Somite Development - Anatomy & Physiology

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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, dermis of the dorsum, 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

1. Paraxial mesoderm forms during gastrulation.
2. It forms a mesenchymal pre - somitic mesoderm tissue starting at the anterior and moving posteriorly.
3. 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.
   • They then separate from the rest of the pre - somitic mesoderm to from somites.
4. As somites "bud off" at the anterior, more pre - somitic mesoderm develops posteriorly. This means that 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 simte two begins; one somite forms every minute.

Somite Maturation

• Somites differentiate to produce:

Scelerotome - develops into axial skeleton, ribs and vertebrae.

Dermomyotome - develops into myotome and dermotome.

Myotome - develops into skeletal muscle of the limbs and body.

Dermotome - develops into the dermis.

• Somites develop in an identical manner, although those at the posterior develop after those at the anterior.
• The chick has 12 somites.

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

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.