Cell death and trophic factors during development

The co-ordinated expression, in space and time, of many genes underlies neurodevelopment. Mutations in these 'neurodevelopmental genes' are increasingly being recognized as causes of developmental neurological disorders(15) such as cortical dysplasia and epilepsy; they may also be relevant to learning disability and schizophrenia. Different gene families are involved in the major component processes of neurodevelopment, such as organogenesis, neurogenesis, neuronal migration, synaptogenesis, and programmed cell death (apoptosis).(1, ,17> The details are beyond the scope of this book, but a few examples are given here.

Homeobox genes are important for the initial segmentation of the brain. Later, when cortical proneurones are migrating radially, their 'stop signal' is reelin, produced by specialized cells (Cajal-Retzius cells). In this way, reelin organizes the inside-to-outside layering of the cortex. Neurotrophins (growth factors) are genes which, as their name suggests, are critical for neuronal growth and survival, especially via their influence on apoptosis which is promoted by insuffiency of neurotrophins such as nerve growth factor and inhibited by enhanced nerve growth factor functioning. The effects of neurotrophins are mediated by specific tyrosine kinase (Trk)

receptors (Table 1). Classical neurotransmitters and their receptors are also involved in neurodevelopment, both directly in the formation of synaptic connections and indirectly through regulation of neurotrophins and Trk receptors; particular roles have been shown for glutamate, acting via N-methyl-D-aspartate receptors, as well as for g-aminobutyric acid and acetylcholine.

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Table 1 Important neurotrophins, their sites of synthesis in the central nervous system, receptors, and target structures

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