Development of the limb bone The limb buds

• Development of the limb bone: • The limb buds project from ventrolateral body wall, they are formed of extensions from somatic mesoderm of lateral plate mesoderm and covered by ectoderm. • At 6 th week, flat plates of hand foot develop distally. • Later on, constrictions appear between parts of limb.

• Tip of limb bud shows a thickening of ectoderm called apical ectodermal ridge(AER) that induces elongation and formation of limb buds. • Segmental death occurs in the AER results in the formation of digits in hand foot. • Development of UL precedes that of LL by about 1 -2 days. During 7 th week, UL rotate 90 degrees laterally, LL rotates 90 degrees medially.

• In 6 th week, mesenchyme of limb buds forms hyaline cartilaginous model of long bones, which ossify in 12 th week. During this time, primary ossification centers will form diaphysis of shaft of the bones.

• After birth secondary ossification centers appear at end sides of shaft of bones forming epiphysis, this epiphyseal plate form a cartilaginous rim between bone of the epiphysis and diaphysis. • The growth of this epiphyseal plate elongates limb bones.

• Development of vertebral column: • vertebrae are derived from sclerotomes of somites that migrate medially and ventrally to surround the notochord and spinal cord during 4 th week. • sclerotomes of each of somite are separated from each other by intersegmental branches of dorsal aorta.

• Later on, caudal part of each of sclerotome invades cephalic part of next sclerotome across the region of the intersegmental vessels between them. • This invasion results in the formation of cartilaginous body of vertebrae.

• mid-region between cephalic and caudal parts of each sclerotome forms intervertebral disc(IVD). • notochord forms Centrum of vertebral body and centrally located nucleus pulposus of IVD. • peripheral annulus fibrosis of IVD is derived from medial regions of sclerotomes.

• Development of the ribs and sternum: • Ribs develop as processes from thoracic vertebrae that are called as the costal processes. • The rib therefore are derived from the thoracic sclerotomes. • The sternum is derived from the somatic layer of lateral plate mesoderm.