Gastrulation is the stage of early development in which the embryo’s single sheet of pluripotent cells reorganizes into three germ layers. In humans it begins about 14 days after fertilization and lasts a little over a week, through the third week of development.[1]
What Is Gastrulation?
During gastrulation, the germ layers are specified, moved into position and shaped into a basic body plan with the beginnings of the organs. Four kinds of tissue movement, conserved across animals generally, do this work: cells moving inside beneath the surface layer (emboly), layers spreading and thinning (epiboly), and tissues narrowing from side to side (convergence) while lengthening from head to tail (extension).[2] By the end of the process, the embryo has an outer, a middle and an inner layer, a head-to-tail axis and a left and right side.
The three layers are the first step toward every tissue in the body. Broadly, the ectoderm gives rise to the nervous system and the skin’s outer layer, the mesoderm to the heart, blood vessels, muscles and skeleton, and the endoderm to the lining of the digestive tract and the lungs.[3]
Before Gastrulation: From Blastocyst to Bilaminar Disc
About five days after fertilization the embryo is a hollow ball called a blastocyst. Its outer layer, the trophectoderm, will form the placenta, and a cluster of cells inside, the inner cell mass, will form the embryo. The inner cell mass separates into two layers: the epiblast, the pluripotent cells from which all three germ layers will come, and the hypoblast (primitive endoderm), which faces the cavity of the blastocyst. The human blastocyst implants in the wall of the uterus 6 to 7 days after fertilization.[3]
During the following week, the epiblast forms a flat, two-layered structure with the hypoblast called the bilaminar disc. The part of the epiblast facing the implanted trophoblast becomes the thin amnion, which encloses the amniotic cavity, while the rest stays as the embryonic epiblast.[3] Gastrulation turns this two-layered disc into three layers.
The Primitive Streak and Node
Gastrulation starts with a visible groove, the primitive streak, which forms in the epiblast at the end of the disc that will become the tail. The streak fixes the head-to-tail axis, and its position also defines the embryo’s left and right sides. As it lengthens toward the future head, a small thickening forms at its front tip, the primitive node. In a human embryo 16 to 19 days after fertilization, the streak runs about half the length of the disc, with the node at its front end.[1]
The node acts as a signaling center, the mammalian version of the organizer first described in amphibians. Cells leaving it form the axial mesoderm: the prechordal plate and the notochord, a rod that runs along the midline and later signals to the tissues around it.[3]
How Cells Move Inward: Epithelial to Mesenchymal Transition
The epiblast is an epithelium, a sheet of cells tightly attached to each other. To move inside, cells at the primitive streak go through an epithelial to mesenchymal transition (EMT). They switch on transcription factors of the SNAIL family, lose the adhesion protein E-cadherin that holds epithelial cells together, gain N-cadherin, and become motile.[3] The cells then slip through the streak and migrate away from it beneath the epiblast.
The cells that pass through the streak follow two broad paths. Some move into the lower layer, where they join and largely take over from the hypoblast and form the definitive endoderm. Others spread out between the two layers and form the mesoderm. Epiblast cells that never enter the streak stay on the surface and become the ectoderm.[1] Each of these paths is the start of a different program of cell differentiation.
The word “mesenchymal” here describes how a cell looks and behaves: loosely connected and able to migrate. It does not mean the cells are mesenchymal stem cells, the adult stromal cells that laboratories grow from bone marrow, fat or birth tissues.
The Signals That Drive Gastrulation
Three families of signaling molecules work in sequence to start mesoderm and endoderm formation. Research in model organisms has defined a BMP to WNT to NODAL cascade. In the mouse, BMP4 from tissue next to the epiblast switches on WNT3, and WNT signaling in turn switches on NODAL, a member of the TGF-beta family. Inhibitors released from the hypoblast side limit these signals to the back of the epiblast, which is why the streak forms at one end only. In primate and cultured human embryos, the amnion appears to be the source of the BMP4 signal.[3]
The Three Germ Layers and What Each Forms
The table below summarizes the main tissues that come from each layer. The detailed lists, including the neural crest and the separate parts of the gut tube, are on the pages for ectoderm and endoderm derivatives and for the mesoderm.
| Germ layer | Position | Main tissues and organs it forms |
|---|---|---|
| Ectoderm | Outer layer | Brain, spinal cord and peripheral nerves; outer layer of the skin, hair, nails and tooth enamel; lens of the eye; neural crest derivatives such as pigment cells and much of the facial skeleton |
| Mesoderm | Middle layer | Heart, blood vessels and blood cells; skeletal, heart and smooth muscle; bone and cartilage; kidneys and gonads; the inner layer of the skin (dermis); the linings of the body cavities; the notochord |
| Endoderm | Inner layer | Lining of the digestive tract; liver and pancreas; lining of the airways and lungs; thyroid; lining of the bladder |
Common Confusions About Germ Layers
Germ layers are not germ cells. Germ layers are the three tissue layers that build the body. Germ cells are the precursors of eggs and sperm. In a human embryo at gastrulation, primordial germ cells are already present near the primitive streak.[1] Where they first arise in humans is still debated: in cynomolgus monkeys they are specified in the young amnion before gastrulation begins.[4]
Textbook drawings often show a different animal. Many gastrulation diagrams show a hollow ball folding inward through an opening called the blastopore, which is how gastrulation looks in sea urchins and frogs. The human embryo at this stage is a flat disc, and its cells move inward through the primitive streak instead. Even the mouse, the most studied mammal, differs: its gastrulating embryo is cylindrical rather than flat, which changes the paths that migrating cells take.[1]
The first endoderm is not the final endoderm. Most of the definitive endoderm that forms the gut and its organs comes from epiblast cells that passed through the streak. The older view was that these cells push the hypoblast aside; live imaging in mouse embryos instead showed the two populations mixing, with some hypoblast-derived cells joining the early gut tube.[5] The rest of the hypoblast lineage forms the lining of the yolk sac.
Why Gastrulation Matters in Stem Cell Science
Human embryonic stem cells and induced pluripotent stem cells resemble the epiblast just before gastrulation, so laboratories make specific cell types by giving them the same signals in the same order.[3] Treating human embryonic stem cells with activin A, which acts like NODAL, produced cultures in which up to 80 percent of the cells were definitive endoderm. The cells went through an epithelial to mesenchymal transition and a pattern of gene activity resembling gastrulation, and this step is the starting point for making endoderm organs such as the pancreas.[6] When the same cells are grown on small round patches and treated with BMP4, they arrange themselves into rings that mimic the germ layers: ectoderm-like cells in the center, a ring of primitive streak-like cells around them, and an outer ring of cells resembling extraembryonic tissue.[7] Physical cues matter too: the stiffness of the scaffold that cells grow on influences which germ layer they choose.[8]
Gastrulation also marks an ethical boundary. Many guidelines, including earlier versions of the International Society for Stem Cell Research guidelines, have prohibited growing intact human embryos in the laboratory beyond 14 days after fertilization or the appearance of the primitive streak, whichever comes first, a limit known as the 14-day rule. In its 2021 update, the society moved such research from the prohibited category to one that requires specialized review, and it recommended broad public discussion before any proposal to go past the limit is considered.[9]
Published Clinical Citations
[1] ^ Tyser RCV, Mahammadov E, Nakanoh S, Vallier L, Scialdone A, Srinivas S. Single-cell transcriptomic characterization of a gastrulating human embryo. Nature. 2021;600(7888):285-289. https://doi.org/10.1038/s41586-021-04158-y
[2] ^ Solnica-Krezel L, Sepich DS. Gastrulation: making and shaping germ layers. Annual Review of Cell and Developmental Biology. 2012;28:687-717. https://doi.org/10.1146/annurev-cellbio-092910-154043
[3] ^ Stringa B, Solnica-Krezel L. Signaling mechanisms that direct cell fate specification and morphogenesis in human embryonic stem cells-based models of human gastrulation. Emerging Topics in Life Sciences. 2023;7(4):383-396. https://doi.org/10.1042/ETLS20230084
[4] ^ Sasaki K, Nakamura T, Okamoto I, et al. The germ cell fate of cynomolgus monkeys is specified in the nascent amnion. Developmental Cell. 2016;39(2):169-185. https://doi.org/10.1016/j.devcel.2016.09.007
[5] ^ Kwon GS, Viotti M, Hadjantonakis AK. The endoderm of the mouse embryo arises by dynamic widespread intercalation of embryonic and extraembryonic lineages. Developmental Cell. 2008;15(4):509-520. https://doi.org/10.1016/j.devcel.2008.07.017
[6] ^ D’Amour KA, Agulnick AD, Eliazer S, Kelly OG, Kroon E, Baetge EE. Efficient differentiation of human embryonic stem cells to definitive endoderm. Nature Biotechnology. 2005;23(12):1534-1541. https://doi.org/10.1038/nbt1163
[7] ^ Warmflash A, Sorre B, Etoc F, Siggia ED, Brivanlou AH. A method to recapitulate early embryonic spatial patterning in human embryonic stem cells. Nature Methods. 2014;11(8):847-854. https://doi.org/10.1038/nmeth.3016
[8] ^ Zoldan J, Karagiannis ED, Lee CY, Anderson DG, Langer R, Levenberg S. The influence of scaffold elasticity on germ layer specification of human embryonic stem cells. Biomaterials. 2011;32(36):9612-9621. https://doi.org/10.1016/j.biomaterials.2011.09.012
[9] ^ Lovell-Badge R, Anthony E, Barker RA, et al. ISSCR Guidelines for Stem Cell Research and Clinical Translation: the 2021 update. Stem Cell Reports. 2021;16(6):1398-1408. https://doi.org/10.1016/j.stemcr.2021.05.012