The
mesoderm forms at the same time as the other two
germ layers, the
ectoderm and
endoderm. The mesoderm at either side of the neural tube is called
paraxial mesoderm. It is distinct from the mesoderm underneath the neural tube, which is called the
chordamesoderm that becomes the notochord. The paraxial mesoderm is initially called the "segmental plate" in the chick embryo or the "unsegmented mesoderm" in other vertebrates. As the
primitive streak regresses and neural folds gather (to eventually become the
neural tube), the paraxial mesoderm separates into blocks called somites.
Formation The pre-somitic mesoderm commits to the somitic fate before mesoderm becomes capable of forming somites. The cells within each somite are specified based on their location within the somite. Additionally, they retain the ability to become any kind of somite-derived structure until relatively late in the process of
somitogenesis. The timing of the interval is not universal. Different species have different interval timing. In the
chick embryo, somites are formed every 90 minutes. In the
mouse the interval is 2 hours. For some species, the number of somites may be used to determine the stage of embryonic development more reliably than the number of hours post-fertilization because rate of development can be affected by temperature or other environmental factors. The somites appear on both sides of the neural tube simultaneously. Experimental manipulation of the developing somites will not alter the rostral/caudal orientation of the somites, as the cell fates have been determined prior to somitogenesis. Somite formation can be induced by
Noggin-secreting cells. The number of somites is species dependent and independent of embryo size (for example, if modified via surgery or genetic engineering). Chicken embryos have 50 somites; mice have 65, while snakes have 500. As cells within the paraxial mesoderm begin to come together, they are termed
somitomeres, indicating a lack of complete separation between segments. The outer cells undergo a
mesenchymal–epithelial transition to form an
epithelium around each somite. The inner cells remain as
mesenchyme.
Notch signalling The Notch system, as part of the clock and wavefront model, forms the boundaries of the somites.
DLL1 and
DLL3 are Notch
ligands, mutations of which cause various defects. Notch regulates
HES1, which sets up the caudal half of the somite. Notch activation turns on
LFNG which in turn inhibits the Notch receptor. Notch activation also turns on the HES1 gene which inactivates LFNG, re-enabling the Notch receptor, and thus accounting for the oscillating clock model.
MESP2 induces the
EPHA4 gene, which causes repulsive interaction that separates somites by causing segmentation. EPHA4 is restricted to the boundaries of somites.
EPHB2 is also important for boundaries.
Mesenchymal-epithelial transition Fibronectin and
N-cadherin are key to the
mesenchymal–epithelial transition process in the developing embryo. The process is probably regulated by Paraxis (
Transcription factor 15) and MESP2. In turn, MESP2 is regulated by Notch signaling. Paraxis is regulated by processes involving the
cytoskeleton.
Specification The
Hox genes specify somites as a whole based on their position along the anterior-posterior axis through specifying the pre-somitic mesoderm before somitogenesis occurs. After somites are made, their identity as a whole has already been determined, as is shown by the fact that transplantation of somites from one region to a completely different region results in the formation of structures usually observed in the original region. In contrast, the cells within each somite retain plasticity (the ability to form any kind of structure) until relatively late in somitic development. and
bone (sclerotomes). Because the sclerotome differentiates before the dermatome and the myotome, the term
dermomyotome refers to the combined dermatome and myotome before they separate out.
Dermatome The
dermatome is the dorsal portion of the paraxial mesoderm somite which gives rise to the skin (
dermis). In the human embryo, it arises in the third week of
embryogenesis. From this vertebral body, sclerotome cells move dorsally and surround the developing
spinal cord, forming the vertebral arch. Other cells move distally to the costal processes of
thoracic vertebrae to form the ribs. The syndetomes are precursors for the tendons and ligaments of the vertebrae. == In arthropods==