Somites are blocks of mesoderm tissue that appear sequentially along the head-to-tail axis in the developing embryos of all vertebrates. These structures form directly alongside the developing neural tube, acting as the foundation for the body’s segmented organization. They are the initial blueprints for the segmented pattern seen in the adult anatomy of the torso, influencing the arrangement of bone, muscle, and skin. Somites exist only for a short time during early development, but their organization is imprinted onto many of the body’s most fundamental components.
The Process of Somite Formation
Somites originate from the paraxial mesoderm, a thick layer of tissue that flanks the neural tube and notochord in the early embryo. The unsegmented portion of this tissue is known as the presomitic mesoderm (PSM). This tissue undergoes a precise process called somitogenesis, which involves the periodic “budding off” of cellular blocks from the anterior end of the PSM, forming new somites in a rhythmic sequence from the head region backward.
The timing and positioning of this segmentation are tightly controlled by an internal biological mechanism often described using the segmentation clock and wavefront model. The “clock” involves oscillating gene expression in the PSM cells, acting as a timer that dictates when a new somite boundary can form. Meanwhile, a “wavefront” of signaling molecules sweeping backward across the PSM determines the exact location where the cells will aggregate and separate.
The total number of somite pairs formed is highly specific to the species, providing a reliable marker for developmental staging. For instance, human embryos typically form around 42 to 44 pairs, while the chick embryo forms approximately 50 pairs. Once a block of cells separates from the PSM, the cells on the outer edge undergo a transformation from a loosely organized mesenchymal state into a tightly packed epithelial layer, which clearly defines the boundary of the new somite.
Primary Somite Derivatives
Immediately after a somite is formed, it begins a rapid process of differentiation and separation into three primary, distinct cell populations. This cellular reorganization is a response to signaling molecules emanating from the surrounding tissues, such as the neural tube and notochord. The most ventromedial (lower and inner) cells of the somite undergo an epithelial-to-mesenchymal transition and migrate away to form the sclerotome.
The remaining dorsal epithelial portion is called the dermomyotome, which later splits to give rise to the other two primary derivatives. The sclerotome cells migrate inward toward the notochord and neural tube, where they will ultimately form the cartilage and bone components of the axial skeleton.
The myotome portion consists of muscle precursor cells that will form the skeletal muscle of the trunk and limbs. The dermatome is the third primary derivative, formed from the most dorsal layer of the original somite structure. These cells are the precursors for the deep layer of the skin, the dermis, in the corresponding region of the body. The myotome and dermatome collectively migrate and expand, carrying the segmental pattern established by the original somite outward into the body wall.
The Adult Structures Built by Somites
The derivatives of the somites establish the segmented blueprint for the adult body, particularly the structures that make up the trunk. The sclerotome cells are responsible for forming the entire vertebral column, including the body of each vertebra and the surrounding arches. They also form the costal cartilages and the bony ribs that articulate with the thoracic vertebrae.
The myotome differentiates into two main muscle groups: the epaxial and hypaxial muscles, which retain the somite’s original segmental organization. The epaxial muscles form the deep, intrinsic muscles of the back, such as the erector spinae group. The hypaxial muscles migrate further to form the muscles of the body wall, the abdominal muscles, and the majority of the skeletal muscles in the limbs.
The dermatome contributes to the dermis of the dorsal skin, creating the connective tissue layer directly beneath the outer epidermis. Furthermore, the segmentation pattern of the dermatome is permanently reflected in the adult nervous system’s sensory map. This pattern dictates which specific spinal nerve supplies sensation to a particular strip of skin, a clinically useful map known as dermatomes.
Clinical Significance of Somite Development
Disruptions during the precise timing and formation of somites can lead to a range of congenital defects, underscoring the process’s importance. Faulty somitogenesis, particularly issues with the segmentation clock or boundary formation, can result in vertebral segmentation anomalies.
Congenital scoliosis, a condition involving an abnormal sideways curvature of the spine, can arise from the incomplete formation or improper fusion of somite-derived vertebral precursors. Other conditions, such as Klippel-Feil syndrome, involve the fusion of two or more cervical (neck) vertebrae, directly resulting from errors in the early differentiation of the sclerotome. The complex interplay between somite formation and surrounding tissues also means that somite defects can be associated with other developmental issues. For example, defects in the somite region can sometimes correlate with neural tube closure issues, which may manifest as certain forms of spina bifida.