Final answer:
Adding a supernumerary limb bud in an embryo may disrupt normal neurulation and limb development, leading to abnormal motoneuron survival due to disrupted signaling, competition for synaptic sites, and possibly affecting the embryo's symmetry and nervous system organization.
Step-by-step explanation:
Adding a supernumerary limb bud to a developing embryo could have multiple consequences on motoneuron survival. During normal development, the apical ectodermal ridge (AER) at the end of a limb bud stimulates outgrowth and limb elongation. This requires a proper balance of proliferation and differentiation signals, which are tightly regulated processes. When a supernumerary limb bud is added, the normal signaling pathways might become disrupted leading to abnormal connectivity patterns and potentially the apoptosis of motoneurons due to inadequate target tissue.
During normal neurulation and organogenesis, developing motoneurons project axons towards the limb buds with which they will form neuromuscular junctions. If an additional limb bud presents, there might be an increase of target tissue for these axons to innervate. This could initially prevent the apoptosis of some motoneurons due to the presence of more target cells. However, the long-term effects could be detrimental as there may be competition for synaptic sites, and the eventual malfunction or death of motoneurons unable to establish correct connections.
The embryo's balance of symmetry is determined by the notochord, as established by Spemann and Mangold's experiments. An extra limb bud could potentially disrupt the axis formation, leading to asymmetrical development that could be manifest in the overall nervous system organization.