Somite Differentiation
somite tissue differentiation
Development of the axial musculoskeletal system & skin: axial refers to the midline appendicular refers to the limbs and limb joints.
- Somites are sections of paraxial mesoderm. They differentiate into (from lateral to medial):
- Dermatome, which forms dermis of the axial body: the trunk (limb dermis derives from lateral plate mesoderm).
- Myotome, which forms muscle of the axial body: paraspinal and abdominal regions.
- Sclerotome, which forms the bone of the axial body: the spine and the posterior skull base.
trilaminar germ disc during the differentiation of the somites (during week 4)
From top to bottom, we see the trilaminar germ disc comprises ectoderm, mesoderm intraembryonic mesoderm, and endoderm.
Within the mesoderm, we see the ectoderm-derived neural tube and the mesoderm-derived notochord.
- Mesoderm divides from medial to lateral into:
- Somites (paraxial mesoderm)
- Intermediate mesoderm (which differentiates into the urogenital system)
- Lateral plate mesoderm (which differentiates into many organ systems and the appendicular muscles and bones, and the sternum).
Somites
- Dermatome differentiates into dermis.
- Myotome differentiates into epimere (posterior myotome), which will develop into paraspinal muscle, and hypomere (hypaxial muscle, ie abdominal muscle).
- Sclerotome develops into the spine (and also the posterior skull base).
- Posteriorly, the spine forms the vertebral arch (bilateral neural arches form its primary ossification centers).
- Anteriorly, the spine forms the vertebral body (the centrum forms its primary ossification center).
Nerve Roots & Rami
- The dorsal nerve root sends sensory fibers to the dermis (and musculoskeletal structures) to receive sensory input (we omit the dorsal root ganglion, for simplicity).
- The ventral nerve root sends fibers to the hypomeric muscles (and also to those of the limbs) via the ventral ramus and to the epimeric muscles (the paraspinal muscles) via the dorsal ramus.
- The rami are NOT nerve roots; they form peripheral to the convergence of the dorsal and ventral nerve roots as the mixed spinal nerve.
segmentation of the spine
We draw a lateral view of the fully-formed spine.
- The spinal cord is the differentiated neural tube.
- Posteriorly, are the vertebral arches.
- Anteriorly, are two levels of vertebral bodies.
- In between them, is an intervertebral disc.
- Spinal nerves exit via intervertebral foramen between the vertebral levels.
Sclerotome Resegmentation in Spine Formation
- Resegmentation originates at von Ebner's fissure (the intrasegmental line).
- Sclerotome from the inferior portion of one segment merges with sclerotome from the superior portion of the one below to form a single vertebra.
- Spinal nerves at the intervertebral foramen extend out peripherally to innervate the myotomal segments.
- Intervertebral discs comprise annulus fibrosus, peripherally – a series of cartilaginous rings, and nucleus pulposus, centrally – a gel, which is degenerated notochord.
Clinical Correlation
- Chordoma is a type of tumor that forms from persistent emryonic remnants of notochord.