Visualization of a Single Myelination Process of an Oligodendrocyte in Culture by Video Microscopy.

Abstract

ABSTRACT. Wedescribed the initial events in the interaction between an oligodendrocyte process and an axon in culture utilizing video time-lapse microscopy. Myelination of an axon by the lamellipodium of an oligodendrocyte was achieved in several steps of cellular process development and coordinated interaction between axon and oligodendrocyte. The initial stage of contact included the formation of a lamellipodium process at the end of an oligodendrocyte process. It appeared that this process contacted the axon several times and was then retracted, and that the filopodia and lamellipodium underwent morphological changes prior to the onset of the myelination. In the second stage, the lamellipodium appeared to thicken and anchor to the axon. Finally, when rippling of the lamellipodial ruffling occurred, the angle between the anchoring filopodium and the axon changed depending on the direction of lamellipodial movement, and the lamellipodium, which was folded in layers, wrapped around the axon like a transverse wave in one motion as observed on the video screen. Thereafter, the lamellipodium assumed a "bursting" form within minutes in real time. This is the first comprehensive overview of howan oligodendrocyte plasma membranewraps around an axon to form myelin.Oligodendrocytes are the myelin-forming cells of the central nervous system (CNS). Myelination is highly specialized and one of the major events occurring during brain development. The myelin is present as a sheathlike structure consisting of tightly packed oligodendrocyte plasma membranewhich surrounds axons in the CNS. Myelin compaction occurs almost immediately during myelination in the CNS (6, 16), but fixation difficulties with immature CNStissue prevent consistent demonstration of intraperiod lines in these tissues (17).Their absence led to speculation that intraperiod lines in CNSmyelin occur only in the mature sheath, which is formed at a later stage. The literature on the ultrastructure of myelinated fibers has been reviewed by Remahl and Hildebrand (20,21). In their review, the fine structural aspects of the axon-oligodendrocyte interaction during initial myelination are clarified, and it is described that at the first step in myelination, the axon becomes ensheathed by one to three uncompactedlayers of oligodendrocyte plasma membrane.In addition, the relationship between the number of myelin lamellae and axon diameter and the diameter for initial myelination are related to axonal fiber size (8, 19). Myelination in the CNScommencesseveral days after is onset in the PNSand is initiated when axons to be myelinated reach a diameter of approximately 1 fjtm (7, 17). Little is knownabout the timing of the induction of differentiation of oligodendrocytes and how they form the myelin sheath (3, 9). Moreover, the initiation and precise mechanism of myelination in the CNSremain a puzzle. In the present study, using video time-lapse microscopy we have clearly demonstrated how oligodendrocyte processes develop, form contacts, and wrap around the axon during in...

In vitro myelin formation using embryonic stem cells

“…Surprisingly, we observed that the process anchored on the axon but continued to survey. This observation is consistent with earlier work by Asou et al (1995), although that work was conducted at much lower resolution and for a shorter time period. Additionally, it has been suggested that the interaction between CASPR and its binding partner on the glial membrane, NF155, is never broken from the initial contact to completion of myelination (Pedraza et al, 2009).…”

“…As a consequence, most of the existing models of myelination predict that the front edge of the oligodendrocyte process continuously grows while wrapping the axon: for example, the 'carpet crawler' model, in which the oligodendrocyte process flattens out and expands after contact with the axon and then wraps the axon like a rolling carpet (Bauer et al, 2009;Bunge et al, 1989); the 'serpent' model, in which the oligodendrocyte process wraps around the axons in a helical fashion first, like a corkscrew, and then expands into overlapping sheets (Asou et al, 1995;Bauer et al, 2009;Ioannidou et al, 2012;Knobler et al, 1976); and the 'liquid croissant' model, in which the oligodendrocyte process continuously wraps and spreads sideways along the axon (Snaidero et al, 2014;Sobottka et al, 2011). Alternatively, through a detailed analysis of CASPR and NF155 (NFASC -Mouse Genome Informatics) localization during myelination, Pedraza et al (2009) proposed that the initial contact between the axon and the oligodendrocyte process persists throughout myelination and the membrane wraps the axon like a yo-yo before expanding laterally along the internode; then, the leading edges of all the overlapping layers expand and draw closer at the paranodes.…”

Subtype‐specific oligodendrocyte dynamics in organotypic culture

“…This is consistent with the concept that oligodendrocyte processes that find appropriate axonal contacts are stabilized and those that do not are pruned. As glialaxon contact is established, membrane ensheathment is believed to be initiated by a dramatic series of process extension and retraction events (Asou et al, 1995b). In some cases, we observed two distinct oligodendrocyte processes appearing to join into a single membrane ensheathment (Fig.…”

N‐WASP regulates extension of filopodia and processes by oligodendrocyte progenitors, oligodendrocytes, and Schwann cells—implications for axon ensheathment at myelination

“…Rumsby et al, 2003;Song et al, 2001), whereas WAVE-1, a regulator of the Arp2/3 complex (Miki and Takenawa, 2003), localizes to OL process tips (Kim et al, 2006). The tips of the thin processes extended by an OL broaden into more traditional lamellipodia on contact with axons and extend filopodia to the axon for anchorage (Asou et al, 1995); these lamellipodia show membrane ruffling, which is due to actin polymerization (Ridley, 1994) regulated by Rac and WAVE proteins (e.g. Miki et al, 1998a,b;Takenawa and Miki, 2001) and IRSp53 (Miki et al, 2000).…”