ABSTRACT
The preceding chapter summarized the capability of magnetic resonance imaging (MRI) to assess normal and abnormal anatomy with the fetal central nervous system in utero. One of the most difficult structures to assess on ante-natal ultrasonography is the developing cerebellum. Therefore, this chapter concentrates on the normal and abnormal development of that structure. It is important to understand some of the detailed sequential development of the structures in the posterior fossa, particularly the cerebellar vermis. We therefore commence with a description of earlier MRI studies of fetal specimens. This allows us to review the anatomical features that will be important when we perform in utero studies described in the second part of the chapter.
The cerebellar vermis starts development as a thickening of the alar plate of the rhombencephalon during the 5th week of gestation (1). This thickening forms the rhombic lip which is continuous across the midline (Fig. 1). The cerebellar hemispheres grow on both sides, and the vermis forms along their line of fusion in the midline, starting in the 9th week of gestation.
