The corpus callosum (CC) is a large bundle of axons that interconnect neurons in the two
cerebral hemispheres. Several years ago we rediscovered something originally noted by
Richard Wimer in the mouse strains BALB/c and 129: often there is no corpus callosum. This
neurological defect is especially interesting because BALB/c mice are highly inbred and
therefore genetically homogeneous. Nonetheless, the outcome of development is highly
variable; littermate embryos adjacent to each other in the uterus often have radically different
brains, one being quite normal and the other having no CC. This variation arises from a third
source of individual differences generated inside the embryo and is neither hereditary nor
environmental. We have utilized this within-strain variation to study the causes of abnormal
axon rerouting and the behavioural consequences of having no CC. Genetic experiments
revealed that the difference between the BALB/c and 129 strains pertinent to CC development
most likely entails only two major loci. We are now working on a genetic linkage study to
localize these genes to chromosomal regions.
Three inbred mouse strains are currently maintained in the laboratory because of their special relevance to the problem of absent CC. Limited numbers of animals are available to qualified investigators for collaborative research and to establish separate breeding colonies.
BALB/cWah1 was originally derived from BALB/cJ via Carworth Farms and has been inbred in my lab since 1976. About 20% of mice have no CC and another 20 to 30% have an unusually small CC. The hippocampal commissure (HC) is almost always normal.
9XC-1/Wah is a recombinant inbred strain originating from the hybrid cross of a 129/ReJ female and a BALB/cWah1 male. The strain shows 100% total absence of the CC and also suffers a greatly reduced HC. These mice breed surprisingly well.
9XC-2/Wah originated from the same cross as 9XC-1/Wah, but most mice have a
normal CC and the HC is virtually always normal.
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