Somites are temporary, segmented blocks of tissue that form during the early embryonic development of vertebrates. These structures are derived from the mesoderm, specifically the paraxial mesoderm, which is the middle layer of embryonic tissue. Somites are found on either side of the developing neural tube and serve as precursors to a variety of tissues that contribute to the segmented body plan of vertebrates.
How Somites Form
The formation of somites, called somitogenesis, begins with the paraxial mesoderm located alongside the neural tube. This tissue undergoes rhythmic, sequential segmentation, budding off into pairs of somites along the embryo’s anterior-posterior axis, from head to tail. In humans, the first pair of somites appears around day 20 of development, with new pairs forming approximately every 90 minutes until an average of 44 pairs are present.
This precise timing and spacing are regulated by a “segmentation clock,” which involves oscillating gene expression within the presomitic mesoderm (PSM), the unsegmented part of the paraxial mesoderm. Genes like Hes7 show synchronized oscillations, influenced by signaling pathways such as Notch, Wnt, and FGF. The Notch pathway helps establish these molecular clock oscillations, while Wnt and FGF signaling play roles in maintaining the undifferentiated state of PSM cells and establishing a “determination front.” The determination front is where cells become competent to form somites, with somite size determined by the number of cells passing this front during one segmentation clock cycle. Following segmentation, the mesenchymal cells of the PSM undergo an epithelialization process, organizing into the epithelial structures of the somites.
What Somites Become
Somites are transient structures that undergo further differentiation, giving rise to distinct tissues. The somite first differentiates into two main parts: the ventromedial sclerotome and the dorsolateral dermomyotome.
The sclerotome, the ventral portion of the somite, provides the cellular building blocks for the axial skeleton. Cells from the sclerotome migrate and condense around the notochord and neural tube, forming the vertebral bodies, intervertebral discs, and ribs through a process called endochondral ossification. Disruptions in sclerotome development can lead to congenital malformations of the spine and ribs.
The dermomyotome further divides into the myotome and the dermatome. The myotome differentiates into skeletal muscle. It is broadly categorized into epaxial and hypaxial muscles. Epaxial muscles, which form from the medial myotome, develop into the deep muscles of the back. Hypaxial muscles, derived from the ventrolateral myotome, form the muscles of the limbs, body wall, and tongue, including intercostal muscles and abdominal wall muscles.
The dermatome, originating from the dorsal part of the somite, forms the dermis of the skin, particularly in the dorsal region of the trunk. The segmented arrangement of dermatomes is reflected in the areas of skin innervated by specific spinal nerves, with neighboring dermatomes often overlapping.
Why Somites Are Important
Somites establish the segmented organization of the vertebrate body plan. This segmentation is evident in the repeating units of the axial skeleton, musculature, and associated nervous system. The precise formation and patterning of somites directly influence the proper development and alignment of these structures.
Beyond forming specific tissues, somites also guide the migratory paths of neural crest cells and the axons of spinal nerves, creating a template for the developing nervous system. The timed and coordinated differentiation of somites ensures that muscles, bones, and skin components develop in their correct positions relative to each other. Any disruptions in the somite formation cycle or their subsequent segmentation can lead to various congenital anomalies, such as vertebral defects. The study of somites continues to provide insights into developmental biology and has implications for understanding congenital abnormalities and advancing regenerative medicine.