Early fetal development of the human cerebellum.

Early cerebellum development in humans is poorly understood. The present study histologically examined sections from 20 human embryos and fetuses at 6 weeks (12-16 mm crown-rump length (CRL); 4 specimens), 7-9 weeks (21-39 mm CRL; 8 specimens), 11-12 weeks (70-90 mm CRL; 4 specimens) and 15-16 weeks (110-130 mm CRL; 4 specimens). During 7-9 weeks (approximate CRL 28 mm), the rhombic lip (a pair of thickenings of the alar plate) protruded dorsally, bent laterally, extended ventrolaterally and fused with the medially located midbrain. During that process, the primitive choroid plexus appeared to become involved in the cerebellar hemisphere to form a centrally located eosinophilic matrix. At that stage, the inferior olive had already developed in the thick medulla. Thus, the term 'bulbo-pontine extension' may represent an erroneous labeling of a caudal part of the rhombic lip. The cerebellar vermis developed much later than the hemisphere possibly from a midline dark cell cluster near the aqueduct. In the midline area after 12 weeks (80 mm CRL), the growing bilateral hemispheres seem to provide mechanical stress such as rotation and shear that cause the development of several fissures much deeper than those on the hemisphere. The rapidly growing surface germinal layer may be a minor contributor to this vermian fissure formation. The vermian fissures seem to enable inside involvement of the surface germinal cells, and to induce cytodifferentiation of the vermis. Consequently, in the early stages, it appears that the cerebellar hemisphere and vermis develop independently of each other.