Apoptosis plays a crucial role in brain development by ensuring that only appropriately growing, migrating, and synapse-forming neurons and their associated glial cells survive. This process involves an intimate relationship between cell-cell interactions and developmental cues and is further impacted by environmental stress during neurogenesis and disease. Oligodendrocytes (OLs), the major myelin-forming cells in the central nervous system, largely form after this wave of neurogenesis but also show a selective vulnerability to cell death stimuli depending on their stage of development. This can affect not only embryonic and early postnatal brain formation but also the response to demyelinating pathologies. In the present review, we discuss the stagespecific sensitivity of OL lineage cells to damage-induced death and how this might impact myelin survival and regeneration during injury or disease. Apoptosis is a major form of programmed cell death required for the normal development of metazoan tissues, including the brain. 1 During embryonic development of the brain, many more cells than ultimately needed are generated and then selection occurs, resulting in the apoptotic depletion of the surplus cells. The importance of the apoptotic pathway in early brain development has been demonstrated by the targeted deletion in mice of the death-specific cysteine proteases caspase-3 or caspase-9, or of the co-activator Apaf-1, all of which cause severe brain overgrowth and perinatal death. 2,3 In the adult brain, 90% of the cells belong to the glial lineage, which includes oligodendrocytes (OLs), astrocytes and microglia. The glia ensures proper development, function and repair of the neuronal network. This is possible through continuous cross-talk between the glia and neurons mediated by neurotransmitters, cytokines, growth and trophic factor secretion and signaling in a reciprocal manner. [4][5][6][7] In the central nervous system (CNS), OLs are responsible for axon myelination, which insulates the electrical signals transmitted between neurons. OL and neuron development is tightly regulated and the myelin sheath is constructed only when OLs reach maturity and neurons have grown appropriately. In response to injury or during the course of neurological diseases, the neuron-glia network can be replenished to some extent but the degree of repair is dependent on the developmental stage of the OL. This complexity is compounded by the differential sensitivity of OL lineage cells to apoptotic stimuli. In the present review, we will first briefly examine the stages of differentiation of OL cells and then discuss several diseases that are impacted by OL apoptosis, noting how the stage of cell differentiation governs the sensitivity to apoptosis.
Defined Stages of OL DevelopmentOligodendrocyte development can be divided into four distinct stages according to the temporal expression of cell surface markers and morphology (Table 1). 8 In the first stage, OL progenitor cells (OPCs) originate from the neuroepithelium of the ventricul...