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==Introduction==

 

== Somitogenesis ==

=== Somites I - III  ===

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

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 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==

=== 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'''.  

:'''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.

*The chick has 12 somites.  

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

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.

===Somites I - III===

=== Somites VI Onward  ===

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; ventro - laterally and dorso - medially. '''Ventro - lateral ingression''' develops into '''hypaxial [[Muscles - Anatomy & Physiology|muscles]]''' forming the muscle of the body and limb. '''Dorso - medial ingression''' develops into the '''epaxial muscle''' forming the muscles of the dorsum. '''Myotomal cells''' do not have the ability to undergo mitosis. To increase cell numbers, cells are recruited from the dermomyotome. 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]].

*Epithelial cells on the ''dorsal'' aspect of the somite develop into the '''dermomyotome'''.

*Epithelial cells of the ''ventral - medial'' portion of the somites develop into the '''sclerotome'''.

**Cells undergo an epithelial to mesenchymal transition.

**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.''

===Somites VI Onward===

=== Somites XI Onward ===

The '''sclerotome''' surrounds the notochord and breaks it down. 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, called '''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.  

*The sclerotome surrounds the notochord and breaks it down.

*''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 [[Bones and Cartilage - Anatomy & Physiology#Structure and Function of Cartilage|cartilage]] and secondly [[Bones and Cartilage - Anatomy & Physiology#Types of Bone|bone]]. It forms the vertebrae.

[[Category:Developmental Biology]]

[[Category:Developmental_Biology]] [[Category:A&P Done]]