An editable svg file of this figure can be dowloaded at In this way, the endoderm will end up lining the gut with the mesoderm overlying it and the ectoderm will cover the entire embryo and form the nervous system. During gastrulation, the ectoderm epibolizes over the other germ layers as the mesoderm turns under itself at the dorsal lip of the blastopore. The process of gastrulation moves them relative to each other and elongates them. The blastula germ layers are stacked from animal pole to vegetal pole. A "fate-map" (top left) shows which regions of the blastula will develop into specific parts of the larva after normal development. A Xenopus blastula (pre-gastrulation embryo) will undergo predictable movements in gastrulation to form it's three tissue layers, the ectoderm, mesoderm, and endoderm. Figure 4: Xenopus gastrulation by Kristina Vu and Ajna Rivera. For example, compare a picture of a fate-map of a blastula-stage frog to the fate map of a gastrulated frog (Figure 4). As cells change their partners over time, they become fated to a specific path. Second, it allows for new interactions between cells and places cells in new morphogenic positions. First, it creates tissue layers (germ layers) from set of fairly homogenous cells. Īs mentioned earlier, gastrulation performs two key functions in a developing animal embryo. The following video shows a live-frog embryo undergoing cleavage and gastrulation and also shows an animated internal view of how the cells move inside the embryo. An editable svg file of this figure can be dowloaded from Convergent extension is cell intercalation and cell elongation to turn a short/wide tissue into a long/narrow tissue.
Epiboly is a set of complex movements including intercalation (cells moving between each other), cells crawling, and cells changing shape that results in a sheet of cells covering a surface. Ingressing cells (blue) delaminate from a sheet of cells. An invaginating sheet of cells pokes inwards at one point to make a lined cavity. In involution, a single layer of cells curves under itself to form two layers. Figure 3: types of cell movements during development. As the cells comprising these tissue layers encounter new morphogens and new signaling partners, they become specified as germ-layers. The movement of cells (Figure 3) over (epiboly), under (involution), between (intercalation), internally (invagination) or away from an epithelial sheet (ingression and delamination) creates layers of tissue. This change in local environment means that the cells may a) be exposed to different extracellular morphogens and b) make contact with new cell-cell signaling partners. As they move, they change their local environment and come into contact with new cells. During gastrulation the cells produced during cleavage begin to move relative to each other. Gastrulation refers to a time of great flux in animal embryos. Read more details about Cleavage here, focus on any section you find interesting and especially figure 8.5. The timing of the MBT and the sorting of maternal proteins and RNAs into different cleavage-stage cells varies widely among different animals. If the drugs are applied to later-stage embryos development halts, suggesting that at this point zygotically derived transcripts are necessary for development and cell division. If these drugs are applied to an early cleavage-stage embryo cleavages proceed normally, suggesting that developmental processes and cell division at this early stage are using maternally-derived factors already present in the embryo. One interesting experiment that shows the extent of this transition is to block transcription using the drugs alpha-amanitin or actinomycin D - two inhibitors of RNA polymerase action. The MBT demarcates the transition between using mostly maternal cell and fate determinants (RNAs and protein) to the embryo producing its own RNAs and proteins. Sometimes this is called the "mid-blastula transition" or MBT. Conditional specification typically begins to occur towards the end of cleavage stages when cell division slows down and cells begin to communicate more extensively with each other. If morphogens are asymmetrically distributed in the egg, then these cells can differentiate via autonomous specification even at early stages. Early cleavage stages are different from later cell divisions in that they quickly produce many very similar looking cells. This ball may be hollow inside, have a hollow region, or be fairly solid. During cleavage a single cell embryo rapidly divides to form a ball of cells, called a blastula. Frog Gastrulation and the Dorsal lip of the BlastoporeĬleavage stages of embryogenesis are typically very similar across animals.