Comparative quantitative analysis of the cellular composition of the prefrontal cortex in the superorder Cetartiodactyla, using DTI and isotropic fractionator technique.

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Brain size between mammals varies by a factor of approximately 100,000X, with an absolute brain size ranging from less than 0.1g (insectivore) up to 9,000g (sperm whale). The size of the brain is determined by a large variety of factors such as: number of neurons, number of glial cells, cell-body size, dendritic and axonal arborisation as well as by the size of the vascular and ventricular systems. It is often assumed that the neuronal number is determinant for neuronal function and hence for behaviour. Until recently, very little was known about the quantitative cellular composition of mammalian brains; however with the development of the isotropic fractionator techniques the total neuronal and glial number has been examined in 4 different mammalian orders to date. Contrary to the traditional view it seems that the neuronal density does not decrease with increasing brain size, rather the brains scale as order-specific functions of their numbers of neurons. Moreover the glia cell density seems to be relatively constant across species of different orders and does not increase with increasing brain size as was previously assumed.The mammalian superorder Cetartiodactyla is comprised of the even-toed ungulates and the cetaceans, as they share a monophyletic ancestry. Cetartiodactyla contains nearly 300 extant species and is the most diverse order of medium to large bodied mammals, with a large variety in brain and body size, making the species of this superorder the ideal specimens in which to study brain scaling rules. The global aim of the present research proposal is to examine how brain size scales across the mammalian superorder Cetartiodactyl as a function of the numbers of neuronal and glial cells in comparison to the previously examined mammalian orders, using the isotropic fractionator technique. Furthermore we are interested in how these cells are organized in different structures, especially the prefrontal cortex, which will be delineated using MRI/DTI. Such a comparative analysis will give insight into cellular brain scaling rules that are shared between different mammalian orders and therefore might be inherited and retained from a common ancestor and what rules are different and thus might reflect phylogenetic variance across these orders.