Gastrulation
Encyclopedia
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
, mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
, and endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...
.
Gastrulation takes place after cleavage
Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
and the formation of the blastula
Blastula
The blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
and primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
. Gastrulation is followed by organogenesis
Organogenesis
In animal development, organogenesis is the process by which the ectoderm, endoderm, and mesoderm develop into the internal organs of the organism. Internal organs initiate development in humans within the 3rd to 8th weeks in utero...
, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues
Tissue (biology)
Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...
and organs in the developing embryo. The ectoderm gives rise to epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...
, and to the neural crest
Neural crest
Neural crest cells are a transient, multipotent, migratory cell population unique to vertebrates that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia....
and other tissues that will later form the nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
. The mesoderm is found between the ectoderm and the endoderm and gives rise to somites, which form muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
; the cartilage
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
of the ribs and vertebrae; the dermis
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
, the notochord
Notochord
The notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
, blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
and blood vessels, bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...
, and connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...
. The endoderm gives rise to the epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
of the digestive system and respiratory system
Respiratory system
The respiratory system is the anatomical system of an organism that introduces respiratory gases to the interior and performs gas exchange. In humans and other mammals, the anatomical features of the respiratory system include airways, lungs, and the respiratory muscles...
, and organs associated with the digestive system, such as the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
and pancreas
Pancreas
The pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
. Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme
Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is a type of undifferentiated loose connective tissue that is derived mostly from mesoderm, although some are derived from other germ layers; e.g. some mesenchyme is derived from neural crest cells and thus originates from the ectoderm...
.
The molecular mechanism and timing of gastrulation is different in different organisms. However some common features of gastrulation across triploblastic
Triploblasty
Triploblasty is a condition of the blastula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. The germ layers form during gastrulation of the blastula. Additionally, the term may refer to any ovum in which the blastoderm splits into three layers.All higher and...
organisms include: (1) A change in the topological structure of the embryo, from a simply connected
Simply connected space
In topology, a topological space is called simply connected if it is path-connected and every path between two points can be continuously transformed, staying within the space, into any other path while preserving the two endpoints in question .If a space is not simply connected, it is convenient...
surface (sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
-like), to a non-simply connected surface (torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...
-like); (2) the differentiation of cells into one of three types (endodermal, mesodermal, and ectodermal); and (3) the digestive
Digestive
Digestive may refer to:*Digestion, biological process of metabolism*Digestive biscuit, a British semi-sweet biscuit*Digestif, small beverage at the end of a meal...
function of a large number of endodermal cells.
Lewis Wolpert
Lewis Wolpert
Lewis Wolpert CBE FRS FRSL is a developmental biologist, author, and broadcaster.-Career:Wolpert was educated at the University of Witwatersrand , at Imperial College London, and at King's College London...
, 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."
The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel
Ernst Haeckel
The "European War" became known as "The Great War", and it was not until 1920, in the book "The First World War 1914-1918" by Charles à Court Repington, that the term "First World War" was used as the official name for the conflict.-Research:...
, in his 1872 work "Biology of Calcareous Sponges".
Overview
In amniotes, gastrulation occurs in the following sequence: (1) the embryoEmbryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
becomes asymmetric
Asymmetry
Asymmetry is the absence of, or a violation of, symmetry.-In organisms:Due to how cells divide in organisms, asymmetry in organisms is fairly usual in at least one dimension, with biological symmetry also being common in at least one dimension....
; (2) the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
forms; (3) cells from the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...
at the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
undergo an epithelial to mesenchymal transition
Epithelial-mesenchymal transition
Epithelial-mesenchymal transition or transformation is a hypothesized program of development of biological cells characterized by loss of cell adhesion, repression of E-cadherin expression, and increased cell mobility...
and ingress
Ingression (biology)
Ingression is one of the many changes in the location or relative position of cells that takes place during the gastrulation stage of animal development. It produces an animal's mesenchyme cells at the onset of gastrulation...
to at the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
to form the germ layers.
Loss of Symmetry
In preparation for gastrulation, the embryo must become asymmetric along both the proximal-distal axis and the anterior-posterior 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 the placentaPlacenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
, at the proximal end and the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...
at the distal end. Many signaling pathways contribute to this reorganization, including BMP
Bone morphogenetic protein
Bone morphogenetic proteins are a group of growth factors also known as cytokines and as metabologens . Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue...
, FGF
Fibroblast growth factor
Fibroblast growth factors, or FGFs, are a family of growth factors involved in angiogenesis, wound healing, and embryonic development. The FGFs are heparin-binding proteins and interactions with cell-surface associated heparan sulfate proteoglycans have been shown to be essential for FGF signal...
, nodal
NODAL
Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation...
, and Wnt
Wnt signaling pathway
The Wnt signaling pathway is a network of proteins best known for their roles in embryogenesis and cancer, but also involved in normal physiological processes in adult animals.-Discovery:...
. Visceral endoderm surrounds the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...
. The distal visceral endoderm (DVE) migrates to the anterior portion of the embryo, forming the “anterior visceral endoderm” (AVE). This breaks anterior-posterior symmetry and is regulated by nodal
NODAL
Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation...
signaling.
Formation of the Primitive Streak
The primitive streakPrimitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
is formed at the beginning of gastrulation and is found at the junction between the extraembryonic tissue and the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...
on the posterior side of the embryo and the site of ingression
Ingression (biology)
Ingression is one of the many changes in the location or relative position of cells that takes place during the gastrulation stage of animal development. It produces an animal's mesenchyme cells at the onset of gastrulation...
. Formation of the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
is reliant upon nodal
NODAL
Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation...
signaling within the cells contributing to the primitive streak and BMP4 signaling from the extraembryonic tissue. Furthermore, Cer1
Cerberus (protein)
Cerberus also known as CER1 is a protein that in humans is encoded by the CER1 gene.- Function :Cerberus is an inhibitor in the TGF beta signaling pathway secreted during the gastrulation phase of the embryogenesis....
and Lefty1
Lefty (protein)
Lefty are proteins that are closely related members of the TGF-beta family of growth factors. These proteins are secreted and play a role in left-right asymmetry determination of organ systems during development...
restrict the primitive streak to the appropriate location by antagonizing nodal
NODAL
Nodal is a protein that in humans is encoded by the NODAL gene. It is a member of the TGF beta superfamily. Like many other members of this superfamily, it is involved in cell differentiation...
signaling. The region defined as the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
continues to grow towards the distal tip.
During the early stages of development, the primitive streak is the structure that will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. 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. The primitive streak, extends through this midline and creates the antero-posterior body axis, becoming the first symmetry-breaking event in the embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
, and marks the beginning of gastrulation. This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position, where they will differentiate into the three germ layers.
Epithelial to Mesenchymal Transition and Ingression
In order for the cells to move from the epitheliumEpithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
of the epiblast
Epiblast
In amniote animal embryology, the epiblast is a tissue type derived either from the inner cell mass in mammals or the blastodisc in birds and reptiles. It lies above the hypoblast.-Mammals:...
through the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
to form a new layer, the cells must undergo an epithelial to mesenchymal transition
Epithelial-mesenchymal transition
Epithelial-mesenchymal transition or transformation is a hypothesized program of development of biological cells characterized by loss of cell adhesion, repression of E-cadherin expression, and increased cell mobility...
(EMT) to lose their epithelial characteristics, such as cell-cell adhesion
Cell adhesion
Cellular adhesion is the binding of a cell to a surface, extracellular matrix or another cell using cell adhesion molecules such as selectins, integrins, and cadherins. Correct cellular adhesion is essential in maintaining multicellular structure...
. FGF
Fibroblast growth factor
Fibroblast growth factors, or FGFs, are a family of growth factors involved in angiogenesis, wound healing, and embryonic development. The FGFs are heparin-binding proteins and interactions with cell-surface associated heparan sulfate proteoglycans have been shown to be essential for FGF signal...
signaling is necessary for proper EMT. FGFR1 is needed for the up regulation of Snail1, which down regulates E-cadherin
CDH1 (gene)
Cadherin-1 also known as CAM 120/80 or epithelial cadherin or uvomorulin is a protein that in humans is encoded by the CDH1 gene. CDH1 has also been designated as CD324 . It is a tumor suppressor gene.- Function :Cadherin-1 is a classical member of the cadherin superfamily...
, causing a loss of cell adhesion. Following the EMT, the cells ingress
Ingression (biology)
Ingression is one of the many changes in the location or relative position of cells that takes place during the gastrulation stage of animal development. It produces an animal's mesenchyme cells at the onset of gastrulation...
through the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
and spread out to form a new layer of cells or join existing layers. FGF8
FGF8
Fibroblast growth factor 8 is a protein that in humans is encoded by the FGF8 gene.-External links:*...
is implicated in the process of this dispersal from the primitive streak
Primitive streak
The primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
.