Somites are transient, segmented blocks of tissue that emerge early in vertebrate embryonic development. They form along the central axis, playing a foundational role in establishing the segmented organization characteristic of vertebrates.
Formation of Somites
Somites begin to form during a process called somitogenesis, where they bud off sequentially from the paraxial mesoderm, a region of embryonic tissue located on either side of the neural tube. This process starts in the anterior (head) region of the trunk and proceeds caudally (towards the tail). The formation of each somite pair occurs with precision and rhythm, often described by a “segmentation clock” mechanism. This clock involves oscillating cycles of gene expression, notably involving the Notch, Wnt, and FGF signaling pathways, which dictate the exact timing and location for new somite formation.
The paraxial mesoderm first organizes into loosely defined segments called somitomeres. As development progresses, these somitomeres compact and undergo a mesenchymal-to-epithelial transition, forming distinct, epithelialized blocks. This transformation creates clear boundaries between individual somites. This process ensures somites form at specific intervals, important for the organized development of the vertebral column and associated structures.
What Somites Develop Into
Once formed, somites undergo further differentiation, giving rise to several specialized tissues that form the axial skeleton, muscles, and parts of the skin. Each somite subdivides into distinct regions: the sclerotome, myotome, and dermatome. A fourth component, the syndetome, also forms, contributing to tendons.
The sclerotome differentiates into the cartilage and bone components of the axial skeleton, specifically forming the vertebrae and ribs. Sclerotomal cells migrate medially towards the notochord and neural tube, where they condense to form the vertebral bodies and neural arches. The myotome is the precursor to most skeletal muscles of the trunk and limbs. Myotome cells proliferate and differentiate into muscle fibers, eventually forming the epaxial (back) and hypaxial (body wall and limb) muscles. The dermatome contributes to the dermis, the inner layer of skin, in specific regions of the body. Cells from the dermatome migrate to their final positions and interact with the overlying ectoderm to form the skin.
Significance of Somites
Somites are important for establishing the segmented body plan characteristic of vertebrates. Their sequential formation along the anterior-posterior axis provides a template for the organization of many body structures. This segmentation is evident in the repeating units of the vertebral column and the arrangement of trunk muscles.
Somites play an important role in patterning and organizing other developing systems. They provide positional information that guides the development of structures along the anterior-posterior axis, influencing gene expression that determines regional identity. Somites also determine the migratory paths of neural crest cells and spinal nerve axons, helping to organize the developing nervous system in a segmented manner. This arrangement ensures spinal nerves properly innervate the muscles and skin derived from the somites.
Somites and Developmental Health
Disruptions during somitogenesis can lead to congenital anomalies affecting the musculoskeletal system, such as defects of the spine, ribs, and associated musculature. For example, conditions like hemivertebrae, where vertebrae are wedge-shaped or incompletely formed, arise from improper somite segmentation.
Such defects can lead to spinal curvatures like scoliosis, which may manifest as a result of early perturbations in muscle formation or disturbed communication between developing vertebrae and muscles. Disorders like spondylocostal dysostosis, characterized by vertebral and rib abnormalities, are linked to mutations in genes involved in the Notch signaling pathway that regulates somitogenesis. The precise formation of somites is important for healthy musculoskeletal development, and any deviation can affect the body’s structure and function.