Axis Formation

Principal axes of the early Xenopus

Axis formation in the early development of any organism is the crucial first step to deciding what goes where in an embryo. It means the embryo can tell its head from its tail, its front from its back and so on. Figure 1 shows the principal axes required in early Xenopus development.

Animal-Vegetal Pole

The first axis formed is the animal-vegetal pole, which only exists at the egg/embryo stages. This is maternally determined, so when the eggs are laid, even before fertilisation, polarity is already established. This polarity is visible as shown in Figure 2; there is a pigmented side of the egg - the animal pole. This gives rise to ectoderm, the outer germ layer of the embryo. The vegetal pole has a slight yellow pigmentation. This pole contains the yolk and gives rise to endodderm, the inner germ layer of the embryo. There are lots of maternal proteins and mRNA in the egg at this point, mainly general housekeeping proteins for the regual cell function. However, there are a few developmental mRNAs present, such as vegetalising factor -1 (Vg-1). Part of the TGF-β family of signalling molecules, Vg-1 is synthesized during early oogenesis, localizing in the vegetal cortex (a gel-like layer just beneath the membrane). Just after fertilation, Vg-1 moves from the cortex to the vegetal cytoplasm, driving the establishment of the animal-vegetal axis.

Animal-vegetal poles of the unfertilised egg

Dorsal-Ventral Axis

Cortical rotation

The next axis that forms is the dorsal-ventral axis. This is established by the position of sperm entry; the dorsal side will be opposite to the entry point. Approximately 90 minutes after fertilization a process called cortical rotation occurs. This is where the vegetal cortex rotates 30° towards the animal pole, normally in orientation towards the sperm entry point. This is acheived through the formation of a microtubule array that lies parallel to the cortex. Motor proteins run along the microtubule tracks, anchoring to the cortex so that it becomes displaced. It is thought that the new interactions taking place between the vegetal cortex and animal cytoplasm is what causes the formation of the dorsal-ventral axis.

Once rotation is complete a group of cells, located in the outer vegetal region opposite the point of sperm entry, acquire a new identity and become the Nieuwkoop center, which induces dorsal fates to neighbouring cells. The importance of the Nieuwkoop center was shown in an experiment in which it was removed from one embryo and pasted onto the ventral side of another embryo. The embryo with no Nieuwkoop center goes on to form a normal embryo, but the embryo with two Nieuwkoop centers forms an embryo with two heads. This shows the dorsalizing effects of the Nieuwkoop center and that both ventral and dorsal cells have the potential to form dorsal tissue at this stage. After cortical rotation, maternal β-catenin mRNAs, localise to the dorsal side of the embryo. This can act like the Nieuwkoop center, which is why when the Nieuwkoop center is removed, a normal embryo still forms.

The key function of the Nieuwkoop center is to specify another dorsalising center - the Spemann Organiser. The Spemann Organiser is located on the dorsal lip and further induces dorsal tissue.

Germ layers

Next the germ layers form: Endoderm, mesoderm and ectoderm. These are the 3 primary cell layers that go on to form all the tissues and organs of the embryo. The vegetal region gives rise to the endoderm, which forms the gut. The equatorial region gives rise to mesoderm which goes onto form bones and muscles. The animal region gives rise to ectoderm, which makes epidermis and the nervous system. 

Germ layers

4-signal model for mesoderm induction

The mesodem is a little differnt from the ectoderm and endoderm, in that it is not induced by maternal factors and needs additional signals from the embryo to specify mesodermal fate. There is a 4 signal model of mesodem induction shown in Figure 5. The first signal comes from the vegetal region and is a general mesoderm inducer that makes ventral mesoderm (Figure 5; 1). The second signal comes from the Nieuwkoop center and is a dorsal mesoderm inducer that makes the Spemann Organiser and the notochord (Figure 5; 2). The third signal comes from the Spemann Organiser and it modifies the patterning of the dorsal mesoderm (Figure 5; 3) and the fourth signal is a ventral mesoderm inducer (Figure 5; 4).

The mesoderm inducing signals are most likely to be members of the TGF-β family, such as Vg-1 and activin. The factors that pattern the mesoderm are related to bone morphogenetic protein (BMP). BMP is expressed in the ventral region, and antagonists of BMP (like noggin, chordin and follistatin) are expressed on the dorsal side. So high BMP levels give rise to ventral structures, and low BMP levels give rise to dorsal structures.

Gastrulation

Gastrulation is the next process that takes place in development and involves the movement of cell sheets. The first step is involution of the endoderm and mesoderm at a point called the blastopore (Figure 6, A Red). At the same time, the ectoderm (Figure 6, Blue) spreads over the outer layer of the embryo (Figure 6, B) via a process called epiboly, until the entire embryo is surrounded by ectoderm. The mesdermal tissue (Figure 6, Red) extends and moves along the anterior-posterior axis inside the embryo by a mechanism called convergent extension. These cell sheets spread right round until it lines the entire inner wall of the embryo as shown in Figure 6, D. The endoderm (Figure 6, yellow) does not flatten like the other layers, but a space forms in the middle of the embryo called the archenteron which later develops into parts of the digestive system. Once these layers are positioned correctly, gastrulation is complete and the embryo continues into neuraltion.

Gastrulation

References

  • BIRSOY, B., KOFRON, M., SCHAIBLE, K., WYLIE, C. & HEASMAN, J. 2006. Vg1 is an essential signaling molecule in Xenopus development. Development, 133, 15-20.
  • GERHART, J., DANILCHIK, M., DONIACH, T., ROBERTS, S., ROWNING, B. & STEWART, R. 1989. CORTICAL ROTATION OF THE XENOPUS EGG - CONSEQUENCES FOR THE ANTEROPOSTERIOR PATTERN OF EMBRYONIC DORSAL DEVELOPMENT. Development, 107, 37-51.
  • HARLAND, R. & GERHART, J. 1997. Formation and function of Spemann's organizer. Annual Review of Cell and Developmental Biology, 13, 611-667.
  • MARRARI, Y., ROUVIERE, C. & HOULISTON, E. 2004. Complementary roles for dynein and kinesins in the Xenopus egg cortical rotation. Developmental Biology, 271, 38-48.
  • ROSZKO, I., SAWADA, A. & SOLNICA-KREZEL, L. 2009. Regulation of convergence and extension movements during vertebrate gastrulation by the Wnt/PCP pathway. Seminars in Cell & Developmental Biology, 20, 986-997.
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