Further investigations were able to identify specific factors such as noggin and chordin as genetic factors in the dorsal lip that are critical for proper neural development. Future experiments using a series of injections of dorsal lip mRNA into irradiated embryos demonstrated that the dorsal lip contained genetic factors that were sufficient for neural induction. Interest then shifted to identifying the chemical mechanisms underlying the dorsal lip’s organizer function. The dorsal lip of the developing gastrula was thus denoted as the Spemann-Mangold organizer for its role in neural induction and organization of developing neural tissues. Transplantation of the dorsal lip from a Xenopus embryo into the ventral region of a different host embryo demonstrated that an entire secondary axis would form using the host embryo’s own tissue, indicating a clear role of the dorsal lip as a neural inducer and organizer. His research focused on the dorsal lip as a possible organizer of these fate specification changes since it is the first structure to fold inwards during gastrulation. Hans Spemann noted this phenomenon and hypothesized that the tissue rearrangements that occurred during gastrulation must somehow be linked to controlling the fates of developing tissue in the embryo. Early transplantation experiments in developing embryos demonstrated that different layers of the embryo, when isolated and transplanted before gastrulation versus after gastrulation, would develop into distinctly different mature tissues. The dorsal lip refers to the section of tissue located at the site of the first invagination in the developing pregastula and is understood to act as both the neural inducer in the early embryo as well as the overall organizer of the entire body axis. One such candidate gene, noggin, is sufficient for rescuing development of irradiated embryos when its mRNA is injected (C). Establishment of cDNA libraries from extracted dorsal lip mRNA identified candidate genes that may be responsible for neural induction. Injection of extracted mRNA from the dorsal lip into irradiated Xenopus embryos rescued neural induction and overall development demonstrating there is a genetic basis of neural induction (B). Figure 1: Mangold's dorsal lip transplant experiment in Xenopus demonstrated that a transplanted dorsal lip could induce the formation of a double axis in the new host embryo, solidifying the dorsal lip's sufficiency in neural induction (A). It is particularly important for its role in neural induction through the default model, where signaling from the dorsal lip protects a region of the epiblast from becoming epidermis, thus allowing it to develop to its default neural tissue. The dorsal lip is formed during early gastrulation as folding of tissue along the involuting marginal zone of the blastocoel forms an opening known as the blastopore. The dorsal lip of the blastopore is a structure that forms during early embryonic development and is important for its role in organizing the germ layers.
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