Deep Veins and Development
- Deep veins accompany arteries and receive blood from perforating veins.
- Anterior and posterior tibial veins unite with fibular veins to form the popliteal vein.
- The popliteal vein becomes the femoral vein after passing through the adductor hiatus.
- The femoral vein becomes the external iliac vein beneath the inguinal ligament.
- Deep lower-limb veins usually follow the arteries with similar names, so anterior and posterior tibial veins drain to popliteal, then femoral, then external iliac veins.
- Lower-limb buds appear during the fourth week from lateral-plate mesoderm covered by ectoderm.
- The apical ectodermal ridge promotes limb outgrowth.
- The zone of polarizing activity establishes the anteroposterior pattern.
- Lateral-plate mesoderm forms much of the limb skeleton and connective tissue, ectoderm covers the limb surface, the apical ectodermal ridge promotes outward growth, and the zone of polarizing activity helps pattern toe-side identity.
- Medial lower-limb rotation places the patella anteriorly and the great toe medially.
- Because the lower limb rotates medially, the extensor surface moves anteriorly and the flexor surface moves posteriorly, unlike the upper limb.
- Digital rays form in the foot plate, and programmed cell death separates the digits.
- Limb-development errors can cause absent or shortened limbs, abnormal digit number, fused digits, or abnormal bones and joints.
- Digital rays are early toe-forming regions, and programmed cell death removes tissue between them; too many rays can contribute to extra digits, while failed separation can contribute to fused digits.
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