Veins and Development
- Deep veins accompany arteries as venae comitantes.
- Venae comitantes are paired deep veins that travel with an artery, often on both sides of it, and help drain the same region the artery supplies.
- Radial and ulnar veins unite as brachial veins, which join the basilic vein to form the axillary vein.
- The axillary vein becomes the subclavian vein at the first rib.
- Superficial palmar and dorsal venous arches drain into cephalic and basilic systems.
- Limb buds appear during the fourth week of development.
- Lateral-plate mesoderm forms limb bones, connective tissue, and vessels.
- Ectoderm forms the limb surface covering.
- The apical ectodermal ridge maintains limb outgrowth.
- The zone of polarizing activity patterns the anteroposterior axis.
- Lateral-plate mesoderm forms much of the limb skeleton and connective tissue, ectoderm covers the surface, the apical ectodermal ridge supports outward limb growth, and the zone of polarizing activity helps pattern thumb-to-little-finger identity.
- The upper limb rotates laterally, placing the elbow and extensor surfaces posteriorly.
- The lower limb rotates medially.
- Digital rays form in the hand plate, and programmed cell death separates the digits.
- Amelia is complete absence of a limb.
- Meromelia is partial absence of a limb.
- Polydactyly produces extra digits, whereas syndactyly produces fused digits.
- Digital rays are early finger-forming regions in the hand plate, and programmed cell death removes tissue between them; too many rays can contribute to polydactyly, while failed separation can contribute to syndactyly.
- Congenital absence or underdevelopment of bones and joints can restrict movement.
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