Mesoderm

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Mesoderm
Mesoderm.png
Tissues derived from mesoderm.
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Section through a human embryo
Details
Days16
Identifiers
MeSH D008648
FMA 69072
Anatomical terminology

The mesoderm is the middle layer of the three germ layers that develop during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm. [1] [2]

Contents

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). [1] Myogenesis is specifically a function of mesenchyme.

The mesoderm differentiates from the rest of the embryo through intercellular signaling, after which the mesoderm is polarized by an organizing center. [3] 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.

Definition

The mesoderm is one of the three germinal layers that appears in the third week of embryonic development. It is formed through a process called gastrulation. There are four important components, the axial mesoderm, the paraxial mesoderm, the intermediate mesoderm and the lateral plate mesoderm. The axial mesoderm gives rise to the notochord. 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). [1] [2] 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 give rise to the heart, blood vessels and blood cells of the circulatory system as well as to the mesodermal components of the limbs. [4]

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, Dentine of teeth, the kidneys and the adrenal cortex. [5]

Development

During the third week a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. This process begins with formation of a primitive streak on the surface of the epiblast. [6] 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 form 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 trunk mesoderm. [4] 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. In 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 take 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. [7]

Paraxial mesoderm

During the third week, the paraxial mesoderm is organized into segments. If they appear in the cephalic region and grow with cephalocaudal direction, they are called somitomeres. If they appear in the cephalic region but establish contact with the neural plate, they are known as neuromeres, which later will form the mesenchyme in the head. The somitomeres organize into somites which grow in pairs. In the fourth week the somites lose their organization and cover the notochord and spinal cord to form the backbone. In the fifth week, there are 4 occipital somites, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 8 to 10 coccygeal that will form the axial skeleton. Somitic derivatives are determined by local signaling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. [8] The correct specification of the deriving tissues, skeletal, cartilage, endothelia and connective tissue is achieved by a sequence of morphogenic changes of the paraxial mesoderm, leading to the three transitory somitic compartments: dermomyotome, myotome and sclerotome. These structures are specified from dorsal to ventral and from medial to lateral. [8] each somite will form its own sclerotome that will differentiate into the tendon cartilage and bone component. Its myotome will form the muscle component and the dermatome that will form the dermis of the back. The myotome and dermatome have a nerve component. [1] [2]

Molecular regulation of somite differentiation

Surrounding structures such as the notochord, neural tube, epidermis and lateral plate mesoderm send signals for somite differentiation [1] [2] Notochord protein accumulates in presomitic mesoderm destined to form the next somite and then decreases as that somite is established. The notochord and the neural tube activate the protein SHH which helps the somite to form its sclerotome. The cells of the sclerotome express the protein PAX1 that induces the cartilage and bone formation. The neural tube activates the protein WNT1 that expresses PAX 2 so the somite creates the myotome and dermatome. Finally, the neural tube also secretes neurotrophin 3 (NT-3), so that the somite creates the dermis. Boundaries for each somite are regulated by retinoic acid (RA) and a combination of FGF8 and WNT3a. [1] [2] [9] So retinoic acid is an endogenous signal that maintains the bilateral synchrony of mesoderm segmentation and controls bilateral symmetry in vertebrates. The bilaterally symmetric body plan of vertebrate embryos is obvious in somites and their derivates such as the vertebral column. Therefore, asymmetric somite formation correlates with a left-right desynchronization of the segmentation oscillations. [10]

Many studies with Xenopus and zebrafish have analyzed the factors of this development and how they interact in signaling and transcription. However, there are still some doubts in how the prospective mesodermal cells integrate the various signals they receive and how they regulate their morphogenic behaviours and cell-fate decisions. [8] Human embryonic stem cells for example have the potential to produce all of the cells in the body and they are able to self-renew indefinitely so they can be used for a large-scale production of therapeutic cell lines. They are also able to remodel and contract collagen and were induced to express muscle actin. This shows that these cells are multipotent cells. [11]

Intermediate mesoderm

The intermediate mesoderm connects the paraxial mesoderm with the lateral plate and differentiates into urogenital structures. [12] In upper thoracic and cervical regions this forms the nephrotomes, and in caudally regions this forms the nephrogenic cord. It also helps to develop the excretory units of the urinary system and the gonads. [4]

Lateral plate mesoderm

The lateral plate mesoderm splits into parietal (somatic) and visceral (splanchnic) layers. The formation of these layers starts with the appearance of intercellular cavities. [12] The somatic layer depends on a continuous layer with mesoderm that covers the amnion. The splanchnic depends on a continuous layer that covers the yolk sac. The two layers cover the intraembryonic cavity. The parietal layer together with overlying ectoderm forms the lateral body wall folds. The visceral layer forms the walls of the gut tube. Mesoderm cells of the parietal layer form the mesothelial membranes or serous membranes which line the peritoneal, pleural and pericardial cavities. [1] [2]

See also

Related Research Articles

Gastrulation Stage in embryonic development in which germ layers form

Gastrulation is the stage in the early embryonic development of most animals, during which the blastula is reorganized into a multilayered structure known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body, and internalized one or more cell types including the prospective gut.

Ectoderm

The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm and endoderm. It emerges and originates from the outer layer of germ cells. The word ectoderm comes from the Greek ektos meaning "outside", and derma meaning "skin".

Neurulation Embryological process forming the neural tube

Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula.

Somite Each of several blocks of mesoderm that flank the neural tube on either side in embryogenesis

The somites 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 into the sclerotomes, myotomes, syndetomes and dermatomes that give rise to the vertebrae of the vertebral column, rib cage and part of the occipital bone; skeletal muscle, cartilage, tendons, and skin.

A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans produce two or three primary germ layers. Some animals, like cnidarians, produce two germ layers making them diploblastic. Other animals such as bilaterians produce a third layer between these two layers, making them triploblastic. Germ layers eventually give rise to all of an animal’s tissues and organs through the process of organogenesis.

Somitogenesis

Somitogenesis is the process by which somites form. Somites are bilaterally paired blocks of paraxial mesoderm that form along the anterior-posterior axis of the developing embryo in segmented animals. In vertebrates, somites give rise to skeletal muscle, cartilage, tendons, endothelium, and dermis.

Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation form the internal organs of the organism.

Embryonic development Process by which the embryo forms and develops

In developmental biology, embryonic development, also known as embryogenesis, is the development of an animal or plant embryo. Embryonic development starts with the fertilization of an egg cell (ovum) by a sperm cell, (spermatozoon). Once fertilized, the ovum becomes a single diploid cell known as a zygote. The zygote undergoes mitotic divisions with no significant growth and cellular differentiation, leading to development of a multicellular embryo after passing through an organizational checkpoint during mid-embryogenesis. In mammals, the term refers chiefly to the early stages of prenatal development, whereas the terms fetus and fetal development describe later stages.

In the developing vertebrate embryo, the somitomeres 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.

Neurula Embryo at the early stage of development in which neurulation occurs

A neurula is a vertebrate embryo at the early stage of development in which neurulation occurs. The neurula stage is preceded by the gastrula stage; consequentially, neurulation is preceded by gastrulation. Neurulation marks the beginning of the process of organogenesis.

The primitive node is the organizer for gastrulation in the vertebrate embryo. The organizer is determined by the Nieuwkoop center in amphibians or the Posterior Marginal zone in amniotes.

Lateral plate mesoderm

The lateral plate mesoderm is the mesoderm that is found at the periphery of the embryo. It is to the side of the paraxial mesoderm, and further to the axial mesoderm. The lateral plate mesoderm is separated from the paraxial mesoderm by a narrow region of intermediate mesoderm. The mesoderm is the middle layer of the three germ layers, between the outer ectoderm and innner endoderm.

Paraxial mesoderm

Paraxial mesoderm, also known as presomitic or somitic mesoderm is the area of mesoderm in the neurulating embryo that flanks and forms simultaneously with the neural tube. The cells of this region give rise to somites, blocks of tissue running along both sides of the neural tube, which form muscle and the tissues of the back, including connective tissue and the dermis.

Epiblast Embryonic inner cell mass tissue that forms the embryo itself, through the three germ layers

In amniote animal embryology, the epiblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast.

Mesenchyme Type of connective tissue found mostly during the embryonic development of bilateral triploblast animals

Mesenchyme is a type of loosely organised animal embryonic connective tissue of undifferentiated cells that gives rise to blood and lymph vessels, bone, and muscle.

Bilaminar blastocyst

Bilaminar blastocyst or bilaminar disc refers to the epiblast and the hypoblast, evolved from the embryoblast. These two layers are sandwiched between two balloons: the primitive yolk sac and the amniotic cavity.

Fish development

The development of fishes is unique in some specific aspects compared to the development of other animals.

Human embryonic development

Human embryonic development, or human embryogenesis, refers to the development and formation of the human embryo. It is characterised by the processes of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, the development of the human body entails growth from a one-celled zygote to an adult human being. Fertilisation occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of development commences. Embryonic development in the human, covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus. Human embryology is the study of this development during the first eight weeks after fertilisation. The normal period of gestation (pregnancy) is about nine months or 40 weeks.

The face and neck development of the human embryo refers to the development of the structures from the third to eighth week that give rise to the future head and neck. They consist of three layers, the ectoderm, mesoderm and endoderm, which form the mesenchyme, neural crest and neural placodes. The paraxial mesoderm forms structures named somites and somitomeres that contribute to the development of the floor of the brain and voluntary muscles of the craniofacial region. The lateral plate mesoderm consists of the laryngeal cartilages. The three tissue layers give rise to the pharyngeal apparatus, formed by six pairs of pharyngeal arches, a set of pharyngeal pouches and pharyngeal grooves, which are the most typical feature in development of the head and neck. The formation of each region of the face and neck is due to the migration of the neural crest cells which come from the ectoderm. These cells determine the future structure to develop in each pharyngeal arch. Eventually, they also form the neurectoderm, which forms the forebrain, midbrain and hindbrain, cartilage, bone, dentin, tendon, dermis, pia mater and arachnoid mater, sensory neurons, and glandular stroma.

The development of the digestive system in the human embryo concerns the epithelium of the digestive system and the parenchyma of its derivatives, which originate from the endoderm. Connective tissue, muscular components, and peritoneal components originate in the mesoderm. Different regions of the gut tube such as the esophagus, stomach, duodenum, etc. are specified by a retinoic acid gradient that causes transcription factors unique to each region to be expressed. Differentiation of the gut and its derivatives depends upon reciprocal interactions between the gut endoderm and its surrounding mesoderm. Hox genes in the mesoderm are induced by a Hedgehog signaling pathway secreted by gut endoderm and regulate the craniocaudal organization of the gut and its derivatives. The gut system extends from the oropharyngeal membrane to the cloacal membrane and is divided into the foregut, midgut, and hindgut.

References

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Further reading