In the human corticospinal tract (CST), the anterior CST is known to be much thinner than the lateral CST, but fetal development of the CST remains obscure. In this study, we examined horizontal histological sections of the cervical spinal cord from 50 midterm fetuses (crown-rump length, CRL, 70-150 mm; 12-18 weeks) as well as 15 late-stage fetuses (CRL 230-250 mm; 28-30 weeks). While the lateral CST appeared at 14-15 weeks in the cervical cord of the midterm fetuses, we found the anterior CST in only 3 fetuses: CRL 117 mm (15 weeks), and CRL 144 and 150 mm (18 weeks). However, in the late-stage fetuses, the anterior CST exhibited a spectrum of variations, including bilaterally thick tracts (3 fetuses), a unilaterally thick tract (5 fetuses) and a thin or absent tract (7 fetuses). In the group with the thickest anterior CST, the cross-sectional area reached 40% of that of the lateral CST. In contrast to the stable morphology of the lateral CST, the shape of the anterior tract varied slightly between the cervical segments. Immunohistochemical observations demonstrated no difference between the lateral and anterior CSTs: (1) in macrophage infiltration, which suggests apoptosis, and (2) in the arrangement of radial glial fibers. Our findings indicate that the development of the anterior CST starts later than that of the lateral tract, with significant individual variations. The functional relevance of the anterior CST, if present, might not be found in an independent cortical input to motor neurons but in a remodeling of the ascending tracts.
© 2016 S. Karger AG, Basel.