BIOL 2P95 Lecture Notes - Brachial Plexus, Iliacus Muscle, Rhomboid Muscles

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Starts in 4th week, finished by 8th week. Upper limb initiated 3-4 days before lower limb and stays ahead. Development of the three limb axes (proximal/distal, anterior/posterior, dorsal/ventral: limb bud initiation (end of 4th week) Limb bud is mesenchyme covered by ectoderm. Induced by fgf8 release from intermediate mesoderm: fgf8 = mitogen that signals lpm to proliferate => limb bud, fgf8 initiates aer (apical ectodermal ridge, then, fgf8 from aer stimulates growth in progress zone. * limb bud malformations often related to urogenital malformations b/c both involve intermediate mesoderm defects. Lateral plate mesoderm (somatic): bone, connective tissue, vasculature. Aer = apical ectodermal ridge; formed by ectodermal thickening. Influences underlying mesenchyme to proliferate (progress zone) Pattern of hox genes (more distally) controls p/d differentiation. Defects (early teratogen) lead to limb truncations: proximal/distal axis, anterior/posterior axis. Shh gradient produced by mesenchymal tissue (posterior aspect known as zpa = zone of polarizing activity); controls a/p patterning.

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