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My embryology textbook states that mesoderm can exist as Mesenchyme or Epithelium. I'm not sure what 'epithelium' means. Here is a quote for context:
Somatic, splanchnic, and somite mesoderm can be temporarily an epithelium. The temporary epithelium transforms to a secondary mesenchyme
What does this quote mean?
Mesenchyme is the meshwork of embryonic connective tissue in the mesoderm; from it are formed the connective tissues of the body as well as blood vessels and lymph vessels. So it's a part of mesoderm
Lecture - Mesoderm Development
Having now reached week 3 in development we will now begin to look separately at the 3 transient germ layers (ectoderm, mesoderm and endoderm) formed by the process of gastrulation. Beginning with the mesoderm layer, the middle embryonic connective tissue (mesenchyme) layer. Transient in terms of temporary structures that will become something else later in development.
Mesoderm initially forms a multilayered cellular layer separating ectoderm and endoderm, mesoderm also lies outside the embryo as extra-embryonic mesoderm (covered in placenta lecture). Embryonic mesoderm will form most of the adult connective tissues and muscle.
Towards the end of week 3 this layer begins to "partition" into different transient components based upon their location within the layer and the signals the cells are receiving. This partitioning process can be either in terms of cell differentiation or structural. This lecture will describe these initial regions and the tissues they will eventually form. Note that later lectures (muscle, skeleton, limb, integumentary and heart) will revisit these tissues later in development.
While the terms mesenchymal stem cell (MSC) and marrow stromal cell have been used interchangeably for many years, neither term is sufficiently descriptive:
- is embryonicconnective tissue that is derived from the mesoderm and that differentiates into hematopoietic and connective tissue, whereas MSCs do not differentiate into hematopoietic cells.  are connective tissue cells that form the supportive structure in which the functional cells of the tissue reside. While this is an accurate description for one function of MSCs, the term fails to convey the relatively recently discovered roles of MSCs in the repair of tissue. 
- The term encompasses multipotent cells derived from other non-marrow tissues, such as placenta, umbilical cord blood, adipose tissue, adult muscle, corneal stroma or the dental pulp of deciduous (baby) teeth.  The cells do not have the capacity to reconstitute an entire organ.
Mesenchymal stem cells are characterized morphologically by a small cell body with a few cell processes that are long and thin. The cell body contains a large, round nucleus with a prominent nucleolus, which is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance. The remainder of the cell body contains a small amount of Golgi apparatus, rough endoplasmic reticulum, mitochondria and polyribosomes. The cells, which are long and thin, are widely dispersed and the adjacent extracellular matrix is populated by a few reticular fibrils but is devoid of the other types of collagen fibrils.   These distinctive morphological features of mesenchymal stem cells can be visualized label-free using live cell imaging.
Bone marrow Edit
Bone marrow was the original source of MSCs,  and still is the most frequently utilized. These bone marrow stem cells do not contribute to the formation of blood cells and so do not express the hematopoietic stem cell marker CD34. They are sometimes referred to as bone marrow stromal stem cells. 
Cord cells Edit
The youngest and most primitive MSCs may be obtained from umbilical cord tissue, namely Wharton's jelly and the umbilical cord blood. However MSCs are found in much higher concentration in the Wharton's jelly compared to cord blood, which is a rich source of hematopoietic stem cells. The umbilical cord is available after a birth. It is normally discarded and poses no risk for collection. These MSCs may prove to be a useful source of MSCs for clinical applications due to their primitive properties and fast growth rate. 
and these have several advantages over bone marrow-derived MSCs. Adipose tissue-derived MSCs (AdMSCs), in addition to being easier and safer to isolate than bone marrow-derived MSCs, can be obtained in larger quantities.  
Molar cells Edit
The developing tooth bud of the mandibular third molar is a rich source of MSCs. While they are described as multipotent, it is possible that they are pluripotent. They eventually form enamel, dentin, blood vessels, dental pulp and nervous tissues. These stem cells are capable of differentiating into chondrocytes, cardiomyocytes, melanocytes, and hepatocyte‐like cells in vitro. 
Amniotic fluid Edit
Stem cells are present in amniotic fluid. As many as 1 in 100 cells collected during amniocentesis are pluripotent mesenchymal stem cells. 
Differentiation capacity Edit
MSCs have a great capacity for self-renewal while maintaining their multipotency. Recent work suggests that β-catenin, via regulation of EZH2, is a central molecule in maintaining the "stemness" of MSC's.  The standard test to confirm multipotency is differentiation of the cells into osteoblasts, adipocytes and chondrocytes as well as myocytes.
MSCs have been seen to even differentiate into neuron-like cells,  but doubt remains about whether the MSC-derived neurons are functional.  The degree to which the culture will differentiate varies among individuals and how differentiation is induced, e.g., chemical vs. mechanical  and it is not clear whether this variation is due to a different amount of "true" progenitor cells in the culture or variable differentiation capacities of individuals' progenitors. The capacity of cells to proliferate and differentiate is known to decrease with the age of the donor, as well as the time in culture. Likewise, whether this is due to a decrease in the number of MSCs or a change to the existing MSCs is not known. [ citation needed ]
Immunomodulatory effects Edit
MSCs have an effect on innate and specific immune cells. MSCs produce many immunomodulatory molecules including prostaglandin E2 (PGE2),  nitric oxide,  indoleamine 2,3-dioxygenase (IDO), interleukin 6 (IL-6), and other surface markers such as FasL,  PD-L1 and PD-L2. 
MSCs have an effect on macrophages, neutrophils, NK cells, mast cells and dendritic cells in innate immunity. MSCs are able to migrate to the site of injury, where they polarize through PGE2 macrophages in M2 phenotype which is characterized by an anti-inflammatory effect.  Further, PGE2 inhibits the ability of mast cells to degranulate and produce TNF-α.   Proliferation and cytotoxic activity of NK cells is inhibited by PGE2 and IDO. MSCs also reduce the expression of NK cell receptors - NKG2D, NKp44 and NKp30.  MSCs inhibit respiratory flare and apoptosis of neutrophils by production of cytokines IL-6 and IL-8.  Differentiation and expression of dendritic cell surface markers is inhibited by IL-6 and PGE2 of MSCs.  The immunosuppressive effects of MSC also depend on IL-10, but it is not certain whether they produce it alone, or only stimulate other cells to produce it. 
MSC expresses the adhesion molecules VCAM-1 and ICAM-1, which allow T-lymphocytes to adhere to their surface. Then MSC can affect them by molecules which have a short half-life and their effect is in the immediate vicinity of the cell.  These include nitric oxide,  PGE2, HGF,  and activation of receptor PD-1.  MSCs reduce T cell proliferation between G0 and G1 cell cycle phases  and decrease the expression of IFNγ of Th1 cells while increasing the expression of IL-4 of Th2 cells.  MSCs also inhibit the proliferation of B-lymphocytes between G0 and G1 cell cycle phases.  
Antimicrobial properties Edit
MSCs produce several antimicrobial peptides (AMPs) including human cathelicidin LL-37,  β-defensins,  lipocalin 2  and hepcidin.  These peptides, together with the enzyme indoleamine 2,3-dioxygenase (IDO), are responsible for the broad-spectrum antibacterial activity of MSCs. 
Mesenchymal stem cells can be activated and mobilized if needed but their efficiency, in the case of muscle repair for example, is currently quite low. Further studies into the mechanisms of MSC action may provide avenues for increasing their capacity for tissue repair.  
Autoimmune disease Edit
Clinical studies investigating the efficacy of mesenchymal stem cells in treating diseases are in preliminary development, particularly for understanding autoimmune diseases, graft versus host disease, Crohn's disease, multiple sclerosis, systemic lupus erythematosus and systemic sclerosis.   As of 2014, no high-quality clinical research provides evidence of efficacy, and numerous inconsistencies and problems exist in the research methods. 
Other diseases Edit
Many of the early clinical successes using intravenous transplantation came in systemic diseases such as graft versus host disease and sepsis. Direct injection or placement of cells into a site in need of repair may be the preferred method of treatment, as vascular delivery suffers from a "pulmonary first pass effect" where intravenous injected cells are sequestered in the lungs. 
The International Society for Cellular Therapy (ISCT) has proposed a set of standards to define MSCs. A cell can be classified as an MSC if it shows plastic adherent properties under normal culture conditions and has a fibroblast-like morphology. In fact, some argue that MSCs and fibroblasts are functionally identical.  Furthermore, MSCs can undergo osteogenic, adipogenic and chondrogenic differentiation ex vivo. The cultured MSCs also express on their surface CD73, CD90 and CD105, while lacking the expression of CD11b, CD14, CD19, CD34, CD45, CD79a and HLA-DR surface markers. 
The majority of modern culture techniques still take a colony-forming unit-fibroblasts (CFU-F) approach, where raw unpurified bone marrow or ficoll-purified bone marrow mononuclear cells are plated directly into cell culture plates or flasks. Mesenchymal stem cells, but not red blood cells or haematopoetic progenitors, are adherent to tissue culture plastic within 24 to 48 hours. However, at least one publication has identified a population of non-adherent MSCs that are not obtained by the direct-plating technique. 
Other flow cytometry-based methods allow the sorting of bone marrow cells for specific surface markers, such as STRO-1.  STRO-1+ cells are generally more homogenous and have higher rates of adherence and higher rates of proliferation, but the exact differences between STRO-1+ cells and MSCs are not clear. 
Methods of immunodepletion using such techniques as MACS have also been used in the negative selection of MSCs. 
The supplementation of basal media with fetal bovine serum or human platelet lysate is common in MSC culture. Prior to the use of platelet lysates for MSC culture, the pathogen inactivation process is recommended to prevent pathogen transmission. 
New research titled Transplantation of human ESC-derived mesenchymal stem cell spheroids ameliorates spontaneous osteoarthritis in rhesus macaques  Various chemicals and methods including low level laser irradiation have been used to increase proliferation of stem cell. 
In 1924, Russian-born morphologist Alexander A. Maximov (Russian: Александр Александрович Максимов ) used extensive histological findings to identify a singular type of precursor cell within mesenchyme that develops into different types of blood cells. 
Scientists Ernest A. McCulloch and James E. Till first revealed the clonal nature of marrow cells in the 1960s.   An ex vivo assay for examining the clonogenic potential of multipotent marrow cells was later reported in the 1970s by Friedenstein and colleagues.   In this assay system, stromal cells were referred to as colony-forming unit-fibroblasts (CFU-f).
The first clinical trials of MSCs were completed in 1995 when a group of 15 patients were injected with cultured MSCs to test the safety of the treatment. Since then, more than 200 clinical trials have been started. However, most are still in the safety stage of testing. 
Subsequent experimentation revealed the plasticity of marrow cells and how their fate is determined by environmental cues. Culturing marrow stromal cells in the presence of osteogenic stimuli such as ascorbic acid, inorganic phosphate and dexamethasone could promote their differentiation into osteoblasts. In contrast, the addition of transforming growth factor-beta (TGF-b) could induce chondrogenic markers. [ citation needed ]
More recently, there has been some debate over the use of the term "mesenchymal stem cells" and what constitutes the most scientifically correct meaning for the MSC acronym. Most mesenchymal cell or "MSC" preps only contain a minority fraction of true multipotent stem cells, while most cells are instead stromal in nature. One of the pioneers in the MSC field, Dr. Arnold Caplan, has proposed re-naming MSCs to mean "medicinal signaling cells."  Within the stem cell field MSC has most commonly now come to refer to "mesenchymal stromal/stem cells" because of the heterogeneous nature of the cellular preparations.
There is also growing concern about the marketing and injection of MSCs and mesenchymal stem cells into patients by for-profit clinics that lack rigorous data to back up these clinical uses.  
Some Recent Findings
- BMP and FGF signaling interact to pattern mesoderm by controlling basic helix-loop-helix transcription factor activityΐ] "The mesodermal germ layer is patterned into mediolateral subtypes by signaling factors including BMP and FGF. How these pathways are integrated to induce specific mediolateral cell fates is not well understood. We used mesoderm derived from post-gastrulation neuromesodermal progenitors (NMPs), which undergo a binary mediolateral patterning decision, as a simplified model to understand how FGF acts together with BMP to impart mediolateral fate. Using zebrafish and mouse NMPs, we identify an evolutionarily conserved mechanism of BMP and FGF mediated mediolateral mesodermal patterning that occurs through modulation of basic helix-loop-helix (bHLH) transcription factor activity. BMP imparts lateral fate through induction of Id helix loop helix (HLH) proteins, which antagonize bHLH transcription factors, induced by FGF signaling, that specify medial fate. We extend our analysis of zebrafish development to show that bHLH activity is responsible for the mediolateral patterning of the entire mesodermal germ layer."
- BRACHYURY directs histone acetylation to target loci during mesoderm developmentΑ] "T-box transcription factors play essential roles in multiple aspects of vertebrate development. Here, we show that cooperative function of BRACHYURY (T) with histone-modifying enzymes is essential for mouse embryogenesis. A single point mutation (TY88A) results in decreased histone 3 lysine 27 acetylation (H3K27ac) at T target sites, including the T locus, suggesting that T autoregulates the maintenance of its expression and functions by recruiting permissive chromatin modifications to putative enhancers during mesoderm specification. Our data indicate that T mediates H3K27ac recruitment through a physical interaction with p300. In addition, we determine that T plays a prominent role in the specification of hematopoietic and endothelial cell types. Hematopoietic and endothelial gene expression programs are disrupted in TY88A mutant embryos, leading to a defect in the differentiation of hematopoietic progenitors. We show that this role of T is mediated, at least in part, through activation of a distal Lmo2 enhancer." blood
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The mesoglea is mostly water. Other than water, the mesoglea is composed of several substances including fibrous proteins, like collagen and heparan sulphate proteoglycans.  The mesoglea is mostly acellular,  but in both cnidaria  and ctenophora  the mesoglea contains muscle bundles and nerve fibres. Other nerve and muscle cells lie just under the epithelial layers.  The mesoglea also contains wandering amoebocytes that play a role in phagocytosing debris and bacteria. These cells also fight infections by producing antibacterial chemicals. 
The mesoglea may be thinner than either of the cell layers  in smaller coelenterates like a hydra or may make up the bulk of the body in larger jellyfish. The mesoglea serves as an internal skeleton, supporting the body. Its elastic properties help restore the shape after it is deformed by the contraction of muscles.  However, without the buoyancy of water to support it, the mesoglea is not stiff enough to bear the weight of the body and coelenterates generally tend to flatten out, or even collapse when they are taken out of water.
In order to differentiate the use of the word mesenchyme in vertebrate embryology (that is, undifferentiated tissue found in embryonic true [ento-]mesoderm from which are derived all connective tissues, blood vessels, blood cells, the lymphatic system, and the heart) and the use in invertebrate zoology (a more-or-less solid but loosely organized tissue consisting of a gel matrix [the mesoglea, in strict sense] with various cellular and fibrous inclusions, located between epidermis and gastrodermis), 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 (2003) discourage this usage, using mesoglea in its strict sense, and preferring to maintain both the embryological and zoological senses for the term mesenchyme. 
- ^ Sarras, M. P. Madden, M. E. Zhang, X. Gunwar, S. Huff, J. K. Hudson, B. G. (1991). "Extracellular matrix (mesoglea) of Hydra vulgaris". Developmental Biology. 148 (2): 481–494. doi:10.1016/0012-1606(91)90266-6. PMID1743396.
- ^ ab
- Josephson, R. (2004). "The Neural Control of Behavior in Sea Anemones". Journal of Experimental Biology. 207 (14): 2371–2372. doi: 10.1242/jeb.01059 . PMID15184508.
- Werner, B. Chapman, D. M. Cutress, C. E. (1976). "Muscular and nervous systems of the cubopolyp (Cnidaria)". Experientia. 32 (8): 1047–1049. doi:10.1007/BF01933964.
- Hernandez-Nicaise, M. L. (1973). "The nervous system of ctenophores III. Ultrastructure of synapses". Journal of Neurocytology. 2 (3): 249–263. doi:10.1007/BF01104029. PMID9224490.
- Hutton, Danielle M. C. Smith, Valerie J. (1996). "Antibacterial Properties of Isolated Amoebocytes from the Sea Anemone Actinia equina". Biological Bulletin. 191 (3): 441–451. doi:10.2307/1543017. JSTOR1543017. PMID29215925.
- Campbell, Richard D. (1976). "Elimination by Hydra interstitial and nerve cells by means of colchicine". Journal of Cell Science. 21 (1): 1–13. PMID932105.
- Kier, W. M. (2012). "The diversity of hydrostatic skeletons". Journal of Experimental Biology. 215 (8): 1247–1257. doi: 10.1242/jeb.056549 . PMID22442361.
- ^ Brusca & Brusca (2003), p. 220.
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The Embryo Project Encyclopedia
Mesenchyme is a type of animal tissue comprised of loose cells embedded in a mesh of proteins and fluid, called the extracellular matrix. The loose, fluid nature of mesenchyme allows its cells to migrate easily and play a crucial role in the origin and development of morphological structures during the embryonic and fetal stages of animal life. Mesenchyme directly gives rise to most of the body’s connective tissues, from bones and cartilage to the lymphatic and circulatory systems. Furthermore, the interactions between mesenchyme and another tissue type, epithelium, help to form nearly every organ in the body.
Although most mesenchyme derives from the middle embryological germ layer, the mesoderm, the outer germ layer known as the ectoderm also produces a small amount of mesenchyme from a specialized structure called the neural crest. Mesenchyme is generally a transitive tissue while crucial to morphogenesis during development, little can be found in adult organisms. The exception is mesenchymal stem cells, which are found in small quantities in bone marrow, fat, muscles, and the dental pulp of baby teeth.
Mesenchyme forms early in embryonic life. As the primary germ layers develop during gastrulation, cell populations lose their adhesive properties and detach from sheets of connected cells, called epithelia. This process, known as an epithelial-mesenchymal transition, gives rise to the mesodermal layer of the embryo, and occurs many times throughout development of higher vertebrates. Epithelial-mesenchymal transitions play key roles in cellular proliferation and tissue repair, and are indicated in many pathological processes, including the development of excess fibrous connective tissue (fibrosis) and the spread of disease between organs (metastasis). The reverse process, the mesenchymal-epithelial transition, occurs when the loose cells of mesenchyme develop adhesive properties and arrange themselves into an organized sheet. This type of transition is also common during development, and is involved in kidney formation.
The concept of mesenchyme has a long history, which has shaped our modern understanding of the tissue in many ways. In 1879, Charles Sedgwick Minot, an anatomist based out of Harvard Medical School in Boston, Massachusetts, first described what he termed mesamoeboids, the cellular portion of what would soon come to be recognized as mesenchyme. Minot found these cells in the context of histological studies of mesoderm. He understood the loose, mobile cells of mesenchyme as primitive representatives of the mesoderm, but did not consider these cells as a type of tissue. Two years after Minot’s recognition of mesamoeboids, Oscar and Richard Hertwig, two brothers and doctoral students of Ernst Haeckel at the University of Jena in Jena, Germany, coined the term mesenchyma in their publication Die Coelomtheorie. Versucheiner Erklärung des mittleren Keimblattes (Coelom Theory: An attempt to explain the middle germ layer), and they used it to describe the type of tissue that was comprised of the amoeboid cells that Minot had portrayed. The Hertwig brothers established that mesenchyme originates from mesoderm, and they situated this relationship in the broader context of the development of the coelom, a fluid-filled body cavity. Their Die Coelomtheorie also advanced the idea that the three germ layers maintain separate identities and develop distinct tissues and organs, a concept known as germ-layer theory.
In 1888, N. Katschenko suggested that mesenchyme found in the region of the head originated from the neural crest, an ectodermal derivative, effectively expanding the tissue’s origins beyond that of a single germ layer. Five years later, Harvard Medical School doctoral student Julia Platt, in Cambridge, Massachusetts, provided evidence based on her studies of Necturus maculosus embryos, a type of aquatic salamander, that the mesenchyme that developed into the skeletal elements of the branchial arches derived from ectoderm. Platt’s 1893 publication, “Ectodermic Origin of the Cartilages of the Head,” and her conclusions about the ectodermal origins of mesenchyme in the head region, and thus skeletal and cartilaginous tissues of the skull, went against the entrenched germ-layer theory and the mesodermal origins of mesenchyme advocated by the Hertwig brothers in their 1881 Die Coelomtheorie. Platt’s findings were rejected by many established embryologists who upheld the theory of integrity of the germ layers.
In the years following Platt’s publication, several other embryologists identified ectodermal origins for mesenchyme and its derivative skeletal elements in the head region of fish and birds. It was not until nearly thirty years after Platt’s initial publication that independent studies demonstrated a major ectodermal contribution to mesenchyme. In 1921, while investigating the limits of neural crest in the formation of cerebral ganglia in Urodeles, commonly known as salamanders, Francis Landacre at the Ohio State University in Columbus, Ohio, showed the ectodermal origin of cranial mesenchyme. Landacre’s work was followed by other studies which further concluded an ectodermal component of mesenchyme. The idea that mesenchyme in the cranial region derived from neural crest was finally abrogated in the 1940s by the independent research of embryologists Sven Hörstadius at Uppsala University in Uppsala, Sweden, and Gavin de Beer at the University College in London, England.
Soon after the debate over ectodermal mesenchyme ended, research on the role of mesenchyme during development erupted. By the 1960s, embryologists realized that mesenchyme, in combination with epithelium, played an essential role in the morphogenesis of many organs during embryonic and fetal development. Epithelio-mesenchymal interactions form nearly every organ of the body, from hair and sweat glands to the digestive tract, kidneys, and teeth.
In 1969, Edward Kollar and Grace Baird from the University of Chicago in Chicago, Illinois, designed a series of experiments to understand how mesenchyme and epithelium work together when cells differentiate, and how the two tissues combine to make embryonic structures. Their research drew on a long history of investigating tissue interactions during morphogenesis, and especially on the 1954 work of John Cairn at the University of Texas in Austin, Texas, and John Saunders, at Marquette University in Milwaukee, Wisconsin. Cairn and Saunders recognized that mesoderm holds the inductive stimulus within interactions between mesoderm and epithelium. Using tooth development as a model system, Kollar and Baird provided evidence that mesenchyme drives both induction and differentiation during epithelio-mesenchymal interactions, and is thus the tissue that confers structural specificity during these interactions, or determines what structure will form. Kollar and Baird published their findings in 1969 in “The Influence of the Dental Papilla on the Development of Tooth Shape in Embryonic Mouse Tooth Germs,” and in 1970 in “Tissue Interactions in Embryonic Mouse Tooth Germs.”
Shortly before Kollar and Baird published their account of epithelio-mesenchymal interactions, Alexander Friedenstein discovered mesenchymal stem cells in mice (Mus musculus). In publications from 1966 through 1987, Friedenstein, in conjunction with his peers at the University of Moscow in Moscow, Russia, provided evidence from transplantation experiments that stem cells taken from bone marrow can differentiate into mesenchymal tissues, such as fat, bone, and cartilage. These cells came to be known as mesenchymal stem cells, and have subsequently been found in blood, cartilaginous, skeletal, and fatty tissues. Mesenchymal stem cells provide a reservoir of reserve cells that the body can use for normal or pathological tissue regeneration and repair. The abilities of mesenchymal stem cells to differentiate into different tissues, known as cell potency, has been a cause of debate in recent years, leading researchers to question whether these cells are truly multipotent, and can give rise to multiple cells types. The potential multipotency of mesenchymal stem cells, in conjunction with their presence in adult organisms, has made them an attractive alternative to embryonic stem cells for research on tissue regeneration.
Current research on mesenchyme spreads across many biological fields. The focus of mesenchyme research, however, divides between two general interests: the role and expression of mesenchyme-specific genes during development, including pathological processes, and the locations and capabilities of mesenchymal stem cells. While some still investigate mesenchyme at the tissue level, the two current focuses reflect a trend towards the analysis and understanding of molecular-level mechanisms by which mesenchyme functions during development. Beginning with the definition by the Hertwig brothers, mesenchyme research has moved from anatomical investigations in developing embryos, to cellular contributions for organ formation and tissue level interactions, and now to the genetic mechanisms of development and tissue repair.
There is historical continuity within mesenchyme research, but there remain vestiges of the controversy that surrounded this tissue in the late nineteenth century. In her 1893 article in which she introduced the biological community to the ectodermal origins of mesenchyme in the head region, Julia Platt also suggested a change in terminology. Mesenchyme of ectodermal origins she specified by the term mesectoderm, while mesodermal mesenchyme she called mesendoderm. The medical community, especially pathologists, still employs this distinction between mesenchymal sources, only referring to a tissue as mesenchyme if it is derived from mesoderm. Pathologists maintain the distinction because the mesenchymal source determines the type and behavior of a disease. Meanwhile, developmental biologists tend to recognize mesenchyme by a single name, regardless of source.
The study of mesenchyme has a long history, from mesenchyme's recognition within the framework of germ-layer theory, to controversy about mesenchyme's origins, to uncovering mesenchyme's roles in morphogenesis and its capacity to produce stem cells. This history is in part due to the fact that mesenchyme is crucial for embryonic growth and development, as well as maintenance of connective tissues in adulthood. The loose nature of cells within mesenchyme allows the tissue to move and to be molded. During embryogenesis, mesenchyme gives rise to the body’s connective tissues, from cartilage and bone to fat, muscle, and the circulatory system. Meanwhile, nearly every organ forms through epithelio-mesenchymal interactions, in which mesenchyme provides both the inductive stimulus and determines the path of differentiation. Although little mesenchyme remains in the body during adulthood, the final remnants of this tissue, mesenchymal stem cells, allow connective tissues to repair and regenerate.
In the developing vertebrate embryo, the somitomeres (or somatomeres)  are cells that are derived from the loose masses of paraxial mesoderm that are found alongside the developing neural tube. In human embryogenesis they appear towards the end of the third gestational week. The approximately 50 pairs of somitomeres in the human embryo, begin developing in the cranial (head) region, continuing in a caudal (tail) direction until the end of week four.
The first seven somitomeres give rise to the striated muscles of the face, jaws, and throat. 
The remaining somitomeres, likely driven by periodic expression of the hairy gene, begin expressing adhesion proteins such as N-cadherin and fibronectin, compact, and bud off forming somites. The somites give rise to the vertebral column (sclerotome), associated muscles (myotome), and overlying dermis (dermatome). There are a total of 37 somite pairs at the end of the fifth week of development, after the first occipital somite and 5-7 coccygeal somites disappear from the original 42-44 somites
Specification and commitment of somitic cell types
Although all the somites look identical, they will form different structures at different positions along the anterior-posterior axis. For instance, the ribs are derived from somites. The somites that form the cervical vertebrae of the neck and the lumbar vertebrae of the abdomen are not capable of forming ribs ribs are generated only by the somites forming the thoracic vertebrae. Moreover, the specification of the thoracic vertebrae occurs very early in development. If one isolates the region of chick segmental plate that will give rise to a thoracic somite, and transplants this mesoderm into the cervical (neck) region of a younger embryo, the host embryo will develop ribs in its neck. Those ribs will form only on the side where the thoracic mesoderm has been transplanted (Figure 14.6 Kieny et al. 1972 Nowicki and Burke 1999). As discussed in Chapter 11 (see Figure 11.41), the somites are specified in this manner according to the Hox genes they express. Mice that are homozygous for a loss-of-function mutation of Hoxc-8 will convert a lumbar vertebra into an extra ribbed thoracic vertebra (see Figure 11.39).
The segmental plate mesoderm is determined as to its position along the anterior-posterior axis before somitogenesis. When segmental plate mesoderm that would ordinarily form thoracic somites is transplanted into a region in a younger embryo (caudal to (more. )
Differentiation within the somite
Somites form (1) the cartilage of the vertebrae and ribs, (2) the muscles of the rib cage, limbs, and back, and (3) the dermis of the dorsal skin. Unlike the early commitment of the mesoderm along the anterior-posterior axis, the commitment of the cells within a somite to their respective fates occurs relatively late, after the somite has already formed. When the somite is first separated from the presomitic mesoderm, any of its cells can become any of the somite-derived structures. However, as the somite matures, its various regions become committed to forming only certain cell types. The ventral medial cells of the somite (those cells located farthest from the back but closest to the neural tube) undergo mitosis, lose their round epithelial characteristics, and become mesenchymal cells again. The portion of the somite that gives rise to these cells is called the sclerotome, and these mesenchymal cells ultimately become the cartilage cells (chondrocytes) of the vertebrae and part (if not all) of each rib (Figures 14.2 and 14.7).
Diagram of a transverse section through the trunk of a chick embryo on days 2𠄴. (A) The 2-day somite can be divided into sclerotome cells and dermamyotome cells. (B) On day 3, the sclerotome cells lose their adhesion to one another and migrate (more. )
Fate mapping with chick-quail chimeras (Ordahl and Le Douarin 1992 Brand-Saberi et al. 1996 Kato and Aoyama 1998) has revealed that the remaining epithelial portion of the somite is arranged into three regions (Figure 14.7). The cells in the two lateral portions of the epithelium (those regions closest to and farthest from the neural tube) are muscle-forming cells. They divide to produce a lower layer of muscle precursor cells, the myoblasts. The resulting double-layered structure is called the dermamyotome, and the lower layer is called the myotome. Those myoblasts formed from the region closest to the neural tube form the epaxial muscles (the deep muscles of the back), while those myoblasts formed in the region farthest from the neural tube produce the hypaxial muscles of the body wall, limbs, and tongue (Figures 14.7 and 14.8 see Christ and Ordahl 1995 Venters et al. 1999). The central region of the dorsal layer of the dermamyotome is called the dermatome, and it generates the mesenchymal connective tissue of the back skin: the dermis. (The dermis of other areas of the body forms from other mesenchymal cells, not from the somites.) The dermamyotome may also produce the distal cartilage of the ribs, its lateral edge producing the most ventral portion of the rib (Figure 14.8 Kato and Aoyama 1998).
Myotome derivatives of the mouse embryo. The epaxial muscles form from the region of the dermamyotome closest to the neural tube. The hypaxial muscles form from the region of dermamyotome furthest from the neural tube. The epaxial myotome will form the (more. )
Mesenchyme is the meshwork of embryonic connective tissue from which all other connective tissues of the body are formed, including cartilage and ultimately bone (see Chapter 4 ). Mesenchymal cells migrate to sites of future osteogenesis and there differentiate into osteogenic cells as a result of cellular interactions and locally generated growth factors ( Hall, 1988 ). This local instruction ensures that bone does not generally develop in inappropriate sites.
The first sign of future potential bone formation occurs in the early embryonic period as a localized condensation of the mesenchyme (skeletal blastema). Cellular condensations may arise as a result of either increased mitotic activity or an aggregation of cells drawn towards a specific site ( Hall and Miyake, 1992 ). Such chondrogenic condensations involve multiple signalling molecules including bone morphogenic proteins and transforming growth factor β ( Hall and Miyake, 2000 Long and Ornitz, 2013 ).
As the mesenchyme begins to condense, the cells become more rounded, concomitant with a reduction in the amount of intercellular substance ( Streeter, 1949 ). This stage is referred to as the precartilage blastema ( Hamilton and Mossman, 1972 Glenister, 1976 Atchley and Hall, 1991 ). The formation of mesenchymal condensations has been associated with the formation of gap junctions that permit intercellular communication ( Hall and Miyake, 1992 ). Each cell begins to secrete a basophilic matrix, rich in type II, IX and XI collagen filaments along with other substances including chondroitin sulphate, and aggrecan indicating a differentiation into chondroblasts. As development continues, the levels of hyaluron decrease following an increase in hyaluronidase. Hyaluron blocks chondrogenesis, so its removal permits cellular differentiation ( Knudson and Toole, 1987 ). As the levels of hyaluron decrease, so the levels of chondroitin sulphate increase ( Toole and Trelstad, 1971 ). It appears that hyaluron may be necessary for cellular aggregation and therefore essential for the accumulation of a sufficient number of precartilage cells to initiate the transition from mesenchyme to cartilage ( Grüneberg, 1963 Ogden, 1979 ). A number of factors, such as a mutation or the introduction of a teratogen, may be responsible for the reduction in size of a mesenchymal condensation. If this condensation does not reach a critical size/mass, then the onset of chondrification may be retarded or even aborted. There is a substantial volume of evidence from in vitro cultures to suggest the requirement of a minimum cell number before prechondrogenic cells can differentiate ( Steinberg, 1963 Flickinger, 1974 Solursh, 1983 ). A similar requirement has also been documented for pre-osteogenic cells ( Thompson et al., 1989 Nakahara et al., 1991 ). Interestingly, this may go some way towards an explanation for the phylogenetic loss of certain skeletal structures ( Hall, 1984 ). The embryonic potential to produce certain skeletal structures that will ultimately be suppressed can be retained. For example, snakes retain the mesenchymal condensations that would indicate limb formation. However, they remain small and so may fail to meet the prerequisite cellular quantity threshold so that they ultimately regress. Occasionally, some of these suppressed structures do develop beyond the condensation stage and are then classified as atavistic traits ( Hall, 1984 ). Conversely, should a condensation become excessively large, it can subsequently lead to abnormally large skeletal elements ( Hall and Miyake, 1992 ).
As the tissue continues to mature, there is a continued separation of the cells by matrix deposition and so the tissue soon takes on the appearance of early hyaline cartilage. Gardner (1963) reported that such cellular condensations, which will ultimately lead to cartilage formation, could be distinguished at a very early age from the predominantly fibrous condensations that lead to the formation of intra-membranous bone. This ease of identification is partly due to the early presence of a well-defined perichondrium.
Archenteron refers to the rudimentary alimentary cavity of an embryo at the gastrula stage while blastocoel refers to the cavity of a blastula, arising in the course of cleavage. Thus, this is the main difference between archenteron and blastocoel.
Moreover, archenteron is formed during gastrulation while blastocoel is formed during blastulation.
Another difference between archenteron and blastocoel is that the archenteron is not a segmentation cavity since it is made up of both endoderm and mesoderm while blastocoel is a segmentation cavity since it separates future endoderm from the inductive influence of the vegetal cells.
Furthermore, blastocoel is important for the formation of archenteron while blastocoel is the first cavity formed during the embryonic development.
Give Rise to
Also, archenteron gives rise to the lumen of the digestive tract while blastocoel diminishes in size and eventually fills up with the mesoderm. Hence, this is another difference between archenteron and blastocoel.
Archenteron is a cavity formed during gastrulation, developing into the lumen of the digestive cavity. However, blastocoel is the cavity formed during blastulation. It reduces its size and eventually fills up with mesoderm. Therefore, the main difference between archenteron and blastocoel is their formation and fate.
1. “Archenteron.” The Columbia Encyclopedia, 6th Ed, Encyclopedia.com, 2019, Available Here
2. “Blastula.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 3 Mar. 2011, Available Here
1. “Protovsdeuterostomes” By WYassineMrabetTalk✉This W3C-unspecified vector image was created with Inkscape. – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Blastocyst English” By Seans Potato Business (derivative of the source cited above) – Blastocyst.png (CC BY-SA 3.0) via Commons Wikimedia
About the Author: Lakna
Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things