Importance of gastrulation
In 1986, Lewis Wolpert described the importance of gastrulation in his famous phrase” It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life. "1.
Gastrulation is the name given to a key stage in embryogenesis, when the simple spherical blastula, performs significant cell movements and adhesions to transform from being a single-layered into a multi-layered structure, where the three germ layers, ectoderm, mesoderm and endoderm are repositioned to where they will develop into specific body systems2. The blastula, during gastrulation, is named gastrula to indicate its development stage. Using fate mapping techniques, a lot of information was obtained on mechanisms related to cell fates during throughout gastrulation, but there is little information on the control and operation of the cellular and molecular mechanisms that bring about gastrulation2.
Some aspects of gastrulation are shared or look similar throughout most animals, which may suggest that evolution preserved important developmental information throughout species2.
In this essay we will look at some details and mechanisms that take place during gastrulation, which are shared or contrasted amongst studied vertebrate model organisms and recent experimental findings. This essay will include the following model organisms, frog Xenopus (amphibian), zebrafish (fish), chicken (bird), and mouse (mammal). “Different vertebrate embryos arrive at gastrulation in a variety of shapes and sizes”3. However, when gastrulation finishes, the patterns of vertebrate embryos is functionally similar (phylotypic stage)3.
2.0Gastrulation in the vertebrates
The characteristic behaviour of cells is instructed by complicated pathways, that guide the cell to their function and fate i.e. signal the cells to migrate to specific destinations, change their shape, proliferate…etc4.
It is observed that, generally, gastrulation starts with an involution or ingression of mesendodermal cells and followed by ingression and extension on the dorsal side of the embryo. As a result, cells accumulate dorsally in the gastrula and as an impact the body extends on the anterior-posterior axis2.
In Xenopus laevis, gastrulation starts with an infolding (blastopore) at the surface of the blastula in a region called marginal. It is also important to mention that this particular region is very necessary for the amphibian development as it will be the site of the Spemann organiser, which is an embryonic organiser whom without, the dorsal and axial development wont take palce5.
The The blastopore acts as a gate, which allows the future mesoderm and endoderm to pass into the gastrula as an organised sheets of cells (involution)5. There, the tissues will undergo convergence and extension (mediolaterally as an organised sheet) beneath the dorsal ectoderm5. During that time epiboly takes place, where the ectoderm spreads downstream to spread all over the embryo surface5.
When the dorsal endoderm involutes, it resides closely near the mesoderm and as a result of the forming space between the mesoderm and the vegetal pole (yolk), the archenteron is formed. Archenteron is a precursor of the gut cavity5. As observed, even though the movement of the involuting mesoderm and endoderm cells started dorsally at early gastrula stage, it spread to circulate the areas around the blastopore. The inward movement of endoderm and mesoderm begin dorsally and then spreads to form a complete circle around the blastopore5. At the ending stages of gastrulation the blastopore shuts and all the layers are positioned into their assigned locations5.
The zebrafish embryo undergoes epiboly to cover the yolk cell, which is a very similar process to that observed in Xenopus embryo. IT also possesses a shield-shaped region which plays a similar role to the Spemann organiser in the Xenopus gastrula7. Another similar aspect to the embryonic development of zebrafish is that gastrulation also starts with the involution of the endoderm and mesoderm cells at the margin of blastoderm8. When those mesodermal and endodermal cells are internalised, they travel towards the later becoming dorsal side of the gastrula. As loosely packed mesenchymal cells, they converge and extend toward the midline of the gastrula8. These movements cause the “elongationof the embryo in an antero-posterior direction” 8.
The zebrafish and Xenopus share some commonalities and differences in gastrulation, some differences like that of involution occurring in the blastopore in an organised manner in the Xenopus while all involution take place all around the periphery of the blastoderm at about the same time in zebrafish.
There are similarities such as, In the Xenopus and zebrafish gastrulation, “convergent extension is take place along the antero-posterior axis and cells become polarised in the medio-lateral intercalation as cells become incorporated into future notochord.”9.
The main path taken by the zebrafish and the Xenopus during gastrulation is somewhat similar when we look into how the mesoderm and endoderm undergo involution (internalisation) or when the mesoderm convergent extention takes place along the antero-posterior axis and the epibolic covering of embryo by ectodermal extension6.
There are also general similarities observed across vertebrate gastrulation such as the regulation of the convergence and extension movements, which play a huge role in shaping the embryo. The movements are regulated by the Wnt/PCP pathway4.
“In vertebrates, the Wnt/Planar Cell Polarity (Wnt/PCP) pathway is a key regulator of C&E movements, essential for several polarized cell behaviors, including directed cell migration, and mediolateral and radial cell intercalation.In vertebrates, defective Wnt/PCP signalingperturbs cell polarity and consequently cell movements. Many of the molecular components of the Wnt/PCP pathway are conservedand are involved in the establishment of cell polarity in the planeof tissues. However, the subcellular localization of the Wnt/PCPcomponents varies, not only between species but also betweentissues“.4
The chick embryo has many differences with the mouse embryo at the very early stages but later and especially during gastrulation they become very similar and for that reason the chick studies were described as complementary to the mouse embryo development studies.
Unlike the zebrafish and Xenopus embryos, the chick embryo develops as a flat disc of cells lying atop a massive ball of yolk.13
Gastrulation begins at a region of thickened epiblast between area opaca and area pallucida posterior to the embryo.14 This area is known as the posterior marginal zone confronted by a group of small cells with the shape of crescent called Koller's sicle.
The development of the primitive streak marks the beginning of gastrulation. The primitive streak sets the body axis of the embryo.14 the primitive streak is visible as a recognisably denser region extends narrowly from Koller's sickle to about halfway through the area pallucida, this resulting furrow is located at the dorsal side of the epiblast and it is considered the earliest sign of antero-posterior embryo axis.14
Unlike Xenopus and zebrafish, the chick and mouse embryos continue to grow and cells maintain proliferation throughout gastrulation.15
“Epiblast cells converge on the primitive streak, and as the streak moves forward from the posterior marginal zone, cells in the furrow move inward and spread put anteriorly and laterally beneath the upper layer, forming a layer of mesenchyme in the subgermincal space.” 15
This shows that the primitive streak is similar in function to the blastopore of the Xenopus. However, at the primitive streak cells move into the embryo in an individual manner compared to the coherent sheet of cells movement observed at Xenopus cell involution, thus instead of involution this stage is called ingression in the chick.15 Similarly, however, the ingressing cells will become mesoderm and endoderm, but cells that remain at the surface of epiblast will become ectoderm.15
“At the anterior end of the primitive streak a condensation of cells known as Hensen's node becomes apparent.” 15 Hensen's node is formed from cells of the cells of posterior marginal zone, epiblast, and Koller's sickle. It acts as an important organiser centre, which has similar functions to the Spemann organiser in Xenopus.15
At the end of gastrulation, the inward movement of cells stops and the primitive streak regresses. The regression of the primitive streak results in the Hensen's node movement toward the embryo's posterior side and in the early development of the notochord from the internalised mesoderm. 15
In the mouse embryo, gastrulation commences after 6.5 days of implantation. As in the chick, “gastrulation starts when epiblast cells converge on the posterior of the epiblast and move under the surface, forming the primitive streak, where the cells are becoming internalised”.17
“When the proliferating cells move inside the embryo, they spread out in a lateral fashion between the epiblast and the visceral endoderm to give a layer of protective mesoderm. Some of the internalised cells will eventually replace the visceral mesoderm to give definitive endoderm, which forms the gut. AS gastrulation proceeds the primitive streak lengthens and reaches the bottom of the cup, with the node at the anterior end.”17
This node will later give rise to the notochord, which forms a structure known as the head process. Compared the frog gastrula the germ layers of the mouse appear inverted due to the topology of the mouse embryo at this period. 17
From the study we can see that the vertebrates have many characteristics in common with each other when undergoing gastrulation, but there are many differences that could overwhelm these similarities. However, we can suggest that, evolution may have played a major role in preserving some key aspects of gastrulation throughout millions of years, and changes took place depending on the requirements of these animals faced with environment and necessities for their survival. The study was vague and could have looked more in depth into similarities between pathways and pathway signals used in gastrulation, but as this field is in an ongoing development there are yet many secrets to be unfold linking all vertebrates to their common ancestors.