Somite Formation - Somitogenesis

When does somitogenesis occur ?

It occurs in week 5 - day 30

What happens ?

Mesoderm reorganisation - segmentation of some not all. Segmentation is reflected in:

1. Organisation of the vertebral column

2. Organisation of the dermatomes - sensory innervation of the dermis.

What is the definition of somitogenesis ?

Somitogenesis is the cranio-caudal reorganisation and segmentation of the paraxial mesoderm (mesoderm on each side of the notochord).

What day does somitogenesis begin ? With what ? 

It begins on day 18 with the formation of somitomeres (the precursor of somites) 

• Somitomeres are loose condensations of the paraxial mesoderm. will form 50 pairs of somitomeres ( 1 on each side of the paraxial mesoderm).

What happens the somitomeres ? Does this happen to all of them ?

The somitomeres will condense and seperate from each other to form somites. Except the 7 most cranially which are needed for musculature of the head or the neck.

When does the first somite appears ? What happens the following day ?

• The first somite is formed on day 20.

• On day 21 the embryo has 4 pairs of somites.

How much pairs are being formed each day ? What is the final number of pairs ?

• 3-4 pairs are being formed each day.

• There is 42-44 final pairs of somites.

What is the first type of somite ? How many ? What are the derivates ?

• Occular somites

• 4 somites

• occular bone of the skull, bone of the nose, eyes, inner ear, extrinsic occular muscles, muscles of the tongue.

second type ?

• Cervical somites

• 8 somites

• Occipital bone, cervical vertebrae and muscles, part of dermis and upper skin musculature.

3rd type ?

• Thoracic somites

• 12 somites

• Vertebra, muscles and bones of the thoracic wall, thoracic dermis, part of abdominal wall and upper limb musculature

4th type ?

• Lumbar somites

• 5 somites

• Lumbar vertebrae, leg musculature, abdominal muscles and dermis

5th type ?

• sacral somites

• 5 pairs

• Sacrum, musculature, dermis

What about the final type ?

• Coccygeal somites

• 8-12 somites

• Musculature, Coccynx, dermis (the majority degenerates 3-5 final)

What can you say about the end of the third week of development in terms of somitogenesis ?

That there is four pairs of somites in the embryo.