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'''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 '''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.

==Somite Maturation==

== Somite Maturation ==

*Somites differentiate to produce:

Somites differentiate to produce cell types. '''Scelerotome''' develop into the axial skeleton, ribs and vertebrae whilst '''d''''''ermomyotome''' develop into '''myotome''' and '''dermotome'''. '''Myotome''' develop into the [[Muscles - Anatomy & Physiology#Skeletal_Muscle|skeletal muscle]] of the limbs and body and '''d''''''ermotome''' develop into the [[Skin - Anatomy & Physiology#Dermis|dermis]]. Somites develop in an identical manner, although those at the posterior develop after those at the anterior. Somitic derivatives form according to the somite's position along the anterior - posterior axis. Formation of sclerotome, myotome and dermotome is not intrinsic, but governed by '''environmental signals'''. However, positional information is '''predetermined and intrinsic''' to the somite. Positional information is important to form the appropriate type of vertebrae, [[Muscles - Anatomy & Physiology|muscle]] and [[Skin - Anatomy & Physiology|skin]] along the anterior - posterior axis. Positional information is governed by '''HOX gene expression'''. Each somite expresses a unique number of HOX genes. The chick has 12 somites and here is an example''s of somite maturation as observed along the anterior - posterior axis of the chick embryo:''

:'''Scelerotome''' - develops into axial skeleton, ribs and vertebrae.

:'''Dermomyotome''' - develops into ''myotome'' and ''dermotome''.

::'''Myotome''' - develops into [[Muscles - Anatomy & Physiology#Skeletal Muscle|skeletal muscle]] of the limbs and body.

::'''Dermotome''' - develops into the [[Skin - Anatomy & Physiology#Dermis|dermis]].

*Somites develop in an identical manner, although those at the posterior develop after those at the anterior.

*Somitic derivatives form according to the somite's position along the anterior - posterior axis.

**Formation of sclerotome, myotome and dermotome is not intrinsic, but governed by '''environmental signals'''.

**However, positional information is '''predetermined and intrinsic''' to the somite. Positional information is important to form the appropriate type of vertebrae, [[Muscles - Anatomy & Physiology|muscle]] and [[Skin - Anatomy & Physiology|skin]] along the anterior - posterior axis.

**Positional information is governed by HOX gene expression. Each somite expresses a unique number of HOX genes.

*The chick has 12 somites.

''Stages of somite maturation as observed along the anterior - posterior axis of the chick embryo:''

=== Somites I - III ===

These are the most newly formed somites, and are therefore at the posterior of the embryo. Somites are simple at this stage, with epithelial cells surrounding an inner cavity that is filled with mesenchymal pre - somitic mesoderm cells. These cells are called '''somitocoel cells'''. All cells are undergoing mitosis.  

*These are the most newly formed somites, and are therefore at the posterior of the embryo.

*Somites are simple at this stage, with epithelial cells surrounding an inner cavity that is filled with mesenchymal pre - somitic mesoderm cells. These cells are called '''somitocoel cells'''.

*All cells are undergoing mitosis.

===Somites IV - X===

=== Somites IV - X ===

Epithelial cells on the ''dorsal'' aspect of the somite develop into the '''dermomyotome '''and epithelial cells of the ''ventral - medial'' portion of the somites develop into the '''sclerotome'''. 

*At the dorsal aspect of the somite, some cells of the dermomyotome are ingressing to form a new layer between the dermomytome and sclerotome; the '''myotome'''.

*Ingression occurs at two points; ventral - laterally and dorsal - medially.

:'''Ventral - lateral ingression develops into ''hypaxial [[Muscles - Anatomy & Physiology|muscle]]''. This forms [[Muscles - Anatomy & Physiology|muscle]] of the body and limb.'''

:'''Dorsal - medial ingression develops intp the ''epaxial [[Muscles - Anatomy & Physiology|muscle]]''. This forms the [[Muscles - Anatomy & Physiology|muscles]] of the dorsum.'''

*After all the myotomal cells have ingressed what remains of the dermomyotome becomes the dermotome.  

*''The dermotome, which lies dorsally in the somite, forms the [[Skin - Anatomy & Physiology#Dermis|dermis]].''

Cells undergo an epithelial to '''mesenchymal transition''' and '''somitocoel cells''' join these new mesenchymal cells to also become part of the sclerotome. The prospective sclerotome migrates to surround the notochord and neural tube. The scelerotome needs to be at the midline to form the vertebrae.  

*''Thus far, somite development has been segmented. Cells need to be adjacent to each other to function appropriately, so the sclerotome, in the ventral part of the somite, resegments.''

#A fissure develops in the middle of the sclerotome, dividing each somite into an anterior and posterior portion.

#The anterior portion of one somite unites with the posterior portion of the adjacent somite - '''resegmentation'''.

#*But; because only the sclerotome resegments, the [[Muscles - Anatomy & Physiology|muscle]] derived from the myotome remains to unite the original anterior and posterior pairs together.

#The sclerotome surrounds the neural tube and differentiates firstly to [[Cartilage - Anatomy & Physiology#Structure and Function of Cartilage|cartilage]] and secondly [[Bones - Anatomy & Physiology#Types of Bone|bone]]. It forms the vertebrae.

[[Category:Developmental Biology]][[Category:To Do - A&P]]

 

 

 

 

[[Category:Developmental_Biology]] [[Category:To_Do_-_A&P]]