Key-notes on Neural Induction
key notes
1. Neural Induction
All the cells of the vertebrate central nervous system derive from the neural plate, a region of columnar epithelium induced from the dorsal ectoderm during gastrulation.
Specification of the ectoderm
The ectoderm is the outermost germ layer, which covers most of the metazoan embryo after gastrulation and give rise to epidermis, neural tissue and (in vertebrate embryos) the neural crest.
In amphibians, the germ layers are specified early by distribution of maternal determinants in the developing oocyte. Factors involved in the specification of both mesoderm and endoderm have been identified, while no specific factors inducing ectodermal fate are known. Although a number of molecules influencing early ectoderm development have been isolated, they all appear to control the choice between epidermal and neural fates once the ectoderm is already specified. Thus, the ectoderm may represent a "default state" where cells do not receive signals that lead to mesodermal or endodermal specification. Transplantation experiments in which animal cap cells (presumptive ectoderm) were grafted to the vegetal hemisphere of an early blastula resulted in endodermal or mesodermal tissue. Conversely, the same experiments performed explanting from a late blastula stage embryo resulted in ectodermal tissue. Thus, although the ectoderm may be specified very early in development, its fate is not determined until the late blastula stage.
In amniotes,fate-mapping shows that presumptive ectoderm is represented by the epiblast anterior to the node and by lateral epiblast. Specification and determination of the ectoderm (as for the other germ layers) occurs later compared to amphibians, starting at gastrulation.The difference in timing between amphibians and amniotes germ layer specification probably relies in the fact that while early development in amphibians is strictly dependent on maternal factors differentially distributed in the egg, in amniotes, zygotic gene transcrition begins during early cleavage and there is extensive cell proliferation in the epiblast layer prior to and during gastrulation, allowing greater plasticity in patterning.
Fate of the ectoderm
Three major components derive from the ectoderm:
1) the neural plate (gives rise to the CNS)
2) the epidermis
3) the neural plate border (in between 1 and 2) - at the most anterior level gives rise to the ectodermal placodes (sensory organ development) - from the midbrain down, forms the neural crest (PNS+different cell types)
What is neural induction?
is the process by which the ectoderm on the dorsal surface of the vertebrate embryo forms the neural plate. The neural plate later folds to form the neural tube through a process called neurulation (Read chapter "Formation of the neural tube") and eventually develops into the brain and spinal cord.
The ectoderm is induced to form the neural plate during gastrulation under the influence of dorsalizing signals secreted by the underlying dorsal mesoderm of the organizer (the organizer is said to dorsalize or neuralize the overlying ectoderm). The neurogenic ectoderm begins to express neural cell markers (e.g. Sox2 and Sox3), and differentiates to form tall columnar cells. The ability of dorsal mesoderm of the organizer to induce overlying ectoderm to form neural plate has been discovered following the "organizer graft experiments" by Spemann and Mangold in 1924. (Watch the video on Spemann's experiments). The research on the molecular interactions underlying neural induction in different vertebrates is still ongoing.