Somites are blocks of mesodermal tissue that serve as the foundational structures for the segmented body plan of vertebrates. These transient embryonic structures are paired masses of cells that form along the developing neural tube. They govern the formation of the body’s axial structures, including the vertebral column, the skeletal muscles, and the dermis of the back. Precise formation and differentiation are necessary for the proper establishment of the body’s organization and segmentation.
What Are Somites and When Do They Form?
Somites emerge from the paraxial mesoderm, the middle layer of embryonic tissue situated on either side of the neural tube. Before forming distinct blocks, this tissue is unsegmented and referred to as the pre-somitic mesoderm. The formation of the first somite pair in a human embryo typically begins around day 20 of development.
They appear sequentially, starting near the head (cranial region) and moving down toward the tail (caudal region) of the embryo. Human embryos form an average of about 42 to 44 pairs of somites, though some may regress later. The consistent number of somite pairs present at a given time is frequently used by embryologists to accurately determine the developmental age of the embryo.
The Process of Somitogenesis
The process of somite formation, known as somitogenesis, is a rhythmic and sequential event driven by complex molecular signals. It involves transforming the diffuse, unsegmented paraxial mesoderm into clearly defined, epithelialized blocks. This precise segmentation is controlled by the “clock and wavefront” model of development.
The “segmentation clock” refers to a molecular oscillator within the pre-somitic mesoderm, involving the periodic, oscillating expression of specific genes, such as those in the Notch and Wnt signaling pathways. These genes cycle between high and low levels of expression in a consistent, timed fashion, much like a clock. In contrast, the “wavefront” is a caudal-to-cranial gradient of signaling molecules, such as Fibroblast Growth Factor (FGF), that slowly moves backward along the embryonic axis.
A somite boundary is created when the oscillating “clock” hits a certain phase, or “permissive” state, at the exact moment the slowly retreating “wavefront” passes through that region. This event triggers a cellular change, causing the cells to condense and separate from the rest of the unsegmented tissue. This interaction translates a temporal rhythm (the clock) into a physical, repetitive spatial pattern (the somites), determining the precise size and placement of each segment.
The Three Fates of Somite Cells
Once a somite is formed, its cells rapidly differentiate into three primary compartments, determined by signals secreted from surrounding tissues like the neural tube and the notochord. The first compartment to emerge is the sclerotome, which migrates medially toward the notochord, the rod-like structure running beneath the neural tube.
The sclerotome cells lose their epithelial organization and become mesenchymal, forming the cartilage and bone components of the axial skeleton. The sclerotome gives rise to the vertebrae, including the vertebral bodies and neural arches, as well as the ribs. A process called resegmentation occurs where the lower half of one sclerotome fuses with the upper half of the adjacent sclerotome to form a single vertebra, positioning it correctly relative to the spinal nerves.
The remaining dorsal portion of the somite is called the dermomyotome, which then splits into two other distinct regions. The myotome forms the skeletal muscle, dividing into the epaxial myotome (deep muscles of the back) and the hypaxial myotome (muscles of the body wall, limbs, and tongue). The third compartment, the dermatome, is the most dorsal region and primarily contributes to the dermis of the back.
Developmental Significance and Clinical Implications
Proper somite formation is essential to establishing the entire segmented body plan of a vertebrate. Beyond forming the main structures of the trunk, somites guide the migratory paths for other cell types, such as neural crest cells and spinal nerve axons. Errors during somitogenesis can lead to a range of structural defects known as segmentation defects.
Disruptions to the timing or signaling of the segmentation clock can result in congenital vertebral anomalies, where the vertebrae are malformed or fused. Examples include congenital scoliosis (abnormal lateral curvature of the spine) or hemivertebrae (where only half of a vertebral body forms). Research suggests that even mild defects in the muscle-forming myotome can contribute to conditions like late-onset scoliosis by causing problems with muscle organization.