Mesenchyme (pointer) stained with H&E
Mesenchyme, in vertebrate embryology, is a type of connective tissue found mostly during the development of the embryo.It is composed mainly of ground substance with few cells or fibers. It can also refer to a group of mucoproteins found in certain types of cysts (etc.), resembling mucus. It is most easily found as a component of Wharton's jelly.
Embryology is the branch of biology that studies the prenatal development of gametes, fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology.
Connective tissue (CT) is one of the four basic types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesoderm. Connective tissue is found in between other tissues everywhere in the body, including the nervous system. In the central nervous system, the three outer membranes that envelop the brain and spinal cord are composed of connective tissue. They support and protect the body. All connective tissue consists of three main components: fibers, ground substance and cells. Not all authorities include blood or lymph as connective tissue because they lack the fiber component. All are immersed in the body water.
Ground substance is an amorphous gel-like substance in the extracellular space that contains all components of the extracellular matrix (ECM) except for fibrous materials such as collagen and elastin. Ground substance is active in the development, movement, and proliferation of tissues, as well as their metabolism. Additionally, cells use it for support, water storage, binding, and a medium for intercellular exchange. Ground substance provides lubrication for collagen fibers.
The vitreous of the eyeball is a similar tissue.
The vitreous body is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates. It is often referred to as the vitreous humor or simply "the vitreous".
In invertebrate zoology, the term refers to free cells loosely arranged in a matrix.
Invertebrate zoology is the subsystem of zoology that consists of the study of invertebrates, animals without a backbone
"Mesenchyme" is a term introduced by Oscar Hertwig in 1881.
Oscar Hertwig was a German zoologist and professor, who also wrote about the theory of evolution circa 1916, over 55 years after Charles Darwin's book The Origin of Species. He was the elder brother of zoologist-professor Richard Hertwig (1850–1937).The Hertwig brothers were the most eminent scholars of Ernst Haeckel from the University of Jena. They were independent of Haeckel's philosophical speculations but took his ideas in a positive way to widen their concepts in zoology. Initially, between 1879–1883, they performed embryological studies, especially on the theory of the coelom (1881), the fluid-filled body cavity. These problems were based on the phylogenetic theorems of Haeckel, i.e. the biogenic theory, and the "gastraea theory".
In order to differentiate the use of the word mesenchyme in invertebrate zoology (an ecto- or entomesodermal middle layer of some invertebrates) and the use in vertebrate embryology (that is, undifferentiated tissue found in embryonic true mesoderm - entomesoderm - from which are derived all connective tissues, blood vessels, blood cells, the lymphatic system, and the heart), some authors prefer to use the term mesoglea (in wider sense) in lieu of mesenchyme when referring to the middle layers of sponges and diploblasts, reserving the term mesenchyme for the embryological sense. However, Brusca & Brusca discourage this usage, using mesoglea in its strict sense (noncellular mesenchyme), and preferring to maintain both the embryological and zoological senses for the term mesenchyme.
Finally, some similar terms used in botany generally are differentiated by the suffix "a": mesenchyma (a tissue between xylem and phloem in roots), collenchyma (primordial leaf tissues) and parenchyma (supportive tissues).
Parenchyma is the bulk of a substance. In animals, a parenchyma comprises the functional parts of an organ and in plants parenchyma is the ground tissue of nonwoody structures.
Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a loose aggregate of reticular fibrils and unspecialized cells.Mesenchymal cells can migrate easily, in contrast to epithelial cells, which lack mobility and are organized into closely adherent sheets, and are polarized in an apical-basal orientation.
The mesenchyme originates from the mesoderm.From the mesoderm, the mesenchyme appears as an embryologically primitive "soup". This "soup" exists as a combination of the mesenchymal cells plus serous fluid plus the many different tissue proteins. Serous fluid is typically stocked with the many serous elements, such as sodium and chloride. The mesenchyme develops into the tissues of the lymphatic and circulatory systems, as well as the musculoskeletal system. This latter system is characterized as connective tissues throughout the body, such as bone, muscle and cartilage. A malignant cancer of mesenchymal cells is a type of sarcoma.
The first emergence of mesenchyme occurs during gastrulation from the epithelial–mesenchymal transition (EMT) process. This transition occurs through the loss of epithelial cadherin, tight junctions, and adherens junctions on the cell membranes of epithelial cells.The surface molecules undergo endocytosis and the microtubule cytoskeleton loses shape, enabling mesenchyme to migrate along the extracellular matrix (ECM). Epithelial–mesenchymal transition occurs in embryonic cells that require migration through or over tissue, and can be followed with a mesenchymal–epithelial transition to produce secondary epithelial tissues. Embryological mesenchymal cells expresses fibroblast-specific protein (Fsp1), which is indicative of their shared properties with the migratory adult fibroblasts, and c-Fos, an oncogene associated with the down-regulation of epithelial cadherin. Both formation of the primitive streak and mesenchymal tissue is dependent on the Wnt/β-catenin pathway. Specific markers of mesenchymal tissue include the additional expression of ECM factors such as fibronectin and vitronectin.
The first cells of the embryo to undergo EMT and form mesenchyme are the extra-embryonic cells of the trophectoderm. These migrate from the body of the blastocyst into the endometrial layer of the uterus in order to contribute to the formation of the anchored placenta.
Primary mesenchyme is the first embryonic mesenchymal tissue to emerge, and it is produced from EMT in epiblast cells. In the epiblast, it is induced by the primitive streak through Wnt signaling, and produces endoderm and mesoderm from a transitory tissue called mesendoderm during the process of gastrulation.
The formation of primary mesenchyme depends on the expression of WNT3. Other deficiencies in the Wnt signaling pathway, such as in Nodal (a TGF-beta protein), will lead to defective mesoderm formation.
The tissue layers formed from the primitive streak invaginate together into the embryo and the induced mesenchymal stem cells will ingress and form the mesoderm. Mesodermal tissue will continue to differentiate and/or migrate throughout the embryo to ultimately form most connective tissue layers of the body.
Embryological mesenchyme is particularly transitory and soon differentiates after migration. Neural mesenchyme forms soon after primary mesenchyme formation.
The interaction with ectoderm and somite-forming morphogenic factors cause some primary mesenchyme to form neural mesenchyme, or paraxial mesoderm, and contribute to somite formation. Neural mesenchyme soon undergoes a mesenchymal–epithelial transition under the influence of WNT6 produced by ectoderm to form somites.These structures will undergo a secondary EMT as the somite tissue migrates later in development to form structural connective tissue such as cartilage and skeletal muscle.
Neural crest cells (NCCs) form from neuroectoderm, instead of the primary mesenchyme, from morphogenic signals of the neural crest. The EMT occurs as a result of Wnt signaling, the influence of Sox genes and the loss of E-cadherin from the cell surface. NCCs additionally require the repression of N-cadherin, and neural cell adhesion molecule. NCCs ingress into the embryo from the epithelial neuroectodermal layer and migrate throughout the body in order form multiple peripheral nervous system (PNS) cells and melanocytes. Migration of NCCs is primarily induced by BMP signaling and its inhibitor, Noggin.
In some invertebrates, e.g., Porifera, Cnidaria, Ctenophora and some triploblasts (the acoelomates), the term "mesenchyme" refers to a more-or-less solid but looselly organized tissue consisting of a gel matrix (the mesoglea) with various cellular and fibrous inclusions, located between epidermis and gastrodermis. In some cases, the mesoglea is noncellular.
When cellular material is sparse or densely packed, the mesenchyme may be sometimes called collenchyme (e.g., cnidarians) or parenchyme (e.g., flatworms), respectively.When no cellular material is present (e.g., in Hydrozoa), the layer is properly called mesoglea.
In some colonial cnidarians, the mesenchyme is perforated by gastrovascular channels continuous among colony members. This entire matrix of common basal material is called coenenchyme.
In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. The other two layers are the ectoderm and endoderm, with the mesoderm as the middle layer between them.
Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.
Ectoderm is one of the three primary germ layers in the very early embryo. The other two layers are the mesoderm and endoderm, with the ectoderm as the most exterior layer. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma, meaning "skin."
Somites are divisions of the body of an animal or embryo. The divisions are also known as metameric segments
Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis.
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation: the ectoderm, endoderm, and mesoderm form the internal organs of the organism.
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. Wnt is an acronym in the field of genetics that stands for 'Wingless/Integrated'.
Neural crest cells are a temporary group of cells unique to chordates of the group Cristozoa that arise from the embryonic ectoderm cell layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.
Catenins are a family of proteins found in complexes with cadherin cell adhesion molecules of animal cells. The first two catenins that were identified became known as α-catenin and β-catenin. A-catenin can bind to β-catenin and can also bind actin. B-catenin binds the cytoplasmic domain of some cadherins. Additional catenins such as γ-catenin and δ-catenin have been identified. The name "catenin" was originally selected because it was suspected that catenins might link cadherins to the cytoskeleton.
The epithelial–mesenchymal transition (EMT) is a process by which epithelial cells lose their cell polarity and cell-cell adhesion, and gain migratory and invasive properties to become mesenchymal stem cells; these are multipotent stromal cells that can differentiate into a variety of cell types. EMT is essential for numerous developmental processes including mesoderm formation and neural tube formation. EMT has also been shown to occur in wound healing, in organ fibrosis and in the initiation of metastasis in cancer progression.
Mesoglea refers to the tissue found in jellyfish that functions as a hydro-static skeleton. Mesohyl generally refers to tissue found in sponges.
Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue running along both sides of the neural tube, which form muscle and the tissues of the back, including connective tissue and the dermis.
Eye formation in the human embryo begins at approximately three weeks into embryonic development and continues through the tenth week. Cells from both the mesodermal and the ectodermal tissues contribute to the formation of the eye. Specifically, the eye is derived from the neuroepithelium, surface ectoderm, and the extracellular mesenchyme which consists of both the neural crest and mesoderm.
The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development The upper limb bud appears in the third week of human development and the lower limb bud appears four days later.
Zinc finger protein SNAI1 is a protein that in humans is encoded by the SNAI1 gene. Snail is a family of transcription factors that promote the repression of the adhesion molecule E-cadherin to regulate epithelial to mesenchymal transition (EMT) during embryonic development.
Cadherin-1 also known as CAM 120/80 or epithelial cadherin (E-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.
A mesenchymal–epithelial transition (MET) is a reversible biological process that involves the transition from motile, multipolar or spindle-shaped mesenchymal cells to planar arrays of polarized cells called epithelia. MET is the reverse process of epithelial–mesenchymal transition (EMT). Unlike epithelial cells – which are stationary and characterized by an apical-basal polarity, tight junctions, and expression of cell-cell adhesion markers such as E-cadherin, mesenchymal cells do not make mature cell-cell contacts, can invade through the extracellular matrix, and express markers such as vimentin, fibronectin, N-cadherin, Twist, and Snail. METs occur in normal development, cancer metastasis, and induced pluripotent stem cell reprogramming.
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-mesenchymal transition (EMT), the primary mesenchyme cells (PMCs) detach from the epithelium and become internalized mesenchyme cells that can migrate freely. Each animal system utilizes an EMT to produce mesenchyme cells.
Neural crest cells are a group of temporary, multipotent cells that are pinched off during the formation of the neural tube and therefore are found at the dorsal (top) region of the neural tube during development. They are derived from the ectoderm germ layer, but are sometimes called the fourth germ layer because they are so important and give rise to so many other types of cells. They migrate throughout the body and create a large number of differentiated cells such as neurons, glial cells, pigment-containing cells in skin, skeletal tissue cells in the head, and many more.
The clock and wavefront model is a model used to describe the process of somitogenesis in vertebrates. Somitogenesis is the process by which somites, blocks of mesoderm that give rise to a variety of connective tissues, are formed.
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