processes may be controlled autonomously by the embryo. Among embryos in the same uterine tube, the polarity of the anterior-posterior axis relative to the transverse axis of the uterus can be randomly antiparallel to each other (Figure 2D). It is not known if the polarity of the anterior-posterior axis is determined by the positioning of the blastocyst relative to the uterine axes as it implants [5] or by signalling activity that controls the direction of rotation of the anterior-posterior axis in the pre-gastrula embryo. Our attention should now be focussed on resolving when and how the development of the anterior-posterior axis is controlled by the interaction with the uterine environment
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