Somitogenesis is the process by which somites are formed during embryogenesis. Somitomeres are formed first, which are essentially segmented blocks of tissue that differentiate to form the skeletal muscle, vertebral column and skin of vertebrate animals.
The somitomeres are actually spirals of concentric mesoderm that go on to become somites in the unsegmented paraxial or the presomitic mesoderm. The presomitic mesoderm then gives rise to a series of somite pairs that are identical in appearance and become the same cell type through differentiation but differ in structure depending on their anteroposterior arrangement. For example, the thoracic vertebrae have ribs, while the lumbar vertebrae do not have ribs.
The position of each individual somite is unique and is thought to be determined by the Hox (homeotic) genes.
Almost two weeks after the ovum is fertilized by a sperm cell, the paraxial mesoderm starts to become segmented, giving rise to a series of well-defined and cubical blocks – the somites. These form along the whole length of the trunk, starting from the back of the head or the occipital region. These segments each contain a central cavity called a myocoel, which is then filled with cells that are angular and spindle-shaped.
The somites lie just under the ectoderm beside the neural tube and notochord. They are linked to the lateral mesoderm by the cell mass inbetween. In the trunk, eight are present in the cervical region, twelve in the thoracic region, five in the lumbar region, five in the sacral region and between five and eight are present in the coccygeal region.