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/Development of the pelvic girdle and lower limb/STAGES OF LOWER LIMB DEVELOPMENT

CHAPTER 85 – Development of the pelvic girdle and lower limb

STAGES OF LOWER LIMB DEVELOPMENT

The lower limb is first recognizable as a laterally projecting ridge at stage 13 and by stage 14 it is closely associated with the wide umbilical cord. During stages 15–17 the limb projects laterally and outgrowth is fairly symmetrical.

By stage 17 the lower limb still has a flattened foot plate and although a hip region can be identified, there is no true knee as yet. In stage 18 the lower limb appears to be flexed and abducted at the hip with the knee bent, giving the appearance that the knee is facing laterally. Very little skin of the thigh is visible. The soles of the feet face the umbilical cord, and the foot plate has digit rays. During stages 20–23 the digit rays separate, and toes are clearly defined by stage 23. The feet can finally touch at stage 21, when the umbilical cord becomes proportionally smaller and the embryo larger.

The pelvis forms from two hemipelves each of which develops from one cartilaginous focus. Ossification of the pelvis starts with the ilium, which undergoes endochondral ossification at 9.5 weeks. The femur and tibia form in cartilage and the sciatic nerve extends distally to the tibia by stage 18. Cavitation of the hip joint has been reported at 7–8 weeks. The sacroiliac joint can be recognized from 7 weeks; its development is slightly different from that of other synovial joints in that the development of the ilium is ahead of that of the sacrum.

VESSELS IN THE LOWER LIMB

The axial artery of the lower limb arises from the dorsal root of the umbilical artery and courses along the dorsal surface of the thigh, knee and leg (Fig. 85.1). Below the knee it lies between the tibia and popliteus, and in the leg it lies between the crural interosseous membrane and tibialis posterior. It gives off a perforating artery that traverses the sinal tarsus to form a dorsal network and ends distally in a plantar network. The femoral artery passes along the ventral surface of the thigh, opening a new channel to the lower limb. It arises from a capillary plexus that is connected proximally with the femoral branches of the external iliac artery and distally with the axis artery. At the proximal border of popliteus the axis artery splits into primitive posterior tibial and fibular branches which run distally on the dorsal surface of popliteus and tibialis posterior to gain the sole of the foot. At the distal border of popliteus the axis artery gives off a perforating branch which passes ventrally between the tibia and the fibula and then courses to the dorsum of the foot, forming the anterior tibial and dorsalis pedis arteries. The primitive fibular artery communicates with the axis artery at the distal border of popliteus and in its course in the leg.

  

Fig. 85.1  Stages in the development of the arteries of the leg. The original path of the axis artery is indicated by a dashed line.

The femoral artery gradually increases in size. Coincidentally, most of the axis artery disappears, but proximal to its communication with the femoral artery, the root of the axis artery persists as the inferior gluteal artery and the arteria comitans nervi ischiadici.

The proximal parts of the primitive posterior tibial and fibular arteries fuse but they remain separate distally. Ultimately, much of the primitive fibular artery disappears; a part of the axis artery is incorporated in the permanent fibular artery. The same considerations apply to anomalies and variations as were described for the developing forelimb (see Ch. 52).

The preaxial vein becomes the long saphenous vein, which drains into the femoral vein at the saphenous opening. The postaxial vein becomes the short saphenous vein, which passes deep and joins the popliteal vein.

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