Development of myelinated nerve fibers in the sixth cranial nerve of the rat: A quantitative electron microscope study

Abstract

Myelination was studied quantitatively in the sixth cranial nerves of rats by counting and measuring all myelinated fibers during the first three postnatal weeks. In transverse semithin and thin sections cut serially at a well-defined anatomical site in the midsphenoid region, only a few axons (mean 12) were myelinated at birth. On days 2, 4, and 8, counts of myelinated fibers were respectively 5 times (mean 57), 20 times (mean 230), and 24 times (mean 273) the number seen at birth. During the second postnatal week, the number of myelinated fibers remained constant, whereas growth of axons and their myelin sheaths continued. By 15 days these fibers were large and relatively uniform in size; they had compact, circular myelin sheaths. During the third postnatal week, myelination of previously unmyelinated, smaller axons began. The number of myelinated fibers increased again and the size distribution of myelinated fibers became bimodal. Axon diameters, fiber diameters, and myelin sheath dimensions for all fibers were calculated from measurements made on electron micrographs. The transverse length of the myelin membrane increased exponentially with time. The growth increased rapidly during the formation of the first 20 spiral layers and remained relatively constant during the subsequent enlargement of the compact sheath. The association of axon diameter and myelin sheath thickness was poor at young ages, but it improved progressively with maturation of the sheath. The results show that myelination begins around axons that have a wide range of diameters. Also, the first axons to be myelinated become the large myelinated fibers of the sixth nerve. The small myelinated fibers originate from axons that do not become myelinated until the third postnatal week. Myelination, though differing in onset by 2 weeks, appeared to be similar in both populations as judged by similarity of sheath morphology and growth rates. It is of interest that at the level studied, the sixth nerve also contains a fascicle of unmyelinated cranial sympathetic fibers.

Schwann cell caveolin‐1 expression increases during myelination and decreases after axotomy

“…In some instances, it closely opposes or surrounds the P 0 -positive myelin sheaths. Caveolin-1 staining of myelinating SC is widespread at P30, when myelination is essentially complete (Hahn et al, 1987). Between P30 and P90, we did not appreciate a change in either the amount of caveolin-1 expression or its localization in myelinating SC: caveolin-1 is present largely in abaxonal membranes and in regions within the SC that surround P 0 -positive myelin sheaths.…”

In VivoNerve–Macrophage Interactions Following Peripheral Nerve Injury

“…Zebrafish motor nerves consist of ~70 motor axons (Myers, 1985; Westerfield et al, 1986), of which the large diameter axons are myelinated by Schwann cells, while the small diameter axons exhibit little or no myelination (Figure 1F). This is roughly equivalent to the degree of myelination seen within the first postnatal week in mouse and rat (Peters and Muir, 1959; Schlaepfer and Myers, 1973; Hahn et al, 1987; reviewed: Garbay et al, 2000). To visualize individual axons in the context of the entire nerve we stochastically labeled individual motor neurons using mnx1:dsRed .…”

Congenital Hypomyelinating Neuropathy with Lethal Conduction Failure in Mice Carrying the Egr2 I268N Mutation

“…Normal peripheral myelination in mice begins at around postnatal day 1 (P1), and is completed from P14-P30 (Hahn et al, 1987; Bermingham et al, 1996; Mirsky and Jessen, 1996). Egr2 I268N/I268N mice weigh the same as their wild-type and heterozygous (Egr2 I268N/+ ) littermates through the first two weeks of life, in contrast to Egr2 null, Egr2 hypomorphic, and Nab1/Nab2 null mice which are runted from birth (Topilko et al, 1994; Le et al, 2005a; Le et al, 2005b).…”