Oligodendrocytes develop from migratory, proliferating progenitor cells called O-2A progenitors. These cells develop from neuroepithelial cells in the walls of the embryonic neural tube. Two markers are used to identify oligodendrocyte precursor cells: the chondroi-

tin sulfate proteoglycan NG2 and platelet-derived growth factor a receptor (PDGFaR). PDGFaR-posi-tive cells show a distribution similar to that of NG2 + cells. NG2+ cells are first detected in the developing ventral spinal cord at E14 and are found in the hindbrain and within the basal forebrain by E16. At birth NG2 cells are distributed widely in the CNS, indicating a rapid increase in cell distribution. From E17 to birth, all cells positive for NG2 also express PDGFaR, strongly suggesting that NG2+ cells are markers for oligodendrial precursors. In vivo NG2 + cells form oligodendrocytes, but in tissue culture these cells can also form astrocytes as well as oligodendro-cytes. However, there is no evidence that this occurs in vivo and it may be an artifact of the culture conditions. It has also been shown that NG2+ cells persist in the mature CNS, indicating that not all of the NG2+ cells form oligodendrocytes but may have some additional function. However, the question remains whether these NG2+ cells are a separate cellular entity than oligodendrocytes. NG2+ cells in the mature CNS have a complex morphology, which is different from that of oligodendrocytes. They have highly irregular cell bodies from which fine branches radiate. In gray matter NG2+ cells lie closely apposed to neuronal cell bodies, whereas in white matter NG2+ cells are tightly packed between myelinated axons and have elongated cell bodies. These NG2+ cells are slowly dividing and have been implicated as having a role to play during remyelination. There are increased numbers ofNG2 + cells at lesion sites, and there is limited evidence that these cells form new remyelinating oligodendrocytes. There is also evidence that separate progenitor cells may give rise to oligodendrocytes. These cells are characterized by the expression of plp/dm-20, and during development evidence exists that plp/dm-20 expressing cells differentiate into oligodendrocytes and that these plp/dm-20 precursors do not depend on PDGFaR to proliferate, as they do not appear to be precursors of PDGFaR progenitors.

Breaking Bulimia

Breaking Bulimia

We have all been there: turning to the refrigerator if feeling lonely or bored or indulging in seconds or thirds if strained. But if you suffer from bulimia, the from time to time urge to overeat is more like an obsession.

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