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Using 2-3 specific examples, discuss how molecular mechanisms underlie neural development.

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Introduction

Using 2-3 specific examples, discuss how molecular mechanisms underlie neural development The entire nervous system is derived from ectoderm, but not all ectoderm gives rise to the nervous system. By this token, there must be some mechanism by which some ectodermal cells undergo neural differentiation, and others fail to follow this path, and yield skin epidermis. Much research has been done into what determines neural differentiation, or which factors direct the expression of specific genes within individual cells. Two major groups of factors have been described in response to this question: inducing factors; and molecules activated or induced in cells upon exposure to an inducing factor themselves. This latter group relies on the ability of the cell to respond to inductive factors (known as competence), and this depends on the precise repository of receptors etc that is expresses. With inducing factors, cells in different positions are going to be exposed to different inducing factors- so it is really the position of the cell in the early on, that is of critical importance in determining its fate. Hans Spemann and Hilde Mangold did an experiment whereby the organiser region of the mesoderm (later gives rise to notochord) was transplanted in amphibian embryos. The startling results showed that the second organiser developed into a notochord as it usual, but also the graft had remarkable effect on the surrounding ectoderm of the host- a duplicate body axis and second nervous system developed. ...read more.

Middle

By making Shh knockouts, one can block the ability of the notochord to induce virtually all cell types in the ventral neural tube. Thus this one protein is both necessary and sufficient for induction of most cell types generated in the ventral half of the neural tube. So how does Shh determine the fate of so many different cell types in the ventral half of the CNS? Well, it can act both as an inducer, and a morphogen (inductive signals that can direct different cell fates at different concentration thresholds). So, at low concentrations of Shh, ventral interneurons are induced. Higher concentrations cause motor neurons to be induced, and even higher Shh concentrations causes the induction of a ventral floor plate. It would be plausible, with Shh being such a potent signalling molecule, that dorsal neural tube patterning would also be induced by Shh, or perhaps by the lack of it (i.e. default pathway). However, a separate class of factors has been elucidated in induction of dorsal cell differentiation. These are the BMPs (same family as those involved in control of neural induction). Dorsal patterning seems to involve several members of the BMP family- each of which may induce a particular set of cells. Patterning of both halves of the neural tube is similar in that inductive signals for both are initially expressed by non-neural cells (epidermal ectoderm and notochord for dorsal and ventral respectively). ...read more.

Conclusion

So how do axons do it? There are a number of techniques employed by the nervous system to ease the pressure: some axons crawl along epithelial or extracellular surfaces, and sometimes 'guidepost' cells mark sites where axons must make divergent choices. Some axons that grew earlier in development, when distances were shorter, can pioneer routes, which growth cones of newer axons may follow, by cell-cell adhesion. There are two important classes of molecules involved in this latter method- N-CAM (of the superimmunoglobulin family) and N-Cadherin (of the Ca++-dependent cadherin family). Both of these molecules are generally present on the surfaces of growth cones, axons, glial cells, muscle cells, and other cell types growth cones may crawl over. I mentioned the growth cone- this is a spiky enlargement of the tip of the process, and is responsible for guiding the neurite to the correct position. The growth cone acts as both a sensory and motor structure. It puts out filopodia and lamellipodia to sense the environment in which it is growing, and these projections may retract if unfavourable conditions are contacted, or persist if favourable. The growth cone has a multitude of receptors, which allow accurate detection of external signals. It also possesses cytoskeletal proteins and actin-based motors that propel it forward. The discoveries made about neurite guidance are making for exciting times in this extraordinary field of neuroscience, and have broad application in the field of neuro-medicine. ...read more.

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