Chapter 2 Neuron–glial cell cooperation From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved.

Figure 2.1 Fibrillary astrocyte. Micrograph of a fibrillary astrocyte stained with a Golgi stain observed through an optical microscope. The processes of this astrocyte make contact with a blood vessel: these are the terminal end feet. Photograph by Olivier Robain. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved.

Figure 2.2 Protoplasmic astrocytes are well positioned between neurons and vessels and organized in domains. (a) Diagram representing the position of the astrocyte in between blood vessels and synapses. (b) Diagram of the covering formed by astrocyte end feet around a capillary. (c) Tripartite synapse model showing that astrocytes detect the synaptic signal (1) and in turn regulate its efficacy (2). Parts (a, c) drawing by Aude Panatier. Part (b) from Goldstein G, Betz L (1986) La barrière qui protège le cerveau. Pour la Science, November, 84–94, with permission. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 3

Figure 2.3 Myelinating oligodendrocyte. Electron micrograph of an oligodendrocyte. The cell body and one of its processes enwrapping several axons can be seen. Section taken through the spinal cord. Photograph by Olivier Robain. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 4

Figure 2.4 Diagram and photomicrographs of myelinating oligodendrocytes and their numerous processes. (a) Each oligodendrocyte process forms a segment of myelin around a different axon in the central nervous system. Two myelin segments are represented, one partially unrolled, the other completely unrolled. (b) Maturation of oligodendrocytes grown on axons of dorsal root ganglionic (DRG) neurons in culture. DRG axons are immunostained for neurofilament (red), and cell nuclei are labeled with DAPI (blue). Upper part: oligodendrocyte progenitor cells (revealed by NG2 labeling in green) plated onto DRG axons (red) after 2 days in co-culture. They extend many processes which contact multiple axons. Lower part: after 7 days in co-culture, oligodendrocyte precursor cells have differentiated into myelinating oligodendrocytes which form multiple segments of compact myelin associated with the axons (myelin is shown in green by myelin basic protein, a component of the myelin sheath, immunolabeling). Scale bar = 15 μm. Part (a) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science33, with permission. Part (b) from Lee PR, Fields RD. (2009) Regulation of myelin genes implicated in psychiatric disorders by functional activity in axons. Front. Neuroanat.3, 4, with permission. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 5

6 From Cellular and Molecular Neurophysiology, Fourth Edition. Figure 2.5 Myelin sheath of central axons. (a) Three-dimensional diagram of the myelin sheath of an axon in the central nervous system (CNS). The sheath is formed by a succession of compact rolls of glial processes from different oligodendrocytes. (b) Cross-section through a myelin sheath. The dark lines, or major dense lines, and clear bands (in the middle of which are found the interperiod lines) visible with electron microscopy are accounted for by the manner in which the myelin membrane surrounds the axon, and by the composition of the membrane. The dark lines represent the adhesion of the internal leaflets of the myelin membrane while the interperiod lines represent the adhesion of the external leaflets. The lines are formed by membrane proteins while the clear bands are formed by the lipid bilayer. (c,d) Electron micrographs of myelinated axons in sciatic (c) and optic (d) nerves. Structural details of myelin ensheathment at the paranodal level (longitudinal section, c). Cross-section of myelinated axons (d). Numerous microtubules and mitochondria are present in the axons. The innermost layer of myelin sheath is often non-compacted. MVBs: multivesicular bodies. PNJ: paranodal junction. Part (a) drawing from Bunge MB, Bunge RP, Ris H (1961) Ultrastructural study of remyelination in an experimental lesion in adult cat spinal cord. J. Biophys. Biochem. Cytol. 10, 67–94, with permission of Rockerfeller University Press. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science 33, with permission. Parts (c,d) from Nave KA (2010) Myelination and the trophic support of long axons. Nat. Rev. Neurosci. 11, 275–283, with permission. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 6

Figure 2.6 Scheme of the different functions of microglia. Microglia (green) constantly move their processes to scan the brain parenchyma. During their movements they contact synapses and neuronal dendrites (orange), as well other brain cells. They can control brain activity and surrounding cells’ fate by releasing several factors. They phagocytose cells and neuronal debris, but also synaptic elements and newborn cells (orange), thus they participate in sculpting the neuronal circuits. Drawing by E. Avignone. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 7

Figure 2.7 Microglia change properties after activation. The images show an example of morphological changes of microglia 48 hours after activation induced by status epilepticus. In control conditions (a) microglial cells have a small body with long and ramified processes. (b) In contrast, activated microglial cells have larger body with shorter and thicker processes. From Menteyne A, Levavasseur F, Audinat E, Avignone E (2009) Predominant functional expression of Kv1.3 by activated microglia of the hippocampus after status epilepticus. PLoS One 4, e6770, with permission. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 8

Figure 2.8 Myelin sheath of a peripheral axon. (a) Three-dimensional diagram of the myelin sheath of an axon of the peripheral nervous system (PNS). The sheath is formed by successive rolled Schwann cells. (b) Process of myelinization. The internal loop wraps around the axon several times. During this process, the axon grows and the myelin becomes compact. Contact between the Schwann cell and axon occurs only at the paranodal and nodal regions. Elsewhere an extracellular, or periaxonal, space always remains. Part (a) drawing adapted from Maillet M (1977) Le Tissu Nerveux, Paris: Vigot, with permission. Part (b) drawing by Tom Prentiss. In Morell P, Norton W (1980) La myéline et la sclérose en plaques. Pour la Science33, with permission. From Cellular and Molecular Neurophysiology, Fourth Edition. Copyright © 2015 Elsevier Ltd. All rights reserved. 9