Changes

==Introduction==

==Introduction==

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 '''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 organised 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, governed 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 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 I - III  ===

=== 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'''. 

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'''.  

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.  

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 VI Onward  ===

=== 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 - Anatomy & Physiology]]''' forming the [[Muscles - Anatomy & Physiology|muscle]] of the body and limb. '''Dorso - medial ingression'''develops into the '''epaxial [[Muscles - Anatomy & Physiology|muscle]]''' forming the [[Muscles - Anatomy & Physiology|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]].

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]].

=== Somites XI 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, 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.  

[[Category:Developmental_Biology]] [[Category:To_Do_- Review]]

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