| Gastrulation | |
|---|---|
 Gastrulation occurs when ablastula, made up of one layer, folds inward and enlarges to create a gastrula. This diagram is color-coded:ectoderm, blue;endoderm, green;blastocoel (the yolk sac), yellow; andarchenteron (the primary gut), purple. | |
| Identifiers | |
| MeSH | D054262 |
| Anatomical terminology | |
Gastrulation is the stage in the earlyembryonic development of mostanimals, during which theblastula (a single-layered hollow sphere ofcells), or in mammals, theblastocyst, is reorganized into a two-layered or three-layered embryo known as thegastrula.[1] Before gastrulation, theembryo is a continuousepithelial sheet of cells; by the end of gastrulation, the embryo has begundifferentiation to establish distinctcell lineages, set up the basic axes of the body (e.g.dorsal–ventral,anterior–posterior), and internalized one or more cell types, including the prospectivegut.[2]
Intriploblastic organisms, the gastrula is trilaminar (three-layered). These threegerm layers are theectoderm (outer layer),mesoderm (middle layer), andendoderm (inner layer).[3][4] Indiploblastic organisms, such asCnidaria andCtenophora, the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as thehypoblast andepiblast.[5]Sponges do not go through the gastrula stage.
Gastrulation takes place aftercleavage and the formation of the blastula, or blastocyst. Gastrulation is followed byorganogenesis, when individualorgans develop within the newly formed germ layers.[6] Each layer gives rise to specifictissues and organs in the developing embryo.
Following gastrulation, cells in the body are either organized into sheets of connected cells (as inepithelia), or as a mesh of isolated cells, such asmesenchyme.[4][8]
Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation:[2][9]
The terms "gastrula" and "gastrulation" were coined byErnst Haeckel, in his 1872 work"Biology of Calcareous Sponges".[10]Gastrula (literally, "little belly") is a neo-Latin diminutive based on the Ancient Greekγαστήρgastḗr ("a belly").
Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life."[2][11]
Gastrulation is highly variable across the animal kingdom but has underlying similarities. Gastrulation has been studied in many animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals that develop outside the mother.Model organisms whose gastrulation is understood in the greatest detail include themollusc,sea urchin,frog, andchicken. A human model system is thegastruloid.
Thedistinction betweenprotostomes anddeuterostomes is based on the direction in which the mouth (stoma) develops in relation to theblastopore. Protostome derives from the Greek word protostoma meaning "first mouth" (πρῶτος + στόμα) whereas Deuterostome's etymology is "second mouth" from the words second and mouth (δεύτερος + στόμα).[citation needed]
The major distinctions between deuterostomes and protostomes are found inembryonic development:
Sea urchins have been importantmodel organisms indevelopmental biology since the 19th century.[12] Their gastrulation is often considered the archetype for invertebrate deuterostomes.[13]
Sea urchins exhibit highly stereotyped cleavage patterns and cell fates. Maternally depositedmRNAs establish the organizing center of the sea urchin embryo. CanonicalWnt andDelta-Notch signaling progressively segregate progressive endoderm and mesoderm.[14]
The first cells to internalize are the primarymesenchyme cells (PMCs), which have askeletogenic fate, which ingress during the blastula stage. Gastrulation – internalization of the prospectiveendoderm and non-skeletogenicmesoderm – begins shortly thereafter with invagination and other cell rearrangements thevegetal pole, which contribute approximately 30% to the finalarchenteron length. Thegut's final length depends on cell rearrangements within the archenteron.[15]
ThefroggenusXenopus has been used as amodel organism for the study of gastrulation.[16]
The sperm contributes one of the twomitotic asters needed to complete first cleavage. The sperm can enter anywhere in theanimal half of the egg but its exact point of entry will break the egg's radial symmetry by organizing thecytoskeleton. Prior to first cleavage, the egg's cortex rotates relative to the internalcytoplasm by the coordinated action ofmicrotubules, in a process known as cortical rotation. This displacement brings maternally loaded determinants of cell fate from the equatorial cytoplasm and vegetal cortex into contact, and together these determinants set up theorganizer. Thus, the area on the vegetal side opposite the sperm entry point will become the organizer.[17]Hilde Mangold, working in the lab ofHans Spemann, demonstrated that this special "organizer" of the embryo isnecessary and sufficient to induce gastrulation.[18][19][20]
Thedorsal lip of the blastopore is the mechanical driver of gastrulation, and the first sign of invagination seen in the frog.[citation needed]
Specification of endoderm depends on rearrangement of maternally deposited determinants, leading to nuclearization ofBeta-catenin. Mesoderm isinduced by signaling from the presumptive endoderm to cells that would otherwise become ectoderm.[17]
In the frog,Xenopus, one of the signals isretinoic acid (RA).[21] RA signaling in this organism can affect the formation of the endoderm and depending on the timing of the signaling, it can determine the fate whether its pancreatic, intestinal, or respiratory. Other signals such as Wnt and BMP also play a role in respiratory fate of theXenopus by activating cell lineage tracers.[21]
Inamniotes (reptiles, birds and mammals), gastrulation involves the creation of the blastopore, an opening into thearchenteron. Note that the blastopore is not an opening into theblastocoel, the space within theblastula, but represents a new inpocketing that pushes the existing surfaces of the blastula together. Inamniotes, gastrulation occurs in the following sequence: (1) theembryo becomesasymmetric; (2) theprimitive streak forms; (3) cells from theepiblast at theprimitive streak undergo anepithelial to mesenchymal transition andingress at theprimitive streak to form thegerm layers.[7]
In preparation for gastrulation, the embryo must become asymmetric along both theproximal-distal axis and theanteroposterior axis. The proximal-distal axis is formed when the cells of the embryo form the "egg cylinder", which consists of the extraembryonic tissues, which give rise to structures like theplacenta, at the proximal end and theepiblast at the distal end. Many signaling pathways contribute to this reorganization, includingBMP,FGF,nodal, andWnt. Visceral endoderm surrounds theepiblast. Thedistal visceral endoderm (DVE) migrates to theanterior portion of the embryo, forming theanterior visceral endoderm (AVE). This breaks anterior-posterior symmetry and is regulated bynodal signaling.[7]
Theprimitive streak is formed at the beginning of gastrulation and is found at the junction between the extraembryonic tissue and theepiblast on the posterior side of the embryo and the site ofingression.[22] Formation of theprimitive streak is reliant uponnodal signaling[7] in theKoller's sickle within the cells contributing to the primitive streak andBMP4 signaling from the extraembryonic tissue.[22][23] Furthermore,Cer1 andLefty1 restrict the primitive streak to the appropriate location by antagonizingnodal signaling.[24] The region defined as theprimitive streak continues to grow towards the distal tip.[7]
During the early stages of development, the primitive streak is the structure that will establishbilateral symmetry, determine the site of gastrulation and initiate germ layer formation.[25] To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation.[26] The primitive streak extends through this midline and creates the antero-posterior body axis,[27] becoming the first symmetry-breaking event in theembryo, and marks the beginning of gastrulation.[28] This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position,[27][29] where they will differentiate into the three germ layers.[26] The localization of the cell adhesion and signaling moleculebeta-catenin is critical to the proper formation of the organizer region that is responsible for initiating gastrulation.
In order for the cells to move from theepithelium of theepiblast through theprimitive streak to form a new layer, the cells must undergo anepithelial to mesenchymal transition (EMT) to lose their epithelial characteristics, such ascell–cell adhesion.FGF signaling is necessary for proper EMT.FGFR1 is needed for the up regulation ofSNAI1, which down regulatesE-cadherin, causing a loss of cell adhesion. Following the EMT, the cellsingress through theprimitive streak and spread out to form a new layer of cells or join existing layers.FGF8 is implicated in the process of this dispersal from theprimitive streak.[24]
During gastrulation, the cells are differentiated into the ectoderm ormesendoderm, which then separates into the mesoderm and endoderm.[21] The endoderm and mesoderm form due to thenodal signaling. Nodal signaling uses ligands that are part ofTGFβ family. These ligands will signal transmembrane serine/threonine kinase receptors, and this will then phosphorylateSmad2 andSmad3. This protein will then attach itself toSmad4 and relocate to the nucleus where the mesendoderm genes will begin to be transcribed. TheWnt pathway along withβ-catenin plays a key role in nodal signaling and endoderm formation.[30]Fibroblast growth factors (FGF), canonical Wnt pathway,bone morphogenetic protein (BMP), andretinoic acid (RA) are all important in the formation and development of the endoderm.[21] FGF are important in producing thehomeobox gene which regulates early anatomical development. BMP signaling plays a role in the liver and promotes hepatic fate. RA signaling also induce homeobox genes such as Hoxb1 and Hoxa5. In mice, if there is a lack in RA signaling the mouse will not develop lungs.[21] RA signaling also has multiple uses in organ formation of the pharyngeal arches, the foregut, and hindgut.[21]
There have been a number of attempts to understand the processes of gastrulation usingin vitro techniques in parallel and complementary to studies in embryos, usually though the use of2D[31][32][33] and 3D cell (Embryonic organoids) culture techniques[34][35][36][37] usingembryonic stem cells (ESCs) orinduced pluripotent stem cells (iPSCs). These are associated with number of clear advantages in using tissue-culture based protocols, some of which include reducing the cost of associatedin vivo work (thereby reducing, replacing and refining the use of animals in experiments;the 3Rs), being able to accurately apply agonists/antagonists in spatially and temporally specific manner[35][36] which may be technically difficult to perform during Gastrulation. However, it is important to relate the observations in culture to the processes occurring in the embryo for context.
To illustrate this, the guided differentiation of mouse ESCs has resulted in generatingprimitive streak–like cells that display many of the characteristics of epiblast cells that traverse through the primitive streak[31] (e.g. transientbrachyury up regulation and the cellular changes associated with anepithelial to mesenchymal transition[31]), and human ESCs cultured on micro patterns, treated withBMP4, can generate spatial differentiation pattern similar to the arrangement of thegerm layers in the human embryo.[32][33] Finally, using 3Dembryoid body- andorganoid-based techniques, small aggregates of mouse ESCs (Embryonic Organoids, or Gastruloids) are able to show a number of processes of early mammalian embryo development such as symmetry-breaking, polarisation of gene expression, gastrulation-like movements, axial elongation and the generation of all three embryonic axes (anteroposterior, dorsoventral and left-right axes).[34][35][36][38]
Invitro fertilization occurs in a laboratory. The process of invitro fertilization is when mature eggs are removed from the ovaries and are placed in a cultured medium where they are fertilized by sperm. In the culture the embryo will form.[39] 14 days after fertilization the primitive streak forms. The formation of the primitive streak has been known to some countries as "human individuality".[40] This means that the embryo is now a being itself, it is its own entity. The countries that believe this have created a 14-day rule in which it is illegal to study or experiment on a human embryo after the 14-day period invitro. Research has been conducted on the first 14 days of an embryo, but no known studies have been done after the 14 days.[41] With the rule in place, mice embryos are used understand the development after 14 days; however, there are differences in the development between mice and humans.
