Final answer:
In the absence of the notochord, the Pax3 and Pax7 genes, which are important for the development of the neural tube, are likely downregulated. This disruption can prevent neural tube closure, potentially leading to anomalies such as spina bifida. The exact expression pattern would depend on compensatory mechanisms within the embryo.
Step-by-step explanation:
During the process of neurulation, a series of developmental events establish the central nervous system. The notochord, derived from the mesoderm, plays a crucial role in signaling to the overlying ectodermal cells that result in the formation of the neural tube. The Pax3 and Pax7 genes are part of this signaling pathway and are crucial for the proper differentiation and development of tissues.
Without the notochord, there is a disruption in the signaling cascade necessary for the normal development of the neural structures. Specifically, the absence of the notochord can lead to a failure in the closure of the neural tube, which could result in congenital anomalies such as spina bifida. While the exact expression pattern of Pax3 and Pax7 without the notochord is not directly stated, one could infer that their expression would be downregulated, given that they are part of the regulatory network initiated by the notochord signals.
That said, it's important to note that the exact response of Pax3 and Pax7 in the absence of notochord signaling would depend on the context and the compensatory mechanisms in place within the embryo. Their normal function is disrupted, and thus they likely do not induce neural tube closure or upregulation in this scenario.