Mesenchyme () is a type of loosely organized animal
embryonic connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
of
undifferentiated cells that give rise to most tissues, such as
skin
Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation.
Other animal coverings, such as the arthropod exoskeleton, have different ...
,
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...
or
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
.
The interactions between mesenchyme and
epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
help to form nearly every organ in the developing embryo.
Vertebrates
Structure
Mesenchyme is characterized morphologically by a prominent
ground substance matrix
Matrix most commonly refers to:
* ''The Matrix'' (franchise), an American media franchise
** '' The Matrix'', a 1999 science-fiction action film
** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
containing a loose aggregate of
reticular fibers and unspecialized
mesenchymal stem cells. Mesenchymal cells can
migrate easily (in contrast to
epithelial cells, which lack mobility), are organized into closely adherent sheets, and are
polarized 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 and
circulatory
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, t ...
systems, as well as the musculoskeletal system. This latter system is characterized as
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
s throughout the body, such as
bone
A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, ...
, and
cartilage. A malignant
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
of mesenchymal cells is a type of
sarcoma
A sarcoma is a malignant tumor, a type of cancer that arises from transformed cells of mesenchymal ( connective tissue) origin. Connective tissue is a broad term that includes bone, cartilage, fat, vascular, or hematopoietic tissues, and sar ...
.
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,
tight junctions, and
adherens junctions on the
cell membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
s of
epithelial cells
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...
.
The surface molecules undergo
endocytosis and the
microtubule cytoskeleton loses shape, enabling mesenchyme to migrate along the
extracellular matrix
In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
(ECM). Epithelial–mesenchymal transition occurs in embryonic cells that require migration through or over tissue, and can be followed with a
mesenchymal–epithelial transition
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 proces ...
to produce secondary
epithelial tissues.
Embryological mesenchymal cells express
Protein S100-A4
Protein S100-A4 (S100A4) is a protein that in humans is encoded by the ''S100A4'' gene.
Function
The protein encoded by this gene is a member of the S100 family of proteins containing 2 EF-hand calcium-binding motifs. S100 proteins are localiz ...
(''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
Fibronectin is a high- molecular weight (~500-~600 kDa) glycoprotein of the extracellular matrix that binds to membrane-spanning receptor proteins called integrins. Fibronectin also binds to other extracellular matrix proteins such as coll ...
and
vitronectin.
Implantation
The first cells of the embryo to undergo EMT and form mesenchyme are the extra-embryonic cells of the
trophectoderm
The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the ...
. These migrate from the body of the
blastocyst
The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the ''embryoblast'' which subsequently forms the embryo, and an outer layer of trophoblast cells called th ...
into the
endometrial layer of the
uterus
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The ...
in order to contribute to the formation of the anchored
placenta
The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...
.
Primary mesenchyme
Primary mesenchyme is the first embryonic mesenchymal tissue to emerge, and it is produced from EMT in
epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is ...
cells. In the
epiblast
In amniote embryonic development, the epiblast (also known as the primitive ectoderm) is one of two distinct cell layers arising from the inner cell mass in the mammalian blastocyst, or from the blastula in reptiles and birds, the other layer is ...
, 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
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
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
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 proces ...
under the influence of
WNT6 produced by
ectoderm to form
somites
The somites (outdated term: primitive segments) are a set of bilaterally paired blocks of paraxial mesoderm that form in the embryonic stage of somitogenesis, along the head-to-tail axis in segmented animals. In vertebrates, somites subdivide in ...
.
These structures will undergo a secondary EMT as the
somite tissue migrates later in development to form structural
connective tissue
Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
such as
cartilage and
skeletal muscle.
Neural crest cells
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, Per ...
(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
Cadherin-1 or Epithelial cadherin (E-cadherin), (not to be confused with the APC/C activator protein CDH1) is a protein that in humans is encoded by the ''CDH1'' gene. Mutations are correlated with gastric, breast, colorectal, thyroid, and ovari ...
from the cell surface. NCCs additionally require the repression of
N-cadherin
Cadherin-2 also known as Neural cadherin (N-cadherin), is a protein that in humans is encoded by the ''CDH2'' gene. CDH2 has also been designated as CD325 (cluster of differentiation 325).
Cadherin-2 is a transmembrane protein expressed in multip ...
, 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
The peripheral nervous system (PNS) is one of two components that make up the nervous system of bilateral animals, with the other part being the central nervous system (CNS). The PNS consists of nerves and ganglia, which lie outside the brai ...
(PNS) cells and
melanocytes. Migration of NCCs is primarily induced by
BMP signaling and its inhibitor,
Noggin.
Invertebrates
In some
invertebrate
Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s, e.g.,
Porifera,
Cnidaria
Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter.
Their distinguishing feature is cnidocytes, specialized cells that ...
,
Ctenophora and some
triploblasts (the
acoelomates), mesenchyme refers to a more-or-less solid but loosely organized tissue consisting 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
sponge
Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...
s, 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.
* In triploblastic acoelomates (such as
flatworms), 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
Endoderm is the innermost of the three primary germ layers in the very early embryo. The other two layers are the ectoderm (outside layer) and mesoderm (middle layer). Cells migrating inward along the archenteron form the inner layer of the gastru ...
).
When cellular material is sparse or densely packed, as in cnidarians, the mesenchyme may sometimes be called
collenchyma, or parenchyma in flatworms.
When no cellular material is present as 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.
References
*
{{Authority control
Connective tissue