Definition Of Mesoderm

Mesoderm: The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells, and coelomocytes. Mesothelium lines coeloms. Mesoderm forms the muscles in a process known as myogenesis, septa (cross-wise partitions), and mesenteries (length-wise partitions); and forms part of the gonads (the rest being the gametes). Myogenesis is specifically a function of mesenchyme.

Mesoderm, the middle of the three germ layers, or masses of cells (lying between the ectoderm and endoderm), appears early in the development of an animal embryo. Invertebrates it subsequently gives rise to muscle, connective tissue, cartilage, bone, notochord, blood, bone marrow, lymphoid tissue, and to the epithelial (surface, or lining, tissues) of blood vessels, lymphatic vessels, body cavities, kidneys, ureters, gonads (sex organs), genital ducts, adrenal cortex, and certain other tissues. See also ectoderm; endoderm.

The mesoderm differentiates from the rest of the embryo through intercellular signaling, after which the mesoderm is polarized by an organizing center. The position of the organizing center is in turn determined by the regions in which beta-catenin is protected from degradation by GSK-3. Beta-catenin acts as a co-factor that alters the activity of the transcription factor tcf-3 from repressing to activating, which initiates the synthesis of gene products critical for mesoderm differentiation and gastrulation. Furthermore, mesoderm has the capability to induce the growth of other structures, such as the neural plate, the precursor to the nervous system.

Mesoderm: One of the three primary germ cell layers, the others being the ectoderm and endoderm. The mesoderm is the middle layer. In the embryo, it differentiates to give rise to a number of tissues and structures including bone, muscle, connective tissue, and the middle layer of the skin.

Mesoderm. The mesoderm is a germ layer that arises during gastrulation and is present between the ectoderm, which will turn into skin and central nervous system cells, and the endoderm, which will produce the gut and the lungs (4). From: Muscle, 2012.

Ectoderm Mesoderm Endoderm

The mesoderm is one of the three germinal layers that appear in the third week of embryonic development. It is formed through a process called gastrulation. There are three important components, the paraxial mesoderm, the intermediate mesoderm, and the lateral plate mesoderm. The paraxial mesoderm forms the somitomeres, which give rise to mesenchyme of the head and organize into somites in occipital and caudal segments, and give rise to sclerotomes (cartilage and bone), and dermatomes (subcutaneous tissue of the skin).

Signals for somite differentiation are derived from surroundings structures, including the notochord, neural tube, and epidermis. The intermediate mesoderm connects the paraxial mesoderm with the lateral plate, eventually, it differentiates into urogenital structures consisting of the kidneys, gonads, their associated ducts, and the adrenal glands. The lateral plate mesoderm gives rise to the heart, blood vessels, and blood cells of the circulatory system as well as to the mesodermal components of the limbs.

Some of the mesoderm derivatives include the muscle (smooth, cardiac, and skeletal), the muscles of the tongue (occipital somites), the pharyngeal arches muscle (muscles of mastication, muscles of facial expressions), connective tissue, dermis, and subcutaneous layer of the skin, bone, and cartilage, dura mater, endothelium of blood vessels, red blood cells, white blood cells, and microglia, the kidneys, and the adrenal cortex. During the third week, a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. This process begins with the formation of a primitive streak on the surface of the epiblast. The cells of the layers move between the epiblast and hypoblast and begin to spread laterally and cranially.

The cells of the epiblast move toward the primitive streak and slip beneath it in a process called invagination.

Some of the migrating cells displace the hypoblast and create the endoderm, and others migrate between the endoderm and the epiblast to create the mesoderm. The remaining cells from the ectoderm. After that, the epiblast and the hypoblast establish contact with the extraembryonic mesoderm until they cover the yolk sac and amnion. They move onto either side of the prechordal plate. The prechordal cells migrate to the midline to form the notochordal plate. The chordamesoderm is the central region of the trunk mesoderm. This forms the notochord which induces the formation of the neural tube and establishes the anterior-posterior body axis. The notochord extends beneath the neural tube from the head to the tail.

The mesoderm moves to the midline until it covers the notochord, when the mesoderm cells proliferate they form the paraxial mesoderm. On each side, the mesoderm remains thin and is known as the lateral plate. The intermediate mesoderm lies between the paraxial mesoderm and the lateral plate. Between days 13 and 15, the proliferation of extraembryonic mesoderm, primitive streak, and embryonic mesoderm takes place. The notochord process occurs between days 15 and 17. Eventually, the development of the notochord canal and the axial canal takes place between days 17 and 19 when the first three somites are formed.

Paraxial Mesoderm


After fertilization, a new soon-to-be organism goes through a few changes. One of those changes is gastrulation. Gastrulation is an early phase of embryonic development. During this phase, the blastula (the embryo when it’s still a hollow ball of cells) rearranges itself into the gastrula (the embryo with three germ layers). What does this have to do with the topic of our lesson- the mesoderm?

Well, the mesoderm is the embryonic tissue layer that sits between the ectoderm and the endoderm in the gastrula. So to understand how the mesoderm forms, we have to understand how gastrulation occurs. During gastrulation, the blastula will form a small tuck in the side of it. The cells around the tuck start pushing further and further inward. As this occurs, the cells that will become endoderm and mesoderm cells move further and further in. At the same time, this is occurring, the cells that eventually will be ectoderm cells begin to stretch and move towards the tuck, covering the outside of the embryo.

As we progress further, the cells that got pulled inward will arrange themselves so the innermost layer is the endoderm, covered by the mesoderm, covered by the ectoderm that got pulled and extended over the entire embryo. The germ layers that form during gastrulation will arrange themselves on the outside, inside, and middle of the newly formed gastrula.

Mesoderm Formation

The soft spongy tissue in the center of bones is the bone marrow. The bone marrow contains the different types of cells (stem cells) that give rise to the red cells, white blood cells, and platelets found in our blood. The marrow may also contain abnormal cells, proteins, or inflammatory reactions that are not normally present, such as cancer cells (for example, Hodgkin or non-Hodgkin lymphoma cells or leukemia cells) in some individuals. Since the production of red cells requires iron, the marrow is one of the places in the body that normally stores a supply of iron. When we are younger, our bone marrow contains very little fat. As we age, the percentage of fat in our marrow increases.

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 A bone marrow aspiration consists of inserting a special needle into a bone and withdrawing the bone marrow fluid by suction. A bone marrow biopsy takes out a larger piece of the bone marrow by coring out a sample of the marrow with a saw (termed a trephine) that cuts a small amount of bone tissue.

This bone marrow examination can provide a more complete exam of the marrow than an aspirate.

Gastrulation is the process in which the three germ layers (ectoderm, mesoderm, and endoderm) are formed by successive waves of epiblast cells migrating through the primitive streak. Formation of the primitive streak, on day 15, marks the first event of gastrulation. Cells from the epiblast migrate into the interior of the embryo, via the primitive streak, in a process termed ingression, which involves a cellular epithelial-to-mesenchymal transition (EMT). The initial wave of migrating cells (day 16) streams through the primitive streak, displacing the hypoblast cells to become definitive endoderm, which ultimately produces the future gut derivatives and gut linings.

The second wave of migrating cells (also on day 16) populates a layer between the epiblast and the definitive endoderm, thereby forming the mesoderm layer. The intraembryonic mesoderm cells later give rise to five subpopulations of cells: paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm, cardiogenic mesoderm, and a population that forms a midline tube called the notochordal process.

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