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The appendicular skeleton is the skeleton of the upper and lower limbs. In the upper limb, the bones include: clavicle, scapula, humerus, radius and ulna, 8 carpal bones, 5 metacarpal bones and 16 phalanges.

In the lower limb, the bones include: hip bone, femur, tibia and fibula, 7 tarsals, 5 metatarsals and 16 phalanges. These bones arise from the mesoderm of the limbs and except for the clavicle, which develops by intramenbranous ossification, they develop by endochondral ossification.

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Development of The Limbs

  • Limb Buds

    Limb buds make their appearance in the fourth week of development. The upper limb bud appears in the cervical region around day 24, followed by the lower limb bud appearing in the lumbar region around day 28. By 5 weeks they are distinct features of the embryo.
    Each limb bud consists of a central axis of mesodermal mesenchyme derived largely from the somatic layer of the lateral plate mesoderm with some contributions from the myotome mesoderm of the somite (abaxial domain) and covered by surface ectoderm. The surface ectoderm is thickened to form what is called the apical ectodermal ride (AER) that is the primary inducer of the limbs.
    The AER guides the development of the limbs by suppressing differentiation of the underlying mesoderm. As the limb elongates, the AER moves further from the proximal parts of the limb which then begin to differentiate into muscle and bone.

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  • Orientation of the LImbs

    Note that initially, each limb projects from the torso in a straight line. The cranial surface of each limb is designated the preaxial border. On the upper limb the thumb is on the preaxial border and on the lower limb, the great toe is on the preaxial border. The caudal surface of each limb is designated the postaxial border. The little finger and little toe would be on the post axial side of the limb.

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  • Elongation of the LImbs
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    The apical ectodermal ridge (AER) secretes factors that prevent the underlying mesenchyme from differentiating into limb structures. As the limb elongates, the AER becomes displaced distally and its influence on the proximal parts of the limbs decreases. Differentiation of the limbs proceeds in a proximal to distal gradient. The hand and foot take on "paddle" shapes and eventually the AER splits into 5 segments in each limb to guide the development of the fingers and toes. If it divides into more than 5 parts, polydactyly (extra digits) can result.

  • Digits

    Initially, each hand and foot is "paddle" shaped. As the AER degenrerates between digits, it is split into 5 pieces, thus guiding the elongation of the digits into distinct entities. As the limb nears completion of development, separation of the digits is accomplished by degeneration of the tissue between the digital rays through the process of apoptosis. Failure of this to occur results in the fusion of digits, a condition known as syndactyly.

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  • Rotation of the LImbs
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    Final positioning of the limbs is the result of rotation of each limb, in different directions. The upper limb undergoes a slight degree of lateral rotation so that the thumb points laterally and the original ventral surface (i.e. flexor surface) of the limb still faces anteriorly. The lower limb undergoes a great degree of medial rotation, however.
    As a result of this rotation, the great toe moves medially and the original ventral surface (i.e. flexor surface) faces posteriorly. Hence, the dermatomes of the lower limb seem to “spiral” down the limbs.