Dissertation Research: Structural Differences in the Corpus Callosum of Macaca fascicularis and Pan Troglodytes by Confocal Microscopy

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This is an investigation of sexual differences in that part of the brain which serves as the major communicating highway between the two cerebral hemispheres, the corpus callosum. This study will examine the brains of two species of higher primates, the cynomolgus monkey (Macaca fascicularis) and the common chimpanzee (Pan troglodytes) using laser scanning confocal microscopy, a recent advancement in microscopic imaging that allows for a level of investigation and analysis not available previously. Data from this project will offer new information on both the basic biology of the nonhuman primate corpus callosum and provide a substrate which will enable reconstruction of the evolution of sex differences in the primate brain. For example, findings from this study may directly relate to observations on the existence of male-female cognitive and/or linguistic differences in Old World monkeys and great apes. The proposed research will address these claims by testing the hypothesis that sex differences in the corpus callosum of Macaca fascicularis and Pan troglodytes are present in gross morphology related to total and regional callosal areas, and also with regard to the number of myelinated and unmyelinated nerve fibers. Knowledge about fiber composition can shed light on the ability of the two cerebral hemispheres to communicate with each other, and the speed at which this communication may occur. For example, the quantity of fibers traversing the corpus callosum will reflect the degree of connectivity between the hemispheres, and offer insight into the lateralization (i.e., left-right compartmentalization) of functions. The size of the myelinated fibers (i.e., those nerve fibers surrounded by a myelin sheath which aids in conduction velocity) which pass through the corpus callosum will offer insight into the speed at which messages are passed between the hemispheres. In general, the larger the diameter of the nerve fiber the faster the signal will be conducted. The ability to address these fundamental questions on a largely unexplored region of the nonhuman primate brain will offer significant new information on how sexual differences in the human brain came to be.