BIOL 2P95 Lecture Notes - Ectrodactyly, Intermediate Mesoderm, Msx

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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: limb initiation mesoderm. Limb bud is mesenchyme covered by ectoderm. Bones originate from somatic mesoderm, muscles from somitic. Then, fgf8 from aer stimulates growth in progress zone: proximal/distal axis, anterior/posterior axis. Pattern of hox genes (more distally) controls p/d differentiation. Defects (early teratogen) lead to limb truncations. Shh from the mesenchymal zpa (posterior) controls a/p patterning. Defects lead to ectrodactyly (too few digits) or polydactyly (too many digits) Webbing of finger and toes removed by apoptosis. Defects (e. g. msx2, bmp4) lead to syndactyly (fused digital soft tissue: dorsal/ventral axis. Not well understood; involves induction by ectoderm. Mesenchyme differentiates into cartilage, then becomes bone by endochondral. Lateral plate mesoderm condensates into cartilganous perichondrium, this model is replaced by bone. Interzones between bones will differentiation into joint tissue. Diaphyses ossified by birth, epiphyses shortly thereafter.

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