Ebook Developmental neurobiology: Part 2
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Ebook Developmental neurobiology: Part 2
Cell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ach neuron, glial cell, sensory cell, and support cell must acquire highly specialized characters tics in order to contribute to the functions of the adult nervous system. The previous chapter discussed how vertebrate neuroepithelial (ells divide, establish neural precursors, and migrate to new loca Ebook Developmental neurobiology: Part 2tions where they will ultimately differentiate into fully' mature neurons, this chapter foe uses on some of the common mechanisms by which ( ells of tEbook Developmental neurobiology: Part 2
he invertebrate and vertebrate nervous systems transition from a precursor stage to acquire a particular cell fate. Processes regulating (ell fate delCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ng early embryogenesis, neuroepithelial (ells have the potential to form numerous cell subtypes. As development progresses, however, cells arc exposed to various signals that restrict their cell rate options. Depending on the specific precursor and the signals available, a given cell may remain mult Ebook Developmental neurobiology: Part 2ipotent that is. retain the ability to develop into more than one cell type—for an extended period. However, this ability only persists up until the lEbook Developmental neurobiology: Part 2
ime of cellular determination, the stage al which further embryonic development or experimental manipulation can no longer alter the type of cell thatCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2 characteristics associated with a particular cellular subtype.For some cell types, cell fate options become restricted early in the cell cycle in response to intrinsic cues, such as those that arise from nuclear or cytoplasmic, signals inherited from a precursor cell. For other cells, tale is large Ebook Developmental neurobiology: Part 2ly regulated by extrinsic cues encountered during migration or at the final destination. These extrinsic cues arc Otten the same types of signals discEbook Developmental neurobiology: Part 2
ussed in earlier chapters, such as extracellular matrix molecules and diffusible factors. A previously held view was that the fate of invertebrate preCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2 in these model systems, it is now recognized that such distinctions do not apply to all cells. Further, many intrinsic and extrinsic cues overlap148 CHAPTER 6 Cell Determination and Early Differentiationtemporally and spatially to influence cell fate, making It difficult to establish what cues pred Ebook Developmental neurobiology: Part 2ominate for any given cell population Despite the inherent challenges of sorting out the types of cues that direct cell fate decisions, several animalEbook Developmental neurobiology: Part 2
model systems have provided considerable insight Into the signaling pathways that establish cell fate.Here In Chapter 6, examples from selected regioCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ells. While the examples provided are by no means all-inclusive, they represent some of the most common and best understood mechanisms underlying cellular determina tion. Many of these basic mechanisms arc conserved across species, as well as across different regions of the nervous system in a given Ebook Developmental neurobiology: Part 2 animal model. Common mechanisms include lateral inhibition. Notch signaling, and temporally regulated transcription factor cascades. In recent yearsEbook Developmental neurobiology: Part 2
the importance of epigenetic modifications in regulating cell fate options has also been highlighted. Epigenetic modifications that lead to changes inCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2le to achieve a wide range of developmental outcomes.LATERAL INHIBITION AND NOTCH RECEPTOR SIGNALINGA cell passes through several stages prior to adopting a particular cell fair. As introduced in chapter i>, during early neurogenesis selected cells within the neuroepitheliuni begin Io express proncu Ebook Developmental neurobiology: Part 2ral genes—I he genes that provide a cell with the potential to become a neural precursor. The expression of proneural genes leads, in turn, Io the actEbook Developmental neurobiology: Part 2
ivation of transcription factors and neuron specific genes that influence the particular characteristics of a neuron. Cells that do not express proncuCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2l versus nonneuronal (ells is lateral inhibition, n pnx:ess that relies on the level of Notch receptor activity in a given cell. This process has been observed in invertebrate and vertebrate animal models, indicating it is an evolutionarily conserved mechanism for neural specification.Lateral inhibi Ebook Developmental neurobiology: Part 2tion designates future neurons in Drosophila neurogenic regionsIn the developing Drosophila nervous system, lhe areas of eclcxlerm that ultimately givEbook Developmental neurobiology: Part 2
e rise to the neurons are called the neurogenic regions. Cells Withili the neurogenic region begin to express low levels of proncural genes, such as aCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2luster I PNC), and at this stage of development each cell in the cluster has the polential Io become a neuron. Thus, al the earliest stages the cells are equivalent, with each cell expressing low levels of proncural genes.Through cell-cell interactions, one cell in the PNC becomes specified as a neu Ebook Developmental neurobiology: Part 2ral precursor, while the surrounding cells in the cluster become nonneuronal cells. An example of how this occurs involves the expression of the liganEbook Developmental neurobiology: Part 2
d Delta and the receptor Notch in cells of the PNC. In this example, the proneural genes of the achaete-scute complex (AS (?) initiate the expression Cell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ITION AND NOTCH KECEK1OR SIGNALING 14V(QFigure 6.1 Specification of neural precursors in Drosophila r.euroectoderm. (A) I o»v level:: of jxonnutal g::ni::;. such It: llitísc of the: actuate scute: complex (AS Q. begin to bi:j in .1 subsi:l of I>::ui:i<:<:k>i:c"ri culls called tin: pitincur.il duster Ebook Developmental neurobiology: Part 2 (I'NQ. All cells in thePNC express AS-C genes that promote expression ot Delta ligands. Notch receptors are also expressed in all cells of the PNC. sEbook Developmental neurobiology: Part 2
o at this stage all have the potential to become neurons. (B) Some cells within the PNC beg ' to express higher leve s of the Delta ligand. In th s exCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ent ofEbook Developmental neurobiology: Part 2
This lisxling to a ncxwKxaonalEbook Developmental neurobiology: Part 2
to Notch on an adjacent cell, initialing a Signal transduction cascade that ultimately leads one cell in the pair to a neuronal fale and lhe other celCell Determination and Early DifferentiationA wide range OÍ cell types is needed to perform the many diverse functions of lhe adult nervous system. Ea Ebook Developmental neurobiology: Part 2ain ÍN1CD) is then transported to the nucleus, where il forms a complex with other proteins and interacts With Suppressor of Hairless (Sull; Figure 6.1C, top). In the nucleus. Sull acts as a DNA binding protein that increases the expression of Enhancer of split |/;fsp/)|, which func lions, in turn, Ebook Developmental neurobiology: Part 2as a suppressor of neural fate by inhibiting the expression of proneural AS c genes. Thus, Delta binding to the Notch receptor initiates the pathway fEbook Developmental neurobiology: Part 2
or inhibiting neural fate in the Notch-activated cell. In addition, the Nolch-aclivalcd cell decreases its own expression of Delta ligand, so it is unGọi ngay
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