Growth of the mouse corpus callosum.

Brains of BALB/cCF inbred mice were examined at 15 ages ranging from 16.5 to 50.5 days from conception and cross-sectional areas of major forebrain fibre tracts at the midsagittal plane were measured. The anterior commissure appeared prior to the corpus callosum (CC), which was first seen at midplane at 17.0 days, and both tracts underwent a very rapid increase in size in the prenatal and early postnatal period, reaching the adult range of size at about 1 week after birth or several days prior to the onset of myelination. The growth spurt of these fibre tracts was much more pronounced than that of whole brain. By comparing BALB/c mice with hybrid mice that always have normal CC, it was found that some BALB/c mice at 18.5 days of age which have very small or absent CC do so because the growth of the whole brain is retarded whereas others have CC that is small for the brain size. Evidence also suggested that many mice with no CC but normal brain size at 18.5 days prenatally do eventually acquire at least a small CC. Observations at 3 postnatal ages of BALB/c mice weighed and marked at birth revealed that the 'runts' with low birth weight, which were presumably retarded prenatally, either die or catch up with mice of normal birth weight and do not have unusually small adult CC.