Changes

In the verterbrate embryo, as the primitive streak is regressing the '''paraxial mesoderm''' divides into blocks of cells called '''somites'''. These divisions can be seen either side of the notochord. Somites are transient structures that will give rise to cells of the vertebrae and ribs, [[Skin - Anatomy & Physiology#Dermis|dermis]] of the dorsum, [[Muscles - Anatomy & Physiology#Skeletal Muscle|skeletal muscle]] of the body wall, back and limbs. Somites begin to develop at the anterior of the embryo first, and appear at regular intervals.

In the verterbrate embryo, as the primitive streak is regressing the '''paraxial mesoderm''' divides into blocks of cells called '''somites'''. These divisions can be seen either side of the notochord. Somites are transient structures that will give rise to cells of the vertebrae and ribs, [[Skin - Anatomy & Physiology#Dermis|dermis]] of the dorsum, [[Muscles - Anatomy & Physiology#Skeletal Muscle|skeletal muscle]] of the body wall, back and limbs. Somites begin to develop at the anterior of the embryo first, and appear at regular intervals.

==Somitogenesis==

== Somitogenesis ==

 

'''Somitogenesis '''is''' '''the formation of somites. The '''paraxial mesoderm''' is formed during [[Gastrulation - Anatomy & Physiology|gastrulation]] and it forms a mesenchymal '''pre - somitic mesoderm''' tissue starting at the anterior and moving posteriorly. At the anterior, the pre - somitic mesoderm becomes organsised into blocks of cells called '''somitomeres'''. Somitomeres are compacted and cells undergo a mesenchymal to epithelial transition. These somitomeres then separate from the rest of the pre - somitic mesoderm to from '''somites'''. As somites "bud off" at the anterior, more pre - somitic mesoderm develops posteriorly. Thus, the total length of the pre - somitic mesoderm remains constant. Pre - somitic mesoderm forms somites at regular intervals; one forms every 90 minutes. This suggests that cells have an intrinsic clock, goverened by the expression of a gene called "hairy". "Hairy" is expressed for 90 minutes at a time, and 12 cycles of 90 minutes each are required to make a somite. Somites are named beginning with the most newly formed somite. Therefore, the greater the number of the somite, the older the and closer to the anterior of the embryo it is. As cycle one of somite one ends, cycle one of somite two begins; one somite forms every minute.

*''Somitogenesis; the formation of somites.''

#Paraxial mesoderm forms during [[Gastrulation - Anatomy & Physiology|gastrulation]].

#It forms a mesenchymal ''pre - somitic mesoderm'' tissue starting at the anterior and moving posteriorly.

#At the anterior, the pre - somitic mesoderm becomes organsised into blocks of cells called '''somitomeres'''.

#*Somitomeres are compacted and cells undergo a mesenchymal to epithelial transition.  

#*Somitomeres then separate from the rest of the pre - somitic mesoderm to from somites.

#As somites "bud off" at the anterior, more pre - somitic mesoderm develops posteriorly. Thus, the total length of the pre - somitic mesoderm remains constant.

 

*Pre - somitic mesoderm forms somites at regular intervals; one forms every 90 minutes.

**This suggests that cells have an intrinsic clock, goverened by the expression of a gene called "hairy".

**"Hairy" is expressed for 90 minutes at a time, and 12 cycles of 90 minutes each are required to make a somite.

**Somites are named beginning with the most newly formed somite. Therefore, the greater the number of the somite, the older the and closer to the anterior of the embryo it is.

**As cycle one of somite one ends, cycle one of somite two begins; one somite forms every minute.

==Somite Maturation==

==Somite Maturation==