The ectoderm germ layer forms a variety of structures in the body. By far the most complicated and interesting structure formed from the ectoderm is the nervous system. From the established ectoderm layer of the gastrula, neural tissue is derived by a series of tissue inductions, movements, and differentiations. There are a huge number of proteins, genes, and other factors which take part in this complex process. New differentiation factors are discovered each day. This site will only cover a few primary factors.
The general differentiation of the neural tissue starts with the notochord. The notochord (derived from the mesoderm) is the primary inducer of the neural plate. Two signaling molecules, noggin and chordin, which are released by the notochord, induce the overlying ectoderm to thinken into the neural plate. The two molecules both function by blocking the action of bone morphogenic protein-4 (BMP-4). BMP-4 is also critical in mesodermic and hematopoietic development. It inhibits ectoderm from differentiating to neural plate tissue. This in vivo action has been reproduced in in vitro experiments. Under these conditions, the neural plate develops forebrain characteristics. Neural ectoderm induced in the presence of Fibroblast Growth Factor-8 (FGF-8) will develop more caudal features of the spinal chord. FGF's also play a role in liver development.
Along the length of the neural tube, neuroepithelial cells proliferate. Within this neuroepithelium exist multipotential stem cells of the nervous system. During the development of the embryo, these ES cells differentiate into a variety of cell lineages which eventually give rise to the multiple types of mature cells of the adult nervous system (see photo below). Many of these stem cells are only found in certain areas of the developing nervous system. As they begin to differentiate, certain cells must migrate from these primordial locations to the proper location of the adult cell. Of particular interest is the O-2A progenitor cell lineage because it gives rise to oligodendrocytes and type-2 astrocytes.
GO to Neural Tissue Differentiation.
