NE101 Lecture Notes

Differentiation occurs when gene expression patterns

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Differentiation occurs when gene expression patterns change Gene expression refers to the pattern of when and where genes are “turned on” or “turned off”. Different cell types express different suites of genes All cells contain the entire genome Subset of genes are expressed If other sets of genes were expressed, cell would differentiate into a different type of cell Differentiation can occur due to an internal program ( cell-autonomous differentiation) or due to signals from other surrounding cells ( induction ). Fig. 7.4 in book (cell-autonomous differentiation) In vertebrates, differentiation is influenced more strongly by where a cell is and what other cells are around it (induction) notochord induces nearby spinal cord neuronal precursors to differentiate into motor neurons October 16, 2012: Nervous System Development I (cont.) Synaptogenesis The process by which differentiating neurons extend processes (axons and dendrites) and form synapses with other neurons chemoattractants and chemorepellants affect growth cones cell types have different responses to these chemotropic factors, helping system to wire up correctly Why? They express different genes Neuronal cell death During and after synaptogensis, man neurons die. Process is adaptive, occurs via programmed cell death ( apoptosis ) Neurons compete for neurotrophic factors 1. Different neurotrophic factors are produced by different target cells 2. Innervating neurons take up particular neurotrophic factors and transport them to their somata. 3. Upon reaching the cell body, neurotrophic factors regulate the expression of various genes, affecting the development of the neuron 4. Early in development, neurons that manage to gather sufficient amounts of the appropriate neurotrophic factor survive. Neurons that gather insufficient trophic factor undergo apoptosis 5. Because the amount of neurotrophic factor matches the number of target cells, this process results in a rough matching of the size of the target and the number of innervating cells. Synapse rearrangement Refers to the loss of some synapses and development of others Neurotrophic factors also regulate the number of synapses in the developing brain Later in development, axonal processes also appear to compete for limited amounts of various neurotrophic factors. Active synapses seem to compete more successfully than inactive synapses. Because experience can modulate synaptic activity, different experiences can result in the maintenance of different patterns of synaptic connectivity. In humans, the density of synapses declines after the first year of life. (pruning) plasticity is reduced when process is completed Neuronal cell death and synaptic rearrangement represent built-in redundancy.
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