Development of the human oculomotor nuclear complex: somatic nuclei.

Background: Precise anatomical data on the development of human oculomotor somatic nuclei (OSN) remain rare.

Design/subjects: This study describes the histology of human OSN in 11 preterm and full-term infants aged 20-43 postmenstrual weeks who died of various causes. Celloidin-embedded serial sections were stained with the Klüver-Barrera and other conventional methods including silver impregnation. To evaluate the growth of OSN quantitatively, the author estimated the nuclear volume and the average neuronal area on morphometry.

Results: Four subnuclei were identified at 20-21 weeks: the fascicular, principal, dorsal median, and ventral median nucleus. Early tigroid Nissl bodies appeared in presumed motoneurons by 27-28 weeks, then resembled adult Nissl bodies at birth. On silver impregnation, the oculomotor nerve roots, crossed or uncrossed fibers at the midline, and a plexus of efferent or afferent axons in the neuropil were observed at 20-21 weeks. Then, the plexus was elaborated to form a perineuronal net of thin axon terminals by 28-29 weeks. The nuclear volume of OSN exponentially increased with age over 20-43 weeks, while the average of neuronal profile areas linearly increased in each subnucleus; the coefficient of regression was largest in the principal nucleus, and the regression lines nearly overlapped among the other subnuclei. Statistical analysis confirmed that the average neuronal area was largest in the principal nucleus in older cases.

Conclusion: This study suggests that four subnuclei can be distinguished in human OSN by mid gestation, and that the principal nucleus may be different in neuronal cytoarchitecture from the others.