The human trochlear and abducens nerves at different ages - a morphometric study

Abstract

The trochlear and abducens nerves (TN and AN) control the movement of the superior oblique and lateral rectus muscles of the eyeball, respectively. Despite their immense clinical and radiological importance no morphometric data was available from a wide spectrum of age groups for comparison with either pathological or other conditions involving these nerves. In the present study, morphometry of the TN and AN was performed on twenty post-mortem samples ranging from 12-90 years of age. The nerve samples were processed for resin embedding and toluidine blue stained thin (1µm) sections were used for estimating the total number of myelinated axons by fractionator and the cross sectional area of the nerve and the axons by point counting methods. We observed that the TN was covered by a well-defined epineurium and had ill-defined fascicles, whereas the AN had multiple fascicles with scanty epineurium. Both nerves contained myelinated and unmyelinated fibers of various sizes intermingled with each other. Out of the four age groups (12-20y, 21-40y, 41-60y and >61y) the younger groups revealed isolated bundles of small thinly myelinated axons. The total number of myelinated fibers in the TN and AN at various ages ranged from 1100-3000 and 1600-7000, respectively. There was no significant change in the cross-sectional area of the nerves or the axonal area of the myelinated nerves across the age groups. However, myelin thickness increased significantly in the AN with aging (one way ANOVA). The present study provides baseline morphometric data on the human TN and AN at various ages.

Keywords: morphometry; myelin thickness; ocular motor nerves; stereology.

Figure 1.

Cross-sections of the trochlear nerve in various age groups. Panel of low magnification (left column) and high magnification (right column) photomicrographs of toluidene blue stained semithin resin sections of trochlear nerves showing the Epineurium (Epi), Perineurium (Peri) and a bundle of Small Thinly Myelinated Axons (STMA). The darkly stained profiles seen in the micrographs are the myelin sheaths surrounding the axons of the trochlear nerve. A. and B. are of a 14 year old female; C. and D. are of a 38 year old female; E. and F. are of a 51 year old male and G. and H. are of a 71 year old male. Scale bar: A = C = E = G = 300 μm. B = D = F = H = 50 μm.

Figure 2.

Cross-sections of the abducens nerve in various age groups. Panel of low magnification (left column) and high magnification (right column) photomicrographs of toluidene blue stained semithin resin sections of trochlear nerves showing the Epineurium (Epi) and Perineurium (Peri). The darkly stained profiles seen in the micrographs are the myelin sheaths surrounding the axons of the abducens nerve. A. and B. are of a 16 year old female; C. and D. are of a 24 year old female; E. and F. are of a 45 year old male and G. and H. are of a 61 year old man. Scale bar: A = 500 μm. C = E = G = 300 μm. B = D = F = H = 50 μm.