Lecture 7: Development of the Axial Skeleton and Trunk Musculature/ Spinal Cord and PNS
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Indicate the germ layers that give rise to bone (125)
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The skeletal system develops from:
 paraxial and lateral plate (somatic layer) mesoderm
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Paraxial mesoderm forms a segmented series of tissue blocks on each side of the neural tube, known as somitomeres in the head region and somites from the occipital region caudally
* Somitomeres arise from paraxial mesoderm situated lateral to the neural tube and arise in a cranial to caudal sequence (around day 18). Between 49 and 51 pairs of somitomeres form. All but the first 7 pairs give rise to somites
* Somites are blocks of mesoderm that sit on the sides of the neural tube and begin to form on day 20. By day 30 between 42 and 44 pairs of somites have formed
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Somites differentiate into the sclerotome and the dermomyotome
* At the end of the fourth week, sclerotome cells become polymorphous and form the mesenchyme, or embryonic connective tissue
* These mesenchymal cells may become fibroblasts, chondroblasts, or osteoblasts
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neural crest
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Indicate the embryological development of bone (125)
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Intramembranous ossification: mesencyme in the dermis differentiates directly into bone
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Endochondral ossification: mesenchymal cells first give rise to hyaline cartilage models which in turn become ossified
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Describe the development of the vertebral column (140-141)
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Vertebrae form from the sclerotome portions of the somites, which are derived from paraxial mesoderm.
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During the fourth week, sclerotome cells migrate around the spinal cord and notochord to merge with cells from the opposing somite on the other side of the neural tube
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The sclerotome portion of each somite undergoes resegmentation where the caudal half of each sclerotome grows into and fuses with the cephalic half of each subjacent sclerotome.
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Precartilaginous vertebrae are formed from the combination of the caudal half of one somite and the cranial half of its neighbor.
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Mesenchymal cells between cephalic and caudal parts of the original sclerotome segment fill the space between two precartilaginous vertebral bodies, thus contributing to formation of the intervertebral disc. Notochord persists and enlarges in the region of the intervertebral disc and contributes to the nucleus pulposus, which is later surrounded by circular fibers of the annulus fibrosus. The annulus fibrosus develops from the sclerotome. The rearrangement of the sclerotomes results in the myotomes spanning the intervertebral discs.
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In short, mesenchymal cells and notochord form the intervertebral disc
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Describe the development of the spinal cord
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Remember that notochord induces the overlying ectoderm to form the neural plate. The neural plate folds and fuses to form the neural tube and the neural tube gives rise to the brain and spinal cord
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The wall of a recently closed neural tube consists of neuroepithelial cells which form a thick pseudostratisfied epithelium. During the neural groove stage and immediately after closure of the tube, they divide rapidly, producing more and more neuroepithelial cells; thus forming the neuroepithelium
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Neuroepithelial cells give rise to primitive nerve cells- neuroblasts. These form the mantle layer, a zone around the neuroepithelial layer which later forms the gray matter of the spinal cord.
 neuroepithelial

neuroblasts
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mantle layer

gray matter
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The outermost layer of the spinal cord, the marginal layer, contains nerve fibers emerging from neuroblasts in the mantle layer. After myelination of the nerve fibers, this layer appears white and is called the white matter of the spinal cord
 neuroblasts are continuously added to the mantle layer thus thickening the ventral and dorsal sides
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The ventral thickenings, the basal plates, contain ventral motor horn cells and form the motor areas of the spinal cord
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The dorsal thickenings, the alar plates, form the sensory areas of the spinal cord
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These two thickenings are separated by a longitudinal groove, the sulcus limitans. The dorsal and ventral midline portions of the neural tube (the roof and floor plates) do not contain neuroblasts but are pathways for nerve fibers crossing from one side to the other
 A small intermediate horn develops when a group of neurons accumulates between the ventral motor horn and the dorsal sensory horn.
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The intermediate horn contains neurons of the sympathetic portion of the ANS and is present only at T1-T12 and L2 or L3 of the spinal cord
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Describe the development of the spinal nerve
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Motor nerve fibers begin to appear in the fourth week, arising from nerve cells in the basal plates
(ventral horns of the spinal cord). They collect into bundles called ventral nerve roots.
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Dorsal nerve roots form as collections of fibers originating from cells in dorsal root ganglia (spinal ganglia)
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Central processes from these ganglia form bundles that grow into the spinal cord opposite the dorsal horns and distal processes join the ventral nerve roots to form a spinal nerve
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Spinal nerves quickly divide into dorsal and ventral primary rami
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Dorsal primary rami innervate dorsal axial musculature, vertebral joints, and the skin of the back
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Ventral primary rami innervate the limbs and ventral body wall and form the major nerve plexuses
(brachial and lumbosacral)
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List the associated congenital defects that can occur during development of the spinal cord (285-295)
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Klippel-feil anomaly: missing one vertebrae
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Scoliosis: lateral curvature sometimes caused by a hemi-vertebrae
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Spina bifida occulta: a defect in the vertebral arches that is covered by skin and usually does not involve underlying neural tissue. Usually marked by a patch of hair overlying the affected region
(lumbosacral region: L4-S1)
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Spina bifida cystic: severe defect where neural tissue and/or meninges protrude through a defect in the vertebral arches and skin to form a cystlike sac
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Spina bifida with meningocele, spina bifida with meningomyelocele, spina bifida with myeloschisis (failure of caudal end to close) or rachischisis)
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Hydrocephaly usually appears too
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Hirschprung disease: neural crest cells do not migrate to intestines; peristalsis cannot occur
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Describe the development of the muscle from somatic mesoderm (143-146)
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With the exception of some smooth muscle tissue, the muscular system develops from the mesodermal germ layer and consists of skeletal, smooth, and cardiac muscle
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Skeletal muscle is derived from paraxial mesoderm, which forms somites from the occipital to the sacral regions and somitomeres in the head
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Somitomeres  muscle of the head
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Somites

muscle of the torso and limbs
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Ectoderm

intrinsic muscle of the eye
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Smooth muscle differentiates from splanchnic mesoderm around the gut and its derivatives and from ectoderm
 Cardiac muscle is derived from splanchnic mesoderm surrounding the heart tube
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In muscle of the trunk, the dermomyotome splits and gives rise to dermas and skeletal muscle
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The cells of the myotomes will give rise to skeletal muscle. The cells of the myotomes become myoblasts which elongate, migrate, and fuse to form myotubes
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Soon after formation of myotubes, contractile filaments appear in the cytoplasm of the myotubes, thus forming a muscle fiber
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Each myotomes of the trunk splits into dorsal and ventral parts
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The dorsal parts of the myotomes migrate further dorsally and become known as the epimere.
These become innervated by dorsal/ posterior rami and give rise to deep back muscles
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The ventral part is known as the hypomere and these become innervated by anterior/ventral ramus
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Modification of muscle occurs when fusion of the myotomes forms the rectus abdominis and splitting of myotomes longitudinally forms the trapezius and sternocleidomastoid and tangential splitting forms the intercostals and obliques
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