All vertebrates are thought to have an organizer that releases inductive signals, responsible for body plan arrangement. This signifies that the results obtained by studying the Spemann–Mangold organizer can be extrapolated to all vertebrates. However, the AVE (anterior visceral endoderm) an extra embryonic tissue also known as second organizer in mammals ( Figure 1). In teleost fish this is an embryonic shield, in avian embryos it called Hensen’s node, and in mammals refer as node. It has been found that the amphibious Spemann–Mangold organizer has developmental analogues in other vertebrates. Since the initial research carried out mainly by Spemann and Harland, in recent decades many experiments have been carried out to provide more knowledge about the function of the Spemann–Mangold organizer during embryonic development. Third, the three germ layers will respond to the organizer’s signals. Second, the three germ layers (endoderm, mesoderm, and ectoderm) of the developing embryo are signaled and affected by the organizer. First, the organizer deactivates or activates the surrounding cells to differentiate and start morphogenesis. Years after the findings of Spemann and Mangold, Harland and Gerhart, using the amphibian Xenopus laevis, further studied the functions of the organizer and concluded the following. A major milestone had been achieved for developmental biology. This discovery also introduced the concept of induction in embryonic development, which refers to the method used by specific cells to affect the fate of other embryonic cells. Spemann and Mangold found the first evidence of the organizing center, thereafter called the “Spemann organizer”, and its major role in the development of vertebrates. When transplanted to the ventral side of the embryo, the center will induce the formation of a secondary axis, promoting the development of the central nervous system, organs, and tissues, as well as the formation of the main body axis. This center consists of a cluster of cells in the developing embryo that have the ability to interact and instruct morphogenesis in the surrounding cells during gastrulation. Spemann and Mangold discovered the organizing center in the dorsal blastopore lip of amphibians. The major findings were that the transplant had altered the fate of the overlying cells and that the neural folds were built from recipient cells and not donor cells. The transplanted dorsal tissue differentiated mostly into a notochord, while the ectoderm of the host dorsal tissue that was sitting above the transplanted region (blastopore lip) was induced and differentiated to form a Siamese twin containing dorsal tissues such as somites and a neural plate, which would form the central nervous system, forming the bulk of a second axis. In 1924, to understand the processes involved in developmental biology, Spemann and Mangold transplanted a blastopore lip between different ectodermal regions of amphibian embryos. ![]() ![]() ![]()
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