The Retina Optic Nerve Junction

The first part of the optic nerve is a site of transition where unmyelinated retinal axons become myelinated and enter the optic nerve proper (Fig. 22A; see Hildebrand and Waxman, 1983, 1984). The first 300 |im of the retina optic nerve junction (ROJ), which passes through the sclera, is composed of unmyelinated axons and fibrous astrocytic processes. Oligodendrocytes are absent. At places, the axons in this part of the optic nerve head present patches of an axolemmal undercoating with external astrocytic processes. As seen with freeze-fracture EM such axolemmal areas show a high density of large E-face particles (Hildebrand et al., 1985; Black et al., 1985b). Behind this segment follows a 250-| m long transition part, where an increasing proportion of the axons achieve myelin sheaths with increasing distance from the eye.

In the transition part of the ROJ, oligodendroglial processes appose some unmyelinated axons along part of their circumferences and form paranodal-like junction patches. Other axons are ensheathed by short cytoplasmic glial sheaths, or by 1- to 120-|m long myelin sheaths (Fig. 22B-E). Sheath

Figure 2 I Electron micrographs from a series of transverse sections through the nerve fiber layer of the adult rat retina. At each level, the same four axons are indicated by numbers 1-4. (A) Axon 1 shows no unusual features. Axon 2 has an undercoating, an axoplasmic cloud of electron dense material (arrow) and associated glial processes. Axons 3 and 4 are also apposed by glial processes and there may be a feeble undercoating at these sites. (B) Seven sections later (section thickness 65 nm) a distinct under-coating has appeared in the lower left half of axon 1 and several glial processes are associated to this area. The undercoating and the glial processes in relation to axon 2 persist. An undercoating is clearly present in the lower half of axon 3 but axon 4 does not show any specialization. (C) Eight sections later undercoated patches and glial profiles are seen in relation to the lower part of axons 1 and 3 but not in axons 2 and 4. Note the cloud of dense material in axon 1. (D) Another 27 sections later axon 3, but not axons 1, 2, and 4, retains an undercoated patch with associated glial adnexa. (x18,400.) (With permission from Hildebrand and Waxman, 1983.) (E) Freeze-fracture electron micrograph from adult rat retinal nerve fiber layer. Glial processes 1 through 4 abut the the axon A1 and the E-face of this axolemmal area is visible. Note that the E-face exhibits a higher occurrence of large intramembranous particles than does the E-face of the adjacent axon A2. Scale bar = 0.5 |m. (With permission from Black et al., 1984.)

Figure 22 (A) Light micrograph from longitudinal section through the retina (RET), the optic nerve head (ONH), the retina-optic nerve junction (ROJ) and the optic nerve proper (ONP) in the rat. In the retina and the ONH, all axons are unmyelinated; in the ROJ the axons gradually become myelinated; in the ONP nearly all axons are myelinated. (x28.) Scale bar = 500 |m. (With permission from Black et al., 1985b.) (B-E) Electron micrographs from longitudinal sections through the ROJ of the adult rat. (B) In this paranode about half the lateral loops face outward instead of apposing the axolemma (arrows). Arrowheads indicate nodal-paranodal border. (x26,100.) (C) Node of Ranvier (R = nodal axon). Note that isolated looplike profiles without obvious continuity with the myelin cover the nodal axolemma (arrows). (x11,500.) (D) This ROJ axon shows three successive myelin sheaths (1, 2, 3, arrowheads indicate sheath edges). The myelin sheath enclosed by the rectangular frame is 1.3 | m long only. (E). Sheath shown at higher magnification has four outer compacted and 2 inner uncompacted lamellae. Some of the lateral loops face outwards. In the 0.8 | m long gap between sheath 2 and 3, a small oligodendroglial profile apposes the axon (asterisk in E. On both sides of this profile the axolemma has an electron-dense undercoating (arrows in (E). (x17,900) D) and (x 37,600 E). (With permission from Hildebrand et al., 1985.)

length and number of myelin lamellae are not related to axon diameter. Between successive myelin sheaths are gaps of varying lengths, with or without a nodally differentiated axolemma. Freeze-fracture EM shows the presence of bands of large E-face particles in this region (Black et al., 1982; see Waxman, 1984). The optic axons have relatively small diameters where they are unmyelinated in the retina and optic nerve head, somewhat larger diameters in the ROJ, and clearly larger sizes in the optic nerve proper (Hildebrand et al., 1985). This suggests a trophic dependence of axon diameter on myelination in the rat optic system, similar to that seen in the PNS (Bray et al., 1981).

These results show that the shift from an unmyelinated to a myelinated state in the ROJ is gradual. In the transition zone, the axon-myelin relations are similar to those seen in remyelinated CNS fibers (Clifford-Jones et al., 1980; Blakemore and Murray, 1981), along myelinated dendrites (see later) and in some dysmyelinating conditions (Aguayo and Bray, 1982; Duncan et al., 1983). Similarly, freeze-frac-ture EM observations show the presence of axon membrane morphologies previously observed only in dysmyelinated mutants (Black et al., 1985a). Dysmyelinating conditions sometimes depend on a deficient oligodendroglial differentiation. It has been suggested that oligodendroglial differentiation in this area is inhibited through a local blood-brain barrier deficiency (Hildebrand et al., 1985).

0 0

Post a comment