APPENDICULAR SKELETON

APPENDICULAR SKELETON Dr. Mujahid Khan

Composition • The appendicular skeleton consists of pectoral girdles and limb bones • Mesenchymal bones form during the fifth week in the limb buds • Chondrification of mesenchymal bone models occurs in the sixth week • Clavicle initially develops from intramembranous ossification • Later forms growth cartilages at both ends

Composition • The models of pectoral girdle and upper limb bones appear slightly before those of the pelvic girdle and lower limbs • The bone models appear in a proximodistal sequence • Ossification begins in the long bones by the eighth week • Initially occurs in the diaphysis

Primary Ossification • By 12 weeks primary ossification centers appear in almost all bones of the limbs • The clavicle begin to ossify before any other bone in the body • The femora are the next bones to show traces of ossification • First indication of ossification in cartilaginous model appear in the center of the future shaft, called primary center of ossification

Primary Ossification • Primary centers appear at different times in different bones • Most of them develop between 7 and 12 weeks • Virtually all primary centers of ossification are present at birth • The part of the bone ossified from a primary center is the diaphysis

Secondary Ossification • Secondary ossification centers of the bones at knee are the first to appear • The centers for the distal end of femur and proximal end of tibia appear during 34 to 38 weeks • Consequently they are present at birth • Most secondary centers of ossification appear after birth, called epiphysis

Secondary Ossification • The bone forms from the primary center in the diaphysis do not fuse with that formed from the secondary centers in the epiphysis until the bone grows to its adult length • The delay enables lengthening of the bone to continue until the final size is reached

Secondary Ossification • During bone growth, epiphysial plate intervenes between the diaphysis and epiphysis • The epiphysial plate is eventually replaced by bone development on each of its two sides, diaphysial and epiphysial • When this occurs, growth of the bone ceases

Limb Development • The limb buds appear as elevations of the ventrolateral body wall by end of 4th week • The limb buds form deep to a thick band of ectoderm • The upper limb buds are visible by 26 to 27 days • Lower limb buds appear 2 days later

Limb Bud • Each limb bud consists of a mass of mesenchyme covered by ectoderm • The mesenchyme is derived from the somatic layer of lateral mesoderm • The limb buds elongate by the proliferation of the mesenchyme • The upper limb buds appear low on the embryo’s trunk

Limb Bud • The early stages of limb development are alike for the upper and lower limbs • Development of upper limb buds occurs 2 days before that of lower limb buds • The upper limb buds develop opposite the caudal cervical segments • Lower limb buds form opposite the lumbar and upper sacral segments

Limb Bud • At the apex of each limb bud the ectoderm thickens to form an apical ectodermal ridge (AER) • AER exerts an inductive influence on the limb mesenchyme that initiates growth of limbs in proximal-distal axis • Mesenchymal cells aggregate at the posterior margin of the limb bud to form the zone of polarizing activity (ZPA)

Digital Rays • By the end of 6th week, mesenchymal tissue in the hand plates has condensed to form digital rays • These mesenchymal condensations or finger buds outline the pattern of the digits • During the 7th week, similar condensations of mesenchyme form digital rays and toe buds in the foot plates

Digital Rays • AER induces development of the mesenchyme into the mesenchymal primordia of the bones in the digits • The intervals between the digital rays are occupied by loose mesenchyme • Soon the intervening regions of mesenchyme break down forming notches between the digital rays

Digital Rays • As the tissue breakdown progresses, separate digits are formed by the end of 8th week • Programmed cell death (apoptosis) is responsible for the tissue breakdown in the interdigital regions • Blocking these cellular and molecular events could account for syndactyly, webbing or fusion of the fingers or toes

Final Stages of Limb Development • As the limbs elongate in the 5th week, chondrification centers appear • By the end of 6th week, the entire limb skeleton is cartilaginous • Osteogenesis of long bones begins in the 7th week from primary ossification centers in the middle of the cartilaginous models of long bones

Final Stages of Limb Development • Primary ossification centers are present in all long bones by the 12th week • Ossification of the carpal (wrist) bones begins during the first year after birth • As the long bones form, myoblasts aggregate and form a large muscle mass in each limb bud • In general this muscle mass separates into dorsal (extensor) and ventral (flexor) components

Final Stages of Limb Development • The mesenchyme in the limb bud gives rise to bones, ligaments, and blood vessels • From dermomyotome regions of somites, myogenic precursor cells also migrate into the limb bud • Later they differentiate into myoblasts or precursors of muscle cells

Rotations of Limbs • The cervical and lumbosacral myotomes contribute to the muscles of the pectoral and pelvic girdles, respectively • Early in the seventh week the limbs extend ventrally • The developing upper limbs rotate in opposite directions and to different degrees

Rotations of Limbs • The upper limbs rotate laterally through 90 degrees on their longitudinal axis • Now the future elbows point dorsally • Extensor muscles lie on the lateral and posterior aspects of the limb • The lower limbs rotate medially through 90 degrees • Now the future knees face ventrally • Extensor muscles lie on the anterior aspect of the lower limb

Cutaneous Innervation of Limbs • Motor axons arising from the spinal cord enter the limb buds during the fifth week • Grow into dorsal and ventral muscle masses • Sensory axons enter the limb buds after the motor axons and use them for guidance

Cutaneous Innervation of Limbs • Neural crest cells, the precursors of schwann cells, surround the motor and sensory nerve fibers in the limbs • Form the neurolemmal and myelin sheaths • A dermatome in this area of skin supplied by a single spinal nerve and its spinal ganglion

Cutaneous Innervation of Limbs • During the 5th week, the peripheral nerves grow from the developing limb plexuses into mesenchyme of limb buds • The spinal nerves are distributed in segmental bands, supplying both dorsal and ventral surfaces of the limb buds • As the limbs elongate, the cutaneous distribution of the spinal nerves migrates along the limbs

Cutaneous Innervation of Limbs • The original dermatomal pattern changes during growth of the limbs • An orderly sequence of distribution can still be recognized in the adult • When the limbs descend they carry their nerves with them • This explains the oblique course of the nerves arising from the brachial and lumbosacral plexuses

Blood Supply to Limbs • The limb buds are supplied by branches of the dorsal intersegmental arteries • They arise from the aorta and form a fine capillary network in the mesenchyme • The primordial vascular pattern consists of a primary axial artery and its branches • The vascular pattern changes as the limbs develop • This occurs by vessels sprouting from existing vessels

Blood Supply to Limbs • The new vessels coalesce with other sprouts to form new vessels • The primary axial artery becomes the brachial artery and common interosseous artery in the forearm • In the thigh the primary axial artery is represented by the profonda femoris artery • In the leg it is represented by the anterior and posterior tibial arteries

Anomalies of Limbs • Minor limb anomalies are common and can be corrected surgically • The most critical period of limb development is from 24 to 36 days after fertilization • Exposure to teratogen before day 33 may cause severe limb defects • Major limb anomalies appear about twice in 1000 newborns

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APPENDICULAR SKELETON