Somites — segmented paraxial mesoderm; innervated by a spinal nerve
dermatomes — migrate to form dermis of skin
myotomes — migrate & merge to form individual muscles
sclerotomes — form axial skeleton
(also, somitomeres = seven less developed somites found in the head)

Skin — consists of dermis (connective tissue) and epidermis (epithelium)
• epidermis — from ectoderm; the cells form hair follicles and skin glands
(melanocytes in epidermis are derived from neural crest)
• dermis — formed by overlap and merger of generally 3 dermatomes
(an area of skin is generally innervated by three spinal nerves)

Muscle — arises from mesodermal myoblasts (except iris muscle from ectoderm)
• cardiac & smooth muscle — from splanchnic mesoderm
• skeletal muscle — from merger of myotomes (typically 3 or more per muscle)
(multinucleated and striated: myoblasts fuse; synthesize myosin & actin)
(innervation & in utero stretch are required for proper development)

Bone — mode of development and tissue of origin varies by location:
• skull: intramembranous development; osteoblasts arise from neural crest (ectomesenchyme)
• vertebrae & ribs: endochondral development; chondroblasts arise from sclerotomes
• appendicular bones: endochondral development; chondroblasts come from local somatic mesoderm

Endochondral bone formation:
— mesenchyme cells differentialte into chondroblast
— chondroblasts produce a cartilage prototype
— primary and secondary ossifications centers expand & replace cartilage

Joints — arise from mesenchyme in the interzone between cartilage models of bones
• synovial joint formation:
— surrounding mesenchyme becomes synovial membrane & joint capsule
(muscles must move joints in utero to prevent joint ankylosis)


Trunk (innervated by a series of individual spinal nerves):
Skin — skin is produced by overlapping dermatomes (generally three per patch)
- individual dermatomes form bands extending from dorsal to ventral midlines
- each dermatome is supplied by one spinal nerve

Skeletal Muscle — migrating myotomes produce:
- epimere (dorsal mass) – becomes epaxial musculature innervated by dorsal branches of
spinal nerves
- hypomere (ventral mass) – innervated by ventral branches of spinal nerves, subdivides
into individual hypaxial muscles

Vertebrae and Ribs — formed by sclerotomes:
- individual sclerotomes migrate and fuse into a continuous mass that surrounds the neural tube
- the continuous mass segments into individual vertebrae, but each vertebra is formed by
sclerotomes from two adjacent somites
- intervertebral discs develop between vertebrae; ribs arise as vertebral processes


Limb (innervated by branches from the nerve plexus that innervates the limb):
Morphogenesis:
— a region of body wall (limb field) becomes a limb bud
— the limb bud grows in proximal-distal order (shoulder first, manus last)
— the manus resembles a paddle, digits emerge by a necrotic process

Skin — individual dermatomes form continuous patches, each supplied by one spinal nerve. Patches
overlap and limb nerves carry axons from multiple spinal nerve.

Skeletal Muscle — migrating myotomes form an extensor mass and a flexor mass; each mass segregates
into individual muscles comprised of typically three myotomes.

Limb bones — formed endochondrally by local mesenchyme

Head (innervated by cranial nerves):
Skull (consists of a cranium and a face):
- floor of the cranium: endochondral development from sclerotomes of occipital somites
- face and roof of cranium (clavaria): intramembranous development from ectomesenchyme

Skeletal Muscle — myotomes from somites/somitomeres receive cranial nerve innervation and
migrate into pharyngeal arches or to the orbit (eye muscles)


Pharyngeal arches (each innervated by a specific cranial nerves):
— contains ectomesenchyme which forms bone, cartilage, & connective tissue
— receives myotomes and specific cranial nerve innervation
first arch: jaw bones, muscles of mastication, trigeminal nerve
second arch: hyoid bones, muscles of facial expression, facial nerve
third arch: hyoid bones, one pharyngeal m., glossopharyngeal nerve
fourth & sixth arches: larynx, pharyngeal & laryngeal muscles, vagus nerve