1. Orientation: two lineages, one axis

This lecture follows two cell lineages that end up nested inside each other. Paraxial mesoderm segments into somites, which build the vertebrae, the back and body-wall muscles, and the dermis of the back. Separately, ectoderm folds into the neural tube, which becomes the spinal cord. The notochord sits between them: it induces the neural plate, then degenerates everywhere except the discs, where it persists as the nucleus pulposus.

Mesoderm region	Position	Builds
Notochord	Midline rod	Induces the neural plate; persists only as the nucleus pulposus
Paraxial	Two columns beside the notochord	Somitomeres and somites → sclerotome, myotome, dermatome
Intermediate	Between paraxial and lateral plate	Urogenital structures (kidneys, gonads)
Lateral plate, somatic	Lines the body wall	Parietal serous membrane; limb skeleton and connective tissue; limb and anterior body-wall dermis
Lateral plate, splanchnic	Wraps the gut	Visceral serous membrane; heart and circulatory system

High yield Axial skeleton = sclerotome. Limb skeleton = somatic lateral plate. All trunk and limb skeletal muscle = somitic myotome.

2. Forming the somites

Paraxial mesoderm Two longitudinal columns of mesoderm flank the notochord.
Sequential segmentation The columns break into somitomeres, loosely organized whorls, starting cranially and continuing caudally.
Epithelialization From the occipital region down, somitomeres compact into somites, paired epithelial blocks beside the neural tube.
~Day 20 The first somite pair appears in the occipital region; new pairs follow cranio-caudally.
Weeks 3–4 42–44 pairs form in total.

Rule Somitomeres 1–7 never become somites. They stay as mesenchyme and form craniofacial muscles.

Somites	Region (count)	Contributes to
1–4	Occipital (4)	Bones of nose, eye, inner ear, calvaria (+ muscles)
5–12	Cervical (8)	Occipital bone; cervical vertebrae; neck and upper-limb muscles; dermis of back
13–24	Thoracic (12)	Thoracic vertebrae; ribs; thoracic-wall and upper-limb muscles; dermis of back
25–29	Lumbar (5)	Lumbar vertebrae; abdominal and lower-limb muscles; dermis of back
30–34	Sacral (5)	Sacrum (+ muscles and dermis)
35–37	Coccygeal (3)	Coccyx
38–44	—	Degenerate in humans

Memory hook 4-8-12-5-5-3, cranial to caudal. The 8 is why there are 8 cervical nerves.

3. Somite differentiation: sclerotome, myotome, dermatome

Once formed, the somite partly reverses its epithelialization. Its ventromedial cells undergo an epithelial-to-mesenchymal transition and leave as the sclerotome. The dorsolateral remainder, the dermomyotome, splits into dermatome and myotome.

Compartment	Builds	Innervation logic
Sclerotome	Vertebrae, ribs, annulus fibrosus	Intersegmental (resegments)
Myotome · epimere	Deep back (epaxial) muscles	Dorsal ramus
Myotome · hypomere	Body-wall and limb (hypaxial) muscles	Ventral ramus
Dermatome	Dermis of the back only	One spinal nerve per strip

Trap Somitic dermatome does not make the dermis of the limbs or the anterior body wall. Those come from somatic lateral plate mesoderm of the same segment, which is why their innervation is still segmental.

Memory hook SDM: Sclerotome = bone, Dermatome = dermis, Myotome = muscle. Epimere = Epaxial = dorsal ramus.

High yield A clinical dermatome is the skin supplied by one spinal nerve level; a myotome is the muscles supplied by one level. Both are segmental. Vertebrae are not.

4. The sclerotome and the intervertebral disc

EMT Ventromedial somite cells become mesenchymal sclerotome.
Migrate During week 4 they move around the notochord and the neural tube.
Fuse They meet cells from the opposite somite in the midline. Around the notochord they form the vertebral body; around the neural tube, the vertebral arch.
Resegment Each sclerotome splits, and the caudal half of one joins the cranial half of the next.

Cells left in the plane where each sclerotome splits become the fibrous annulus fibrosus. The notochord inside each vertebral body degenerates, but notochordal tissue trapped between the bodies expands into the gelatinous nucleus pulposus.

High yield Disc = annulus fibrosus (sclerotome) + nucleus pulposus (notochord). The soft material in a herniated disc is the notochord remnant.

5. Resegmentation: intersegmental vertebrae, segmental nerves

Each spinal nerve grows out to its own somite’s myotome, so nerves are segmental. If the vertebrae were segmental too, each nerve would run into the middle of a vertebra. Resegmentation slides the bone half a segment: every vertebra is the caudal half of one sclerotome plus the cranial half of the next, so it is intersegmental, and each nerve now lies at the gap between two vertebrae, the intervertebral foramen.

Rule Vertebrae = intersegmental. Spinal nerves, dermatomes and myotomes = segmental.

The cervical off-by-one

8 cervical sclerotomes → 7 cervical vertebrae + base of the occipital bone.
8 cervical spinal nerves still form, one per somite.
C1 exits between the occiput and the atlas; C1–C7 exit above their same-numbered vertebra.
C8 exits between C7 and T1.
From T1 down, each nerve exits below its same-numbered vertebra.

Memory hook Resegmentation exists to give each segmental nerve a doorway to its muscle.

6. Atlas, axis and the dens

C1 and C2 are highly modified because their sclerotome halves migrate unusually.

C1 cranial half Migrates superiorly into the chondrocranium → occipital bone (anterior border of the foramen magnum).
C1 caudal half Migrates inferiorly and joins the C2 cranial half (which stays in place) → the atlas.
C1 body Separates from the atlas and fuses onto the body of the axis → the dens.
C2 caudal half Joins the C3 cranial half → the axis.

High yield C1 lacks a vertebral body. The dens of C2 is the missing C1 body.

The transverse ligament holds the dens against the anterior arch of C1, forming the atlanto-axial pivot joint for head rotation (“no”). Nodding (“yes”) happens mainly at the atlanto-occipital joint.

7. Neurulation

Neural plate The notochord induces the overlying ectoderm to thicken.
Neural folds The plate edges elevate around a neural groove.
Neural tube The folds fuse in the midline. Neural crest cells detach from the fold tips and migrate.
Zipper Fusion begins in the cervical region and proceeds cranially and caudally.
~Day 25 Anterior (cranial) neuropore closes.
~Day 28 Posterior (caudal) neuropore closes.

Memory hook Two-way zipper from the neck; cranial end shuts first (25), caudal second (28). The last two spots to close are where NTDs happen.

8. Neural tube defects

NTDs result when neurulation fails and are associated with maternal folate deficiency. Spina bifida as a group means a split vertebral arch, usually lumbosacral, which may or may not involve neural tissue.

Defect	What is wrong	AFP
Anencephaly	Cranial neuropore fails to close; most of the brain fails to form. Lethal.	↑
Spina bifida occulta	Incomplete vertebral arch, no neural tissue; skin dimple or hair tuft. Most common, least serious.	Normal
Meningocele	Incomplete arch + CSF-filled sac of meninges	↑ if not skin-covered
Meningomyelocele	Incomplete arch + sac of meninges and neural tissue	↑
Rachischisis	Neural folds fail to elevate/fuse or posterior neuropore fails to close; open, flattened cord with no skin. Most severe; lower-limb paralysis.	↑

High yield Severity: occulta < meningocele < meningomyelocele < rachischisis. Prevent with periconceptional folate; open defects raise maternal serum and amniotic AFP.

Trap Occulta is technically an arch-formation defect, not a neural tube closure defect: the tube closes, but the sclerotome doesn’t finish the arch around it.

9. Extra: ascent of the spinal cord

The vertebral column and dura grow faster than the spinal cord, so the cord’s end appears to climb. Lower nerve roots then run obliquely down to their exit levels, forming the cauda equina.

Stage	Cord ends at
Embryo	Full length of the canal
24 weeks	S1
Newborn	L3
Adult	L1/L2 (conus medullaris); dural sac to S2

High yield Lumbar puncture at L4–L5: below the conus, inside the dural sac, only floating roots.