The paraxial mesoderm is a tissue layer that forms early in the embryonic development of vertebrates. It is a region of the mesoderm, the middle of the three primary germ layers, situated on either side of the developing neural tube and notochord. This tissue plays an organizing role in establishing the segmented body plan of the embryo. It acts as a precursor for numerous structures throughout the body.
Where the Paraxial Mesoderm Comes From
The paraxial mesoderm originates from the general mesoderm layer during gastrulation, a process where cells undergo complex movements to establish the three primary germ layers. This tissue initially forms as thick bands of cells flanking the notochord and neural tube.
Following its formation, the paraxial mesoderm undergoes a process called somitogenesis, which involves its segmentation into somites. In humans, the first somite pair emerges around day 20 of development, with additional pairs forming sequentially in a head-to-tail direction. This sequential “budding off” from the anterior end of the presomitic mesoderm results in an average of 44 somite pairs, although some later regress, leaving approximately 31 pairs that correspond to the spinal nerves in the adult.
What the Paraxial Mesoderm Becomes
Each somite undergoes further differentiation into distinct components. These components include the sclerotome, myotome, and dermatome. This differentiation process is regulated by signaling pathways from surrounding tissues, such as the neural tube and notochord.
The sclerotome is the ventromedial portion of each somite that surrounds the notochord and developing spinal cord. These cells contribute to the axial skeleton, forming the vertebrae, ribs, and portions of the skull. The sclerotome’s mesenchymal cells first differentiate into cartilage, which then ossifies through endochondral ossification to form bone, a process that continues postnatally into the mid-20s.
The myotome develops into the skeletal muscles of the trunk and limbs. It forms from the dorsal portion of the somite, specifically from the dermomyotome, which is the combined dermatome and myotome. Myogenic cells from the myotome proliferate and become myoblasts, which then fuse to form myotubes, ultimately developing into muscle fibers. The myotome divides into an epaxial part, forming deep back muscles, and a hypaxial part, which contributes to muscles of the body wall, intercostal muscles, and limb muscles.
The dermatome originates from the dorsal part of the somite and contributes to the dermis, which is the inner layer of the skin. As the limbs grow, the dermis associated with these developing structures stretches, contributing to the segmental innervation pattern seen in adult dermatome maps. Each dermatome is an area of skin predominantly supplied by sensory nerve fibers from a single spinal nerve root.
The Importance of Paraxial Mesoderm Development
The development of the paraxial mesoderm and its derivatives is important for establishing the basic body plan and enabling various physiological functions. The structures formed from the paraxial mesoderm, including the axial skeleton (vertebrae and ribs), skeletal muscles, and parts of the skin, provide support, facilitate movement, and offer protection for the body. The segmented pattern created by the somites is also important for the organization of neural crest cell migration and spinal nerve axon paths.
Formation of the paraxial mesoderm is also necessary for the coordinated development of other systems. For example, some cells derived from the paraxial mesoderm contribute to the developing kidney, forming stromal cells like myofibroblasts and vascular smooth muscle. Disruptions during this complex developmental stage can lead to structural abnormalities, such as vertebral segmentation defects or muscular dystrophies, underscoring the delicate nature of its formation.