In 1924, Hans Spemann and Hilde Mangold experimentally demonstrated that, when explants of tissue were removed from the dorsal blastopore lip of developing embryos and transplanted to their ventral surface, a second set of neural folds developed from the recipient tissue and proceeded to form a secondary central nervous system.
At the time, it was assumed that the default fate of the ventral tissue was epidermal and that some substance within the dorsal blastopore lip - now known as Spemann´s Organizer - must be acting to induce the formation of neural tissue.
The next seventy years were spent pursuing an exhaustive survey of potential neural inducers, but the collected data appeared only to complicate and confuse our understanding mechanisms and substances involved in this process.
Over the last decade the mechanistic mysteries of neural tissue formation during embryogenesis have been resolved and clarified.
Contrary to the long held assumption that neural tissue formation is secondarily induced by signals from surrounding tissues - it has now been demonstrated that the neuralization is the default developmental pathway for embryonic cells at the blastula stage unless they are specifically acted upon by inducers for the formation of epidermal, mesodermal, and endodermal tissues ( ).
BMP signalling has been shown to be necessary for the inductive formation of epidermal tissue.
The BMP signalling pathway is known to be inhibited by noggin, follistatin, and chordin - all substances once suspected of being Spemann´s neural inducer.
The diffusible substances noggin and chordin are know to directly bind to BMP4, as well as with BMP2 and BMP7, preventing it from binding to it´ receptor.
The central nervous system of vertebrates arises from the neural tube that forms early in chordate development.
Dorsal-ventral patterning of the neural tube appears to be a conserved feature of chordate development, with similar genes and gene products playing similar roles in all chordates.
Unlike vertebrates who undergo regulative development, ascidian tunicates demonstrate a highly determinative mode of development with cell fates being largely predetermined during embryogenesis.
The early embryonic neural plate of chordates is induced to form from the overlying ectoderm via signals received from the notochordal plate.
Both the neural plate and the notochordal plate begin to form tubular structures and this process eventually result in the formation of a hollow dorsal neural tube and notochord, respectively.
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Neural crest tissue emerges from the margins of the folding neural tube as a population of migratory cells that will travel throughout the developing vertebrate bodyplan and differentiate into a wide variety of cell types involved in integrating the somatic and visceral bodies into a single functional unit.
In 1983, Carl Gans and Glenn Northcutt suggested that the neural placodes and neural crest tissue were definitive of vertebrates and associated with the vertebrate origin event (, ).
The neural crest tissue is a uniquely vertebrate developmental tissue, arising at the dorsal folds of the neural tube, that is responsible for the generation of a range of diverse tissue types that facilitate organismal integration and are characteristic components of vertebrate novelties.
Brian K. Hall has suggested that the neural crest is the equivalent of a fourth germ layer and that vertebrates should be considered to be "quadro-blastic" rather than "triplo-blastic" animals (, ).
The neural crest cells provide many of the embryological tissues and cells required to bring about the "functional welding" of the "somatic" and "visceral" bodies.
Neural crest tissue gives rise to the following tissue types:
The neural crest tissue contributes unmyelinated neurons to the parasympathetic nervous system and in gnathostomes will also provide the tissues necessary for the development of the myelinated sympathetic system.
In vertebrates, neural crest tissue is derived from HNK-1 and Zic positive cells that migrate from the dorsal margins of the closing neural tube.
Recently Jeffrey et al. (, ) have suggested that the vertebrate neural crest may have it´s evolutionary homolog in cells that "emerge from the neural tube, migrate into the body wall and siphon primordia, and subsequently differentiate as pigment cells." within tunicates - and possibly chordates more broadly.
This suggestion is supported by the fact that these tunicate pigment cells express the HNK-1 antigen and Zic gene markers that are localized to neural crest in vertebrates, thereby allowing identification of neural crest cells in extant vertebrate development.
Emergence of the neural crest at such early stages of vertebrate neural development and the emergence of migratory pigment cells in metamorphosizing tunicates suggests that the timing of gene expression leading to the differentiation of the tunicate pigment cells may have been shifted to an earlier stage of development, perhaps facilitating the emergence of neural crest pluripotentiality as the embryonic cells adopted to the novel internal physiological ecology of the somatoviscerally fused organism and the genome duplication event.
Recently it has been suggested ( ) that there is a population of "VENT cells" that migrate from the ventral neural tube in the cranial region during development, out along the path of the cranial nerves, past the migrating neural crest cells, and into the following regions:
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VENT cells are able to differentiate into a wide range of tissues, becoming virtually indistinguishable from other cells in the tissue, but are only sparsely present in any given tissue.
It is postulated that during development VENT cells are responsible for guiding migratory neural crest neurons to their destinations suggesting that they may play an important role in developing the integration between somatic and visceral divisions of the body.