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Đối với các thuộc tính của tế bào nguyên bản vỏ não, cân nhắc về sinh lý cũng xuất hiện để phù hợp với các quan sát của chúng tôi. Vỏ não là một trong những khu vực cuối cùng của hệ thần kinh trung ương, trong đó myelin hóa được bắt đầu, và quá trình myelin hóa cũng có thể tiếp tục trong thời gian | The Oligodendrocyte Chapter 6 167 optic nerves of embryonic rats and postnatal rats have been compared Gao and Raff 1997 . With respect to the properties of cortical progenitor cells physiological considerations also appear to be consistent with our observations. The cortex is one of the last regions of the CNS in which myelination is initiated and the process of myelination can also continue for extended periods in this region Macklin and Weill 1985 Kinney et al. 1988 Foran and Peterson 1992 . If the biology of a precursor cell population is reflective of the developmental characteristics of the tissue in which it resides then one might expect that O-2A OPCs isolated from this tissue would not initiate oligodendrocyte generation until a later time than it occurs with O-2A OPCs isolated from structures in which myelination occurs earlier. In addition cortical O-2A OPCs might be physiologically required to make oligodendrocytes for a longer time due to the long period of continued development in this tissue at least as this has been defined in the human CNS e.g. Yakovlev and Lecours 1967 Benes et al 1994 . The observation that O-2A OPCs from different CNS regions express different levels of responsiveness to inducers of differentiation adds a new level of complexity to attempts to understand how different signaling molecules contribute to the generation of oligodendrocytes. This observation also raises questions about whether cells from different regions also express differing responses to cytotoxic agents and whether such differences can be biologically dissected so as to yield a better understanding of this currently mysterious form of biological variability. If there are multiple biologically distinct populations of O-2A OPCs it is important to consider whether similar heterogeneity exists among oligodendrocytes themselves. Evidence for morphological heterogeneity among oligodendrocytes is well established. Early silver impregnation studies identified four .
