STAGES OF UPPER LIMB DEVELOPMENT

The upper limb bud is first visible as a ridge along the lateral longitudinal axis of the body wall opposite somites 8–10, at the level of the entrance to the cranial intestinal portal during stage 12. It enlarges, protruding laterally from its elliptical base at the body wall as a flattened plate, with a curved border and an apical ectodermal ridge forming its distal tip. It also has initially equal and relatively flat dorsal and ventral ectodermal surfaces, and a somatopleuric mesenchymal core.

By stage 13 the upper limb bud is curved ventrally. In stage 14 the bud has elongated and the preaxial border has started to lengthen. At this stage the limb bud is opposite the developing ventricles of the heart. In stage 15, the upper limb can be subdivided into definite regions. The proximal portion of the limb which still shows the dorsal bulge and ventral curve is the shoulder and upper arm region; the next distal portion (which was derived from the increase in the length of the preaxial border) can now be identified as the forearm; the most distal portion is now expanded into a flattened hand plate.

At stage 16, the upper limb appears much more substantial. It is sometimes close to the body wall and sometimes abducted. The hand plate has the first indications of digit rays. By stage 17, the upper limb has an elbow region and digit rays; in advanced members of this stage, the hand plate has a crenated rim indicating the beginning of tissue removal between the digits (see Fig. 51.1). In stage 18 embryos (44 days), there is further crenation of the hand plate between the digit rays. Changes during stages 19–23 are concerned with growth of the limb and separation of the digits. The hands now curve over the cardiac region. The distal phalangeal portions of the fingers enlarge at stage 21, forming the nail beds.

Most of the bones in the appendicular skeleton are derived from somatopleuric mesenchyme. Within the upper limb although the clavicle and coracoid portion of the scapula arise from somatopleuric mesenchyme, the body and spine of the scapula are derived from the somites. Prechondroblasts are present in the upper limb at stage 13, and condensations of cartilage can be detected at stage 16, when the humeral anlage can be recognized. Cavitation of the shoulder and elbow joints occurs at 7–8 weeks. By stage 17, when the radius and ulna chondrify, the branched tips of the radial, median and ulnar nerves have migrated to the distal hand plate. The carpal bones chondrify at stage 18, when the hand plate shows notching of the digital rays.