Somite Development - Anatomy & Physiology

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

Somitogenesis is the formation of somites. The paraxial mesoderm is formed during 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

Somites differentiate to produce certain cell types. Scelerotome develop into the axial skeleton, ribs and vertebrae whilst dermomyotome develop into myotomeand dermotome. Myotome develop into the skeletal muscle of the limbs and body and dermotome develop into the 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, muscle and 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 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.

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.

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

At the dorsal aspect of the somite, some cells of the dermomyotomeare 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 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 dermis.

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 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 and secondly bone. It forms the vertebrae.

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