Gastrulation is a procedure of highly coordinated cell and tissue movements whereby cells of blastula are dramatically rearranged. Blastula comprises of several cells, positions of which were established during cleavage. During gastrulation, these cells are provided new positions and new neighbors, and multilayered body plan of organism is established. Cells that will form endodermal and mesodermal organs are brought inside embryo, while cells which will form skin and nervous system are spread over its outside surface. Therefore, three germ layers outer ectoderm, inner endoderm, and interstitial mesoderm are first generated during gastrulation. Additionally, phase is set for interactions of these newly positioned tissues.
Movements of gastrulation involve the complete embryo, and cell migrations in one part of gastrulating embryo should be intimately coordinated with other movements occurring concurrently. Though patterns of gastrulation vary extremely throughout animal kingdom, there are only a few basic kinds of cell movements. Gastrulation generally involves some combination of the following kinds of movements:
i) Invagination: the sheet of cells (known as epithelial sheet) bends inward.
ii) Ingression: individual cells leave the epithelial sheet and become freely migrating mesenchyme cells.
iii) Involution: the epithelial sheet rolls inward to form an underlying layer.
iv) Epiboly: the sheet of cells spreads by thinning.
v) Intercalation: rows of cells move between one another, creating the array of cells which is longer (in one or more dimensions) but thinner.
vi) Convergent Extension: rows of cells intercalate, but intercalation is very directional.
Gastrulation in Sea Urchin:
Late sea urchin blastula comprises of single layer of approx 1000 cells which form the hollow ball, fairly flattened at the vegetal end. Blastomeres, derived from different regions of zygote, have different sizes and properties. Fates of different regions of blastula as it develops through gastrulation to pluteus larva stage characteristic of sea urchins. Fate of each cell layer can be seen through movements during gastrulation.
Function of primary mesenchyme cells:
Soon after the blastula hatches from fertilization envelope, vegetal side of spherical blastula starts to thicken and flatten. At center of this flat vegetal plate, the cluster of small cells starts to change. These cells start extending and contracting long, thin (30 × 5 μm) processes known as filopodia from their inner surfaces. Cells then dissociate from epithelial monolayer and ingress in blastocoel. These cells, derived from micromeres, are known as primary mesenchyme. They will form larval skeleton, so they are at times known as skeletogenic mesenchyme. At first cells seem to move arbitrarily along inner blastocoel surface, vigorously making and breaking filopodial connections to wall of the blastocoel. Finally, though, they become localized inside prospective ventrolateral region of blastocoel. Here they fuse in syncytial cables that will form axis of calcium carbonate spicules of larval skeleton.
First phase of archenteron invagination:
As ring of primary mesenchyme cells leaves vegetal region of blastocoel, significant changes are happening in cells which remain at vegetal plate. These cells remain bound to one another and to hyaline layer of egg, and they move to fill gaps caused by ingression of primary mesenchyme. Furthermore, vegetal plate bends inward and invaginates about one-fourth to one- half the way in blastocoel. Then invagination abruptly stops. Invaginated region is known as archenteron (primitive gut), and opening of archenteron at vegetal region is known as blastopore.
Fibropellins are stored in secretory granules inside oocyte, and are secreted from those granules after cortical granule exocytosis releases hyalin protein. By blastula stage, fibropellins have created meshlike network over embryo surface. At the time of invagination, vegetal plate cells (and only those cells) secrete chondroitin sulfate proteoglycan in inner lamina of hyaline layer directly under them. This hygroscopic (water-absorbing) molecule swells inner lamina, but not outer lamina. This causes vegetal region of hyaline layer to buckle. The endodenual cells adjacent to micromere-derived mesenchyme become foregut, migrating the farthest distance in blastocoel. Next layer of endodenual cells becomes midgut, and the last circumferential row to invaginate forms hindgut and anus.
Second and third phases of archenteron invagination:
Invagination of vegetal cells takes place in three discrete stages. After the short pause, second stage of archenteron formation starts. During this time, archenteron extends dramatically, at times tripling its length. In this procedure of extension, wide, short gut rudiment is transformed in the long, thin tube. To achieve this extension, cells of archenteron rearrange themselves by migrating over one another and by flattening themselves. This phenomenon, in which cells intercalate to narrow tissue and at same time move it forward, is known as convergent extension.
In at least some species of sea urchins, the third phase of archenteron elongation takes place. This last stage is started by tension given by secondary mesenchyme cells that form at tip of archenteron and remain there. Filopodia are extended from the cells through blastocoel fluid to contact inner surface of blastocoel wall. Filopodia join to wall at the junctions between blastoderm cells and then shorten, pulling up the archenteron. Secondary mesenchyme cells with the laser with the result that archenteron could lengthen to only about two-thirds of normal length. If the few secondary mesenchyme cells were left, elongation continued, though at the slower rate. Secondary mesenchyme cells, then, play the necessary role in pulling archenteron up to blastocoel wall during last stage of invagination.
Gastrulation in Amphibian:
The study of amphibian gastrulation is both one of the oldest and one of the newest areas of experimental embryology. Although amphibian gastrulation has been widely studied for past century, most of our theories concerning mecbnnismc of the developmental movements have been revised over past decade. Study of amphibian gastrulation has been complicated by the fact that there is no single way amphibians gastrulate. Different species employ different means toward same goal.
The fate map of Xenopus:
Amphibian blastulae are faced with the similar tasks as invertebrate blastulae to bring inside embrya-those areas destined to create endodermal organs, to surround embryo with cells able to form ectoderm, and to place mesodermal cells in the correct positions between them. Movements whereby this is achieved can be visualized by method of vital dye staining. Fate mapping has illustrated that cells of Xenopus blastula have different fates depending on whether they are located in deep or superficial layers of embryo.
In Xenopus, mesodermal precursors exist mostly in deep layer of cells, whereas ectoderm and endoderm arise from superficial layer on surface of the embryo. Most precursors for notochord and other mesodermal tissues are situated under surface in equatorial (marginal) region of embryo. In urodeles (salamanders like Triturus and Ambystoma) and in some frogs other than Xenopus, several more of notochord and mesoderm precursors are among surface cells.
The midblastula transition: preparing for gastrulation:
The first precondition for gastrulation is an activation of genome. In Xenopus, nuclear genes are not recorded until late in twelfth cell cycle. At that time, different genes start to be recorded in different cells, and blastomeres attain capacity to become motile. This dramatic change is known as the midblastula transition. It is believed that different transcription factors become active in different cells at this time, giving cells new properties. For example, vegetal cells (maybe under direction of maternal VegT protein) turned out to be endoderm and start secreting factors which cause cells above them to become mesoderm.
Positioning the blastopore:
Vegetal cells are vital in finding the location of blastopore, as is point of sperm entry. Microtubules of sperm direct cytoplasmic movements which authorize vegetal cells opposite point of sperm entry to induce blastopore in mesoderm above them. This region of cells opposite the point of sperm entry will create blastopore and turn out to be dorsal portion of the body. In this way, the new state of symmetry is attained. While unfertilized egg was radially symmetrical about the animal-vegetal axis, the fertilized egg now has dorsalventral axis. It has turn out to be bilaterally symmetrical (having right and left sides). The side where sperm goes into marks future ventral surface of embryo; the opposite side, where gastrulation is started, marks future dorsum of embryo. If cortical rotation is blocked, there is no dorsal development, and embryo dies as the mass of ventral (primarily gut) cells.
Invagination and involution in Amphibian:
Amphibian gastrulation is initial visible when the group of marginal endoderm cells on dorsal surface of blastula sinks in embryo. Outer (apical) surfaces of the cells contract dramatically, whereas their inner (basal) ends expand. Apical-basal length of the cells greatly increases to yield features bottle shape. In salamanders, these bottle cells seem to have active role in early movements of gastrulation, found that bottle cells from early salamander gastrulae could join to glass cover slips and lead movement of those cells attached to them.
The convergent extension of the dorsal mesoderm:
Involution starts dorsally, led by pharyngeal endomesoderm and prechordal plate. These tissues will migrate most anteriorly under surface ectoderm. Next tissues to enter dorsal blastopore lip have notochord and somite precursors. In the meantime, as lip of the blastopore expands to have dorsolateral, lateral, and ventral sides, prosepective heart mesoderm, kidney mesoderm, and ventral mesoderm enter in embryo.
The IMZ is initially numerous layers thick. Soon before their involution through blastopore lip, the numerous layers of deep IMZ cells intercalate radially to form one thin, broad layer. This intercalation further extends IMZ vegetally. At the same time, superficial cells spread out by dividing and flattening. When deep cells reach the blastopore lip, they involute in embryo and start the second kind of intercalation. This intercalation causes convergent extension along mediolateral axis which integrates numerous mesodermal streams to form the long, narrow band. This is reminiscent of traffic on the highway when numerous lanes should merge to form the single lane.
Migration of the involuting mesoderm:
As mesodermal movement progresses, convergent extension continues to narrow and extend involuting marginal zone. IMZ has prospective endodermal roof of archenteron in superficial layer (IMZS) and the prospective mesodermal cells, comprising those of notochord, in its deep region (IMZD). During middle third of gastrulation, expanding sheet of mesoderm converges toward midline of the embryo. This procedure is driven by continued mediolateral intercalation of cells along anterior-posterior axis, thereby further narrowing the band.
Epiboly of ectoderm:
While involution is happening at blastopore lips, the ectodermal precursors are increasing over the entire embryo have used scanning electron microscopy to examine the changes in both superficial cells and deep cells of animal and marginal regions. The main mechanism of epiboly in Xenopus gastrulation seems to be the increase in cell number (through division) coupled with the concurrent integration of numerous deep layers into one.
Gastrulation in Mammals:
Birds and mammals are both descendants of reptilian species. Thus, it is not surprising that mammalian development parallels that of reptiles and birds. What is surprising is that gastrulation movements of reptilian and avian embryos that evolved as the adaptation to yolky eggs are maintained even in lack of large amounts of yolk in mammalian embryo.
Change for development within another organism:
Mammalian embryo gets nutrients directly from mother and doesn't depend on stored yolk. This adaptation has involved dramatic restructuring of maternal anatomy (like expansion of oviduct to form the uterus) and the development of the fetal organ capable of absorbing maternal nutrients. This fetal organ the chorion is derived mainly from embryonic trophoblast cells, supplemented with mesodermal cells derived from inner cell mass. Chorion forms fetal portion of placenta. It will induce uterine cells to create maternal portion of placenta, the decidua. Decidua becomes rich in blood vessels which will give oxygen and nutrients to embryo.
Formation of extraembryonic membranes:
As the embryonic epiblast is going through cell movements reminiscent of those seen in reptilian or avian gastrulation, extraembryonic cells are making the noticeably mammalian tissues which allow the fetus to survive inside maternal uterus. Though initial trophoblast cells of mice and humans divide like most other cells of body, they give rise to the population of cells in which nuclear division happens in absence of cytokinesis. The original kind of trophoblast cells constitutes a layer known as the cytotrophoblast, while multinucleated kind of cell forms syncytiotrophoblast. Cytotrophoblast originally sticks to endometrium through the series of adhesion molecules.
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