Week 4 limb buds begin to protrude form the body as if little paddles were covered with a sheath. The movement is guided from the apical ridge (apical ectodermal ridge) - the seam that runs along the tips of your fingers ( see images below) - so growth starts peripherally even beyond the fingertips! As the hands grow out the wrist joints become mobile, as the arm grows further, the elbow can move, and finally at the end of the spiraling growth: the shoulder is a movable joint. Note here that the shoulder blade develops in the last phase of the arm development - so in your dancing,: try on that thought, put that embryonic knowing in your movement and see what changes! At the end of this process the skin between the fingers breaks down to allow for the transition from fin/ paddle to fingers.
The limb buds grow out of the vertebral column at C4,5 and L4,5 + sacrum, whereas we do not yet have a neck or a sacrum yet. Bone starts out as pre-cartilage, that grows along the bloodvessel flow through the developing limb ( remember blood first!) and replaces these. This cartilage then starts to become bone in the middle of each cartilaginous form - only later do blood vessels invade the bone again, nourish this growth and then become marrow.
From Week 4- 8 the limbs develop and rotate. As the apical ridge is orienting this growth, the cartilage is creating forms to become bone, myoblasts - early muscle cells, wander into these areas and align into muscle cell strands, connecting bone to bone. Shortly thereafter nerve axons "smell" their way (following chemical signals of attraction and repulsion) out into the developing limb. As they reach their target muscle fibers they embed their motor nerve heads (called growth cones) into the muscle tissue, literally breaking down tissue to attach and make a synapse. Sensory nerves also travel out, but from the outside of the spinal cord and reach the developing muscle spindles hanging in the connective tissue of the muscle. Their cell bodies lay down connecting nerves into the sensory moter loop. The muscle starts to develop without nerves, but then needs both the innervation and MOVEMENT itself to complete its full growth to health.
Watch 1st video all the way, 2nd video start at 7:45. adn 3rd is an anthropologist looking for the missing link between fish and land animals - and how flippers turned into limbs.
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