Limb Development

By the end of the 4th week, the limbs are oriented as elevations or bulges on the ventrolateral side of the embryo called Limb Buds. The upper limb buds lie opposite somites C4 – T2 whereas the lower limb buds appear opposite L2 – S2 segments. The upper limb buds firstly appear (day 26-27) 1-2 days before the lower limb buds appear.
Each limb bud comprised of a core of mesenchyme, differentiated from the parietal layer of lateral plate mesoderm forms connective tissues and bones of the limb. It is covered by a layer of ectoderm which thickens at the apex of each limb bud to form an apical ectodermal ridge (AER ) which produces fibroblast growth factor (FGF) to interact with the limb mesenchyme, which enhances the growth, elongation and proliferation of the limb
At the 5th week, Limb musculature is derived from somites (myotomes) as, anterior and posterior condensations. During the 6th week, mesenchymes in the terminal ends of the buds flatten to form hand and footplates which are separated from the future forearm and leg by first circular constriction (future wrist and ankle joints) respectively.
By the end of the 6th week, the hand and footplates condense to form digital rays, notches between digits appear as a result of apoptosis (programmed cell death) within the AER, apoptosis continues in the interdigital spaces. Finally, digits separation is completed at week 8.
Rotation of the limbs occur in the opposite directions early in the 7th week, the upper limb rotates 90◦ laterally resulting in lateral position of the thumb, extensors compartment lies on the posterior and lateral side of the limb, flexors compartment lies anterior and elbow joints point dorsally. On the other side, the lower limb rotates 90◦ medially resulting in the future knee joint points ventrally, big toe lies medially, extensor muscles lie anteriorly and flexor muscles lie posteriorly.

Ref: ( Langman's Medical Embryology, 13th edition, Page NO:167 )

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