Age changes in the human oculomotor nerve - a stereological study

The oculomotor nerve (ON) provides motor innervation to the eyeball and exhibits alterations in various physiological and pathological conditions, which may result in abnormal ocular movement. Although the nerve has been studied in detail, very few data are available regarding its morphology and changes with aging. Hence, in the present investigation, the neural and connective tissue organizations of the pre-cavernous part of the ON were studied in order to provide sequential data regarding age-related morphological changes. Thirty-eight ON from cadavers aged from 40 post-natal days to 78 years were studied. Cross-sections of the nerve revealed a poorly defined multifascicular arrangement with predominant myelinated fibres of various calibres randomly intermingled with unmyelinated fibres. Small- and medium-sized fibres (probably parasympathetic) were mainly located at the junction of the central and the paracentral zones of the nerves. Using unbiased stereological techniques, the total number of the axons, the area of the myelinated fibres and myelin sheath thickness were estimated. The cross-sectional area of the nerve increased significantly up to the third decade. The minimal and maximal total number and area of myelinated axons varied from 17,000 to 21,000 and from 8.95 to 14.02 microm(2), respectively. There was a significant increase in the myelin thickness of axons with age. Connective tissue gradually increased in later decades and was more pronounced in the eighth decade. The present study provides novel baseline morphometric data on the ON that would be of help to future studies.