We have studied the temporal and spatial characteristics of the development of 
unipolar brush cells (UBCs) in the human cerebellar vermis. Consistently with 
previous studies in rodents and cat, we have found that unipolar brush cells 
appear at a relatively late phase of cerebellar development and their 
development continues up to and beyond the first postnatal year. A series of 23 
normal human brains, including 5 adult and 18 fetal or infant brains (between 
the 24th gestational week and the 11th postnatal month) were used. In order to 
visualize unipolar brush cells, calretinin-immunocytochemistry was performed on 
formaldehyde-fixed, paraffin-embedded blocks of the cerebellar vermis. Our 
results show that calretinin-immunoreactive unipolar brush cells are not yet 
present in the cerebellar vermis at the 28th gestational week. At birth, they 
are present in a relatively small number, mostly in the vestibular lobules. At 
the 3rd, 5th, 8.5th and 11th postnatal months the number of 
calretinin-immunoreactive unipolar brush cells gradually increase, first 
appearing in the vestibular lobules, followed by the invasion of the later 
developing vermal lobules, spreading in a rostro-caudal and proximo-distal 
direction. Although at the 11th postnatal month unipolar brush cells exhibited 
adult-like morphological and distributional features, their number appeared to 
be lower than in the adult cerebellum. The late maturation of unipolar brush 
cells implies that the cytoarchitectonical development of the human cerebellum 
is not completed by the end of the first postnatal year.