This page was contributed by David Boshell

1. Cylindrical, paraxial mesoderm, just lateral to the notochord
2. Less pronounced, slenderer intermediate mesoderm, lateral to the paraxial mesoderm
3. A flat sheet of lateral plate mesoderm, filling the remainder of the disc

• The paraxial mesoderm of each pharyngeal arch, derived from cranial somitomeres and occipital somites, gives rise to the branchiomeric muscles.

1. The sclerotome medially, that migrates to surround the developing notochord and neural tube
2. The dermamyotome that is displaced laterally, and will itself split into:

• A dermotome
• A myotome

• Ventrally, the notochord induces the surrounding sclerotome cells to develop into vertebral bodies, whilst the neural tube, dorsally, induces surrounding sclerotome cells to form vertebral arches.

• Abnormal induction by the notochord and neural tube causes spinal defects such as scoliosis from the vertebral body rudiments, or spina bifida from the vertebral arch rudiments.

• The dermotome cells migrate to the surface ectoderm of the corresponding segmental region to form the dermis of the dorsal neck and trunk, with the remainder of the dermis of the body being derived from somatopleuric lateral plate mesoderm.

• The myotome, which will differentiate into myogenic cells, splits to form:

1. A dorsal epimere, that will form the deep muscles of the back (erector spinae etc.)
2. A ventral hypomere that will form the anterolateral thoracic and abdominal wall muscles

• Myoblasts also invade the limb buds for limb musculature.

• The muscles from each myotome are innervated by the corresponding spinal nerve.
• The area of skin from each dermatome receives sensory innervation from the corresponding spinal nerve, although there is some overlap.

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• Differentiation of the limb buds occurs over the 5^th - 8^th weeks, with the upper limb developing in advance of the lower limb.

• The joints are formed by movements of the limb portions.
• The webbing of the fingers and toes is removed by programmed cell death.
• The connective tissues of the limb (bones, joints, tendons, ligaments, vasculature) develop from splanchnopleuric lateral plate mesoderm.
• The musculature of the limb is derived from somite mesoderm.
• The dermis is derived from dermatomes and lateral plate mesoderm
• The melanocytes and Schwann cells are derived from neural crest.

• The limbs are innervated by the branches of the ventral primary rami of spinal nerves C5 – T1 (upper limb) and L4 – S3 (lower limb), with axons mingling at the base of the limb to form the brachial and sacral plexuses, respectively.

• In the 6^th - 8^th weeks, the limbs rotate:
• The upper limb undergoes a slight lateral rotation, so that the elbow points caudally.
• The lower limb undergoes medial rotation, so the knee points cranially; and the foot rotates 180 degrees around its long axis, making its original dorsal surface face ventrally.

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1. Endochondral ossification
2. Membranous ossification

• All bones of the appendicular skeleton (girdles and limbs) except the clavicle form by endochondral ossification, whereby each bone is preformed as cartilage by chondrocytes (chondrification), before ossification via osteocytes and remodelling by osteoclasts.
• Ossification begins at a primary ossification centre in the bone, and in the case of the limbs, secondary ossification centres also, at the periphery of the bone.
• Long bones grow by the metaphyses (ends) of their diaphysis (shaft), until the epiphyseal cartilagenous plates fuse with the diaphysis, with the last epiphyses fusing by about 20 years of age.
• The facial bones, jaws, calvaria, and clavicle, form by membranous or direct ossification from condensations of dermal mesenchyme.

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