| Mesenchyme | |
|---|---|
 Transmissionelectron micrograph of mesenchyme displaying theultrastructure of a typicalcell andmatrix. | |
 Mesenchyme (pointer) stained withH&E | |
| Details | |
| Carnegie stage | 6b |
| Precursor | Lateral mesoderm |
| Identifiers | |
| TE | E5.16.4.0.3.0.18 |
| Anatomical terminology | |
Mesenchyme (/ˈmɛsənkaɪmˈmiːzən-/[1]) is a type of loosely organized animalembryonicconnective tissue ofundifferentiated cells that give rise to most tissues, such asskin,blood, orbone.[2][3] The interactions between mesenchyme andepithelium help to form nearly everyorgan in the developingembryo.[4]
Mesenchyme is characterized morphologically by a prominentground substancematrix containing a loose aggregate ofreticular fibers and unspecializedmesenchymal stem cells.[5] Mesenchymal cells canmigrate easily (in contrast toepithelial cells, which lack mobility, are organized into closely adherent sheets, and arepolarized in anapical-basal orientation).[citation needed]
The mesenchyme originates from themesoderm.[6] 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 thelymphatic andcirculatory systems, as well as the musculoskeletal system. This latter system is characterized asconnective tissues throughout the body, such asbone, andcartilage. A malignantcancer of mesenchymal cells is a type ofsarcoma.[7][8]
The first emergence of mesenchyme occurs duringgastrulation from theepithelial–mesenchymal transition (EMT) process. This transition occurs through the loss ofepithelial cadherin,tight junctions, andadherens junctions on thecell membranes ofepithelial cells.[9] The surface molecules undergoendocytosis and themicrotubule cytoskeleton loses shape, enabling mesenchyme to migrate along theextracellular matrix (ECM). Epithelial–mesenchymal transition occurs in embryonic cells that require migration through or over tissue, and can be followed with amesenchymal–epithelial transition to produce secondaryepithelial tissues.Embryological mesenchymal cells expressProtein S100-A4 (S100A4)[10] also known asfibroblast-specific protein,[11] which is indicative of their shared properties with the migratory adultfibroblasts, andc-Fos, anoncogene associated with the down-regulation of epithelial cadherin.[12][13] Both formation of theprimitive streak and mesenchymal tissue is dependent on theWnt/β-catenin pathway.[14] Specific markers of mesenchymal tissue include the additional expression of ECM factors such asfibronectin andvitronectin.[15]
The first cells of the embryo to undergo EMT and form mesenchyme are the extra-embryonic cells of thetrophectoderm. These migrate from the body of theblastocyst into theendometrial layer of theuterus in order to contribute to the formation of the anchoredplacenta.[16]
Primary mesenchyme is the first embryonic mesenchymal tissue to emerge, and it is produced from EMT inepiblast cells. In theepiblast, it is induced by theprimitive streak throughWnt signaling, and producesendoderm andmesoderm from a transitory tissue calledmesendoderm during the process ofgastrulation.[17]
The formation of primary mesenchyme depends on the expression ofWNT3. Other deficiencies in signaling pathways, such as inNodal (a TGF-beta protein), will lead to defectivemesoderm formation.[9]
The tissue layers formed from the primitive streak invaginate together into the embryo and the inducedmesenchymal stem cells will ingress and form themesoderm. Mesodermal tissue will continue to differentiate and/or migrate throughout the embryo to ultimately form mostconnective tissue layers of the body.[18]
Embryological mesenchyme is particularly transitory and soon differentiates after migration. Neural mesenchyme forms soon after primary mesenchyme formation.[19]
The interaction withectoderm and somite-forming morphogenic factors cause some primary mesenchyme to form neural mesenchyme, orparaxial mesoderm, and contribute tosomite formation. Neural mesenchyme soon undergoes amesenchymal–epithelial transition under the influence ofWNT6 produced byectoderm to formsomites.[20] These structures will undergo a secondary EMT as thesomite tissue migrates later in development to form structuralconnective tissue such ascartilage andskeletal muscle.[21]
Neural crest cells (NCCs) form fromneuroectoderm, instead of the primary mesenchyme, from morphogenic signals of theneural crest. The EMT occurs as a result ofWnt signaling, the influence ofSox genes and the loss ofE-cadherin from the cell surface. NCCs additionally require the repression ofN-cadherin, andneural cell adhesion molecule. NCCs ingress into the embryo from the epithelial neuroectodermal layer and migrate throughout the body in order form multipleperipheral nervous system (PNS) cells andmelanocytes. Migration of NCCs is primarily induced byBMP signaling and its inhibitor,Noggin.[22][23]
In someinvertebrates, such asPorifera,Cnidaria,Ctenophora, and sometriploblasts (namely theacoelomates), the term "mesenchyme" refers to a more-or-less solid but loosely organized tissue that consists of a gel matrix (themesoglea) with various cellular and fibrous inclusions, located between theepidermis and thegastrodermis (non-triploblast animals usually are considered to lack "connective" tissue). In some cases, the mesoglea is noncellular.[24]
When cellular material is sparse or densely packed, as in cnidarians, the mesenchyme may sometimes be calledcollenchyma, or parenchyma in flatworms.[25] When no cellular material is present as inHydrozoa, the layer is properly calledmesoglea.[25]
In somecolonial cnidarians, the mesenchyme is perforated by gastrovascular channels continuous among colony members. This entire matrix of common basal material is calledcoenenchyme.[25]
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