Development of the human trochlear nucleus: a morphometric study

Summary Background The trochlear nucleus, the smallest of the extraoculomotor nuclei, is unique or even curious, because the nerve roots emerge dorsally from the superior medullary velum after decussation. Little information is available on the developmental anatomy of this nucleus in humans. Design/subjects We examined serial brain sections from 10 premature infants aged 20–39 weeks of gestation to document the histology and morphometry. Results The trochlear nucleus was composed of three parts: the rostral tip, the main body, and the caudal division. The rostral tip was a rostral continuation of the main body, being closely related to the oculomotor nucleus; the main body was enveloped by a fibrous capsule; the caudal division was a small separate cluster of neurons in the medial longitudinal fasciculus or the root fibers with individual variations. Tigroid Nissl bodies first appeared at 28 weeks in presumed motoneurons. Various sizes of motoneurons were recognized; medium-sized to small motoneurons were preferentially accumulated in the rostral tip. Among the motoneurons, presumed non-motor neurons were infrequently scattered. Morphometric analysis showed that the nuclear volume exponentially increased with age, about 15 fold over 20–39 weeks, while the average profile area of the neurons linearly increased. Statistical analysis confirmed that cell area was smallest in the rostral tip among the three parts. Conclusion Although the sample number is small in this study, it suggests that the human trochlear nucleus can be divided into three parts, and that the overall growth may be accelerated at about 30 weeks of gestation.