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Chapter 9: Summary of Cranial Nerves Essay | Medicine …
The superficial-to-deep difference appears to be more extreme when growth cone percentages per fascicle are plotted instead of absolute densities (Figs. 9A, squares). For instance, at E49 the percentage of growth cone profiles averages about 0.5% in central fascicles and is as high as 15% in superficial fascicles, a 30-fold difference. In comparison, the absolute density difference is only 4–5-fold. The explanation is that the average size of fibers in superficial and deep fascicles differs greatly. Central fascicles typically contain many more small fibers than do superficial fascicles (Fig. 10B, C). This in turn leads to an increase in axon packing density and a sharp decrease in the percentage of growth cone profiles in the total fiber population in these central fascicles. The reason there are more small axons in central fascicles is related to axon age: small axons are typically older; they have been in the nerve for a longer time, and their growth cones have progressed farther into the brain. In contrast, large axons are relatively young axons or even the trailing part, the shank, of the growth cone (Fig. 5; Williams and Rakic, 1985).
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Between E39 and E45, the optic nerve is transformed rapidly by the addition of thousands of fibers. The lumen of the stalk is obliterated, and the dorsal half of the nerve becomes filled with fibers. During this period, fiber number doubles approximately every 24 hr, and by E45 the nerve contains 380,000 ± 10,000 fibers and has a cross-sectional area of about 40,000 µm. Of this area, 70–75% is occupied by fibers; the remainder is occupied by glial cells and a few blood vessels.