Ingression (biology)
Encyclopedia
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. During the epithelial-to-mesenchymal transition (EMT), the primary mesenchyme cells (PMCs) detach from the epithelium
and become internalized mesenchyme cells that can migrate freely (Figure 1). Each animal system utilizes an EMT to produce mesenchyme cells.
While the mechanisms of ingression are not fully understood, studies using the sea urchin as a model organism have begun to shed light on this developmental process, and will be the focus here. There are three main changes that must occur within a cell to enable the process of ingression. The ingressing PMCs must first alter their affinity for the neighboring epithelial cells that will remain in the vegetal plate (vertebrate PMCs ingress from the primitive streak
). During this time, these cells must lose their affinity for the hyaline layer to which their apical surface is attached. The ingressing cells will then apically constrict and alter their cellular architecture through a dramatic reorganization of their cytoskeleton
. Lastly, these cells will modify their mode of motility and presumably gain affinity for the basal lamina which composes the lining of the blastocoel, the future migration substrate of the PMCs,.
Changes in the adhesion properties of these cells are the best characterized and understood mechanism of ingression. In sea urchins, epithelial cells adhere to one another as well as the hyaline layer through classic adherens and cadherin
junctions. Ingression is a very dynamic process however, and the first sign of an ingressing cell is seen when a future PMC loses its adhesion to hyaline, loses adhesion and cadherin, and increase their adhesion to a basal laminal substrate. These processes occur rapidly, over approximately 30 minutes. It is not understood how the PMCs penetrate the basal lamina. The basal lamina is a loose matrix, therefore it is possible that the ingressing cells squeeze through the matrix. It is also hypothesized that the PMCs use a protease
. EMT is determined by a dynamic gene regulatory network (GRN). snail and twist are two key transcription factors that makes up the GRN. Within an hour of ingression, numerous transcript factors are activated. It is known that beta-catenin
(β-catenin) plays a key role in EMT. When β-catenin function is blocked, no EMT results. If β-catenin is over-expressed, too many cells undergo EMT. The vascular endothelial growth factor receptor (VEGFR) is also necessary for the PMCs to function as mesenchymal cells. Lastly, it is thought that the ingression of PMCs is further facilitated simply through the simultaneous ingression neighboring cells.
Within birds and mammals, epiblast
cells converge at the midline and ingress at the primitive streak. Ingression of these cells results in formation of the mesoderm
. The use of ingression to internalize presumptive mesoderm is considered a major evolutionary change in mesoderm morphogenesis within chordates. Within chordate embryos, there is an evolutionary trend exhibited in the mechanisms use to internalize presumptive mesoderm. Basal chordates rely predominantly on invagination, anamniote vertebrates and reptiles on a varying combination of involution and ingression, and birds and mammals primarily on ingression.
Besides ingression, two other types of internalizing cell movements may occur during gastrulation: invagination
and involution
.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
that takes place during the gastrulation
Gastrulation
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
stage of animal development. It produces an animal's 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...
cells at the onset of gastrulation. During the epithelial-to-mesenchymal transition (EMT), the primary mesenchyme cells (PMCs) detach from 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...
and become internalized mesenchyme cells that can migrate freely (Figure 1). Each animal system utilizes an EMT to produce mesenchyme cells.
While the mechanisms of ingression are not fully understood, studies using the sea urchin as a model organism have begun to shed light on this developmental process, and will be the focus here. There are three main changes that must occur within a cell to enable the process of ingression. The ingressing PMCs must first alter their affinity for the neighboring epithelial cells that will remain in the vegetal plate (vertebrate PMCs ingress 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:...
). During this time, these cells must lose their affinity for the hyaline layer to which their apical surface is attached. The ingressing cells will then apically constrict and alter their cellular architecture through a dramatic reorganization of their cytoskeleton
Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within a cell's cytoplasm and is made out of protein. The cytoskeleton is present in all cells; it was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton...
. Lastly, these cells will modify their mode of motility and presumably gain affinity for the basal lamina which composes the lining of the blastocoel, the future migration substrate of the PMCs,.
Changes in the adhesion properties of these cells are the best characterized and understood mechanism of ingression. In sea urchins, epithelial cells adhere to one another as well as the hyaline layer through classic adherens and cadherin
Cadherin
Cadherins are a class of type-1 transmembrane proteins. They play important roles in cell adhesion, ensuring that cells within tissues are bound together. They are dependent on calcium ions to function, hence their name.The cadherin superfamily includes cadherins, protocadherins, desmogleins, and...
junctions. Ingression is a very dynamic process however, and the first sign of an ingressing cell is seen when a future PMC loses its adhesion to hyaline, loses adhesion and cadherin, and increase their adhesion to a basal laminal substrate. These processes occur rapidly, over approximately 30 minutes. It is not understood how the PMCs penetrate the basal lamina. The basal lamina is a loose matrix, therefore it is possible that the ingressing cells squeeze through the matrix. It is also hypothesized that the PMCs use a protease
Protease
A protease is any enzyme that conducts proteolysis, that is, begins protein catabolism by hydrolysis of the peptide bonds that link amino acids together in the polypeptide chain forming the protein....
. EMT is determined by a dynamic gene regulatory network (GRN). snail and twist are two key transcription factors that makes up the GRN. Within an hour of ingression, numerous transcript factors are activated. It is known that beta-catenin
Beta-catenin
Beta-catenin is a protein that in humans is encoded by the CTNNB1 gene. In Drosophila, the homologous protein is called armadillo...
(β-catenin) plays a key role in EMT. When β-catenin function is blocked, no EMT results. If β-catenin is over-expressed, too many cells undergo EMT. The vascular endothelial growth factor receptor (VEGFR) is also necessary for the PMCs to function as mesenchymal cells. Lastly, it is thought that the ingression of PMCs is further facilitated simply through the simultaneous ingression neighboring cells.
Within birds and mammals, 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:...
cells converge at the midline and ingress at the primitive streak. Ingression of these cells results in formation of the 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...
. The use of ingression to internalize presumptive mesoderm is considered a major evolutionary change in mesoderm morphogenesis within chordates. Within chordate embryos, there is an evolutionary trend exhibited in the mechanisms use to internalize presumptive mesoderm. Basal chordates rely predominantly on invagination, anamniote vertebrates and reptiles on a varying combination of involution and ingression, and birds and mammals primarily on ingression.
Besides ingression, two other types of internalizing cell movements may occur during gastrulation: invagination
Invagination
Invagination means to fold inward or to sheath. In biology, this can refer to a number of processes.* Invagination is the morphogenetic processes by which an embryo takes form, and is the initial step of gastrulation, the massive reorganization of the embryo from a simple spherical ball of cells,...
and involution
Involution
In mathematics, an involution, or an involutary function, is a function f that is its own inverse:-General properties:Any involution is a bijection.The identity map is a trivial example of an involution...
.