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DaysMesoderm_header_cell_0_2_0 16Mesoderm_cell_0_2_1
MeSHMesoderm_header_cell_0_4_0 Mesoderm_cell_0_4_1
FMAMesoderm_header_cell_0_5_0 Mesoderm_cell_0_5_1

In all bilaterian animals, the mesoderm is one of the three primary germ layers in the very early embryo. Mesoderm_sentence_0

The other two layers are the ectoderm (outside layer) and endoderm (inside layer), with the mesoderm as the middle layer between them. Mesoderm_sentence_1

The mesoderm forms mesenchyme, mesothelium, non-epithelial blood cells and coelomocytes. Mesoderm_sentence_2

Mesothelium lines coeloms. Mesoderm_sentence_3

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). Mesoderm_sentence_4

Myogenesis is specifically a function of mesenchyme. Mesoderm_sentence_5

The mesoderm differentiates from the rest of the embryo through intercellular signaling, after which the mesoderm is polarized by an organizing center. Mesoderm_sentence_6

The position of the organizing center is in turn determined by the regions in which beta-catenin is protected from degradation by GSK-3. Mesoderm_sentence_7

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. Mesoderm_sentence_8

Furthermore, mesoderm has the capability to induce the growth of other structures, such as the neural plate, the precursor to the nervous system. Mesoderm_sentence_9

Definition Mesoderm_section_0

The mesoderm is one of the three germinal layers that appears in the third week of embryonic development. Mesoderm_sentence_10

It is formed through a process called gastrulation. Mesoderm_sentence_11

There are three important components, the paraxial mesoderm, the intermediate mesoderm and the lateral plate mesoderm. Mesoderm_sentence_12

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). Mesoderm_sentence_13

Signals for somite differentiation are derived from surroundings structures, including the notochord, neural tube and epidermis. Mesoderm_sentence_14

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. Mesoderm_sentence_15

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. Mesoderm_sentence_16

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. Mesoderm_sentence_17

Development of the mesodermal germ layer Mesoderm_section_1

During the third week a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. Mesoderm_sentence_18

This process begins with formation of a primitive streak on the surface of the epiblast. Mesoderm_sentence_19

The cells of the layers move between the epiblast and hypoblast and begin to spread laterally and cranially. Mesoderm_sentence_20

The cells of the epiblast move toward the primitive streak and slip beneath it in a process called invagination. Mesoderm_sentence_21

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. Mesoderm_sentence_22

The remaining cells form the ectoderm. Mesoderm_sentence_23

After that, the epiblast and the hypoblast establish contact with the extraembryonic mesoderm until they cover the yolk sac and amnion. Mesoderm_sentence_24

They move onto either side of the prechordal plate. Mesoderm_sentence_25

The prechordal cells migrate to the midline to form the notochordal plate. Mesoderm_sentence_26

The chordamesoderm is the central region of trunk mesoderm. Mesoderm_sentence_27

This forms the notochord which induces the formation of the neural tube and establishes the anterior-posterior body axis. Mesoderm_sentence_28

The notochord extends beneath the neural tube from the head to the tail. Mesoderm_sentence_29

The mesoderm moves to the midline until it covers the notochord, when the mesoderm cells proliferate they form the paraxial mesoderm. Mesoderm_sentence_30

In each side, the mesoderm remains thin and is known as the lateral plate. Mesoderm_sentence_31

The intermediate mesoderm lies between the paraxial mesoderm and the lateral plate. Mesoderm_sentence_32

Between days 13 and 15, the proliferation of extraembryonic mesoderm, primitive streak and embryonic mesoderm take place. Mesoderm_sentence_33

The notochord process occurs between days 15 and 17. Mesoderm_sentence_34

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. Mesoderm_sentence_35

Paraxial mesoderm Mesoderm_section_2

During the third week, the paraxial mesoderm is organized into segments. Mesoderm_sentence_36

If they appear in the cephalic region and grow with cephalocaudal direction, they are called somitomeres. Mesoderm_sentence_37

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. Mesoderm_sentence_38

The somitomeres organize into somites which grow in pairs. Mesoderm_sentence_39

In the fourth week the somites lose their organization and cover the notochord and spinal cord to form the backbone. Mesoderm_sentence_40

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. Mesoderm_sentence_41

Somitic derivatives are determined by local signaling between adjacent embryonic tissues, in particular the neural tube, notochord, surface ectoderm and the somitic compartments themselves. Mesoderm_sentence_42

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. Mesoderm_sentence_43

These structures are specified from dorsal to ventral and from medial to lateral. Mesoderm_sentence_44

each somite will form its own sclerotome that will differentiate into the tendon cartilage and bone component. Mesoderm_sentence_45

Its myotome will form the muscle component and the dermatome that will form the dermis of the back. Mesoderm_sentence_46

The myotome and dermatome have a nerve component. Mesoderm_sentence_47

Molecular regulation of somite differentiation Mesoderm_section_3

Surrounding structures such as the notochord, neural tube, epidermis and lateral plate mesoderm send signals for somite differentiation Notochord protein accumulates in presomitic mesoderm destined to form the next somite and then decreases as that somite is established. Mesoderm_sentence_48

The notochord and the neural tube activate the protein SHH which helps the somite to form its sclerotome. Mesoderm_sentence_49

The cells of the sclerotome express the protein PAX1 that induces the cartilage and bone formation. Mesoderm_sentence_50

The neural tube activates the protein WNT1 that expresses PAX 2 so the somite creates the myotome and dermatome. Mesoderm_sentence_51

Finally, the neural tube also secretes neurotrophin 3 (NT-3), so that the somite creates the dermis. Mesoderm_sentence_52

Boundaries for each somite are regulated by retinoic acid (RA) and a combination of FGF8 and WNT3a. Mesoderm_sentence_53

So retinoic acid is an endogenous signal that maintains the bilateral synchrony of mesoderm segmentation and controls bilateral symmetry in vertebrates. Mesoderm_sentence_54

The bilaterally symmetric body plan of vertebrate embryos is obvious in somites and their derivates such as the vertebral column. Mesoderm_sentence_55

Therefore, asymmetric somite formation correlates with a left-right desynchronization of the segmentation oscillations. Mesoderm_sentence_56

Many studies with Xenopus and zebrafish have analyzed the factors of this development and how they interact in signaling and transcription. Mesoderm_sentence_57

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. Mesoderm_sentence_58

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. Mesoderm_sentence_59

They are also able to remodel and contract collagen and were induced to express muscle actin. Mesoderm_sentence_60

This shows that these cells are multipotent cells. Mesoderm_sentence_61

Intermediate mesoderm Mesoderm_section_4

The intermediate mesoderm connects the paraxial mesoderm with the lateral plate and differentiates into urogenital structures. Mesoderm_sentence_62

In upper thoracic and cervical regions this forms the nephrotomes, and in caudally regions this forms the nephrogenic cord. Mesoderm_sentence_63

It also helps to develop the excretory units of the urinary system and the gonads. Mesoderm_sentence_64

Lateral plate mesoderm Mesoderm_section_5

The lateral plate mesoderm splits into parietal (somatic) and visceral (splanchnic) layers. Mesoderm_sentence_65

The formation of these layers starts with the appearance of intercellular cavities. Mesoderm_sentence_66

The somatic layer depends on a continuous layer with mesoderm that covers the amnion. Mesoderm_sentence_67

The splanchnic depends on a continuous layer that covers the yolk sac. Mesoderm_sentence_68

The two layers cover the intraembryonic cavity. Mesoderm_sentence_69

The parietal layer together with overlying ectoderm forms the lateral body wall folds. Mesoderm_sentence_70

The visceral layer forms the walls of the gut tube. Mesoderm_sentence_71

Mesoderm cells of the parietal layer form the mesothelial membranes or serous membranes which line the peritoneal, pleural and pericardial cavities. Mesoderm_sentence_72

See also Mesoderm_section_6


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