Mesenchyme (
Vertebrates
Structure
Mesenchyme is characterized morphologically by a prominent
ground substance matrix containing a loose aggregate of
and unspecialized mesenchymal stem cells.
Mesenchymal cells can migrate easily (in contrast to
, which lack mobility, are organized into closely adherent sheets, and are
cell polarity in an apical-basal orientation).
Development
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 system and circulatory systems, as well as the musculoskeletal system. This latter system is characterized as connective tissues throughout the body, such as bone, and
cartilage. A malignant
cancer of mesenchymal cells is a type of
sarcoma.
Epithelial to mesenchymal transition
The first emergence of mesenchyme occurs during
gastrulation from the epithelial–mesenchymal transition (EMT) process. This transition occurs through the loss of epithelial cadherin,
, and adherens junctions on the
of
epithelium.
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
epithelium.
Embryological mesenchymal cells express Protein S100-A4 (
S100A4)
also known as
fibroblast-specific protein,
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.
Implantation
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
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 signaling pathways, 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.
Neural mesenchyme
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.
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.
Invertebrates
In some
, such as
Porifera,
Cnidaria,
Ctenophora, and some
(namely the
acoelomates), the term "mesenchyme" refers to a more-or-less solid but loosely organized tissue that consists of a gel matrix (the
mesoglea) with various cellular and fibrous inclusions, located between the
epidermis and the
gastrodermis (non-triploblast animals usually are considered to lack "connective" tissue). In some cases, the mesoglea is noncellular.
-
In , the mesenchyme is called mesohyl.
-
In diploblasts (Cnidaria and Ctenophora), the mesenchyme is fully ectodermally derived. This kind of mesenchyme is called ectomesodermal, and is not considered true mesoderm.
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In triploblastic acoelomates (such as ), the term parenchyma is sometimes used for the middle (mesenchymal) layer, in which the dense layer includes tissues derived from both ectoderm, and entomesoderm (true mesoderm, derived from entoderm).
When cellular material is sparse or densely packed, as in cnidarians, the mesenchyme may sometimes be called collenchyma, or parenchyma in flatworms.
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.
