Somites segmentation

M. Maroto and O. Pourquie, A molecular clock involved in somite segmentation. Curr. Top. Dev. Biol. 51, 221-248 (2001). 

Y. Masamizu, T. Ohtsuka, Y. Takashima, H. Nagahara, Y. Takenaka, K. Yoshikawa, H. Okamura, and R. Kageyama, Real-time imaging of the somite segmentation clock Revelation of unstable oscillators in the individual presomitic mesoderm cells. Proc. Natl. Acad. Sci. USA 103, 1313-1318 (2006). 

J. Dubrulle, M. J. McGrew, and O. Pourquie, FGF signaling controls somite boundary position and regulates segmentation clock control of spatiotemporal Hox gene activation. Cell 106,219— 232 (2001). 

Hester SD, Belmonte JM, Gens JS, Clendenon SG, Glazier JA (2011) A multi-cell, multi-scale model of vertebrate segmentation and somite formation. PLoS Comput Biol 7( 10 ) e 1002155. doi 10.1371/journal.pcbi. 1002155... 

Somite One of the longitudinal series of segments into which the body of some organisms is divided. 

Somite block-like mass of mesoderm alongside neural tube forms vertebral column and segmental musculature... 

Stern, C. and Keynes, R.J. (1987) Interactions between somite cells-the formation and maintenance of segment boundaries in the chick embryo. Development 99 261-272. 

The blastoderm comprises two layers of cell, the upper layer or epiblast known as the ectoderm, and the lower layer or hypoblast known as the endoderm (Fig. 12.3fl). Gastrulation begins with the formation of the middle layer, known as the mesoderm, and it is from blocks of mesoderm that the somites arise and the segmentation pattern is developed. Segmentation... 

Jouve C, limura T, Pourquie 0.(2002) Onset of the segmentation clock in the chick embryo evidence for oscillations in the somite precursors in the primitive streak. Development. 129 5) lVy7-llll... 

The neural plate is a pseudostratified epithelivun. Between its caudal tip and the prospective infundibulum, its midline overlies the notochord, which induces the overlying neuroepithelial cells to become the floor plate. Anterior to the infundibulum the neural plate overlies prechordal mesoderm. Lateral to the midline, the neural plate overlies paraxial mesoderm the segmented somitic mesoderm posteriorly (behind the prospective otocyst) and unsegmented cranial paraxial mesoderm anteriorly. 

Fig. 7. Stage 7 (HH) (26-30 h) shows the segmentation of the first pair of somites from the para-notochrodal mesoderm. This is in fact the second pair of somites. The first somite is a phantom somite. It never forms fully and will disperse again in the next few hours of development. It is a slight mesodermal condensation anterior to the cleavage for the first somite proper. Not all embryos have a visible first somite. Subsequent pairs of somites will segment in two hourly intervals from the para-axial mesoderm. Neural folds reach to the mesencephalon. The wall of the foregut emerges close to the lateral head fold.

Fig. 11. Stage 10 (HH), 10 pairs of somites (36-42h) dorsally anterior neuropore closed, prominent optic vesicles, rhombomere boundary 4/5 formed, and neural folds are closed to almost the level of the node. Ventrally, Hensen s node has regressed almost to the end of the primitive streak (the 10th pair of somites has not fully segmented caudally in this illustration), pronephric tubules develop between somites 6 and 10, heart tube turns asymmetrical bulging out to the right and contractions can be seen, and bilateral vitelline veins fan out toward the area opaca, which shows large blood islands to establish circulation.

The somites are an intriguing invention of vertebrate embryos. They represent the most overtly segmented structures of the body plan, but they give rise to both obviously segmental (e.g., the axial skeleton) as well as not-so-obviously metameric (dermis and skeletal muscle) elements. In addition, they play a key role in controlling several aspects of the organization of the central and peripheral nervous system of the trunk and participate in several different types of inductive interactions, both within themselves and with neighboring tissues, like the neural tube, the notochord, the metanephric and lateral plate mesoderm, and the ectoderm and endoderm see [1,2] for reviews). 

Here the host emhryo is operated in ovo, which is suitahle with ease only for embryos older than about 36 8 h incubation. As an example, the operation described here consists of a graft of presomitic mesoderm (segmental plate) with a quail embryo as the donor and a chick embryo as the host. An identical procedure can be followed to transplant paraxial mesoderm that has already formed somites, but it should be remembered that this is much easier when it... 

Break the vitelline membrane over the region to be operated on using several nicks of the tip of a fine hypodermic needle (Fig. 2B-D). The segmental plates are the rodlike structures lying on either side of the neural tube at the tail end of the embryo, just behind the last somite. The portion to be rotated in this example is the most anterior third of the plate. 

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