Our objective is to measure spatial, temporal, and directional variations in mechanical properties of the ferret brain during cortical folding. Disturbances of folding have serious and lasting consequences, but the mechanism is not understood. Van Essen (1997) hypothesized that mechanical tension in axons drives cortical folding. According to this hypothesis, tension between strongly interconnected regions of the cortex pulls these regions together, generating an outward fold (gyrus);weakly-connected regions end up separated by an inward fold (sulcus). This hypothesized mechanism encourages compact wiring (Van Essen, 1997). To evaluate such theoretical models, accurate measurements of residual stress, stiffness, and anisotropy in the developing brain are needed. Approach: Studies will be performed in the neonatal ferret. The ferret has a gyrencephalic brain which undergoes folding during the first post-natal month. Mechanical properties will be found from experimental data combined with finite element modeling. Residual stress will be estimated by measuring deformation after local cuts. Stiffness properties of the brain will be measured from shear wave speed and analysis of indentation. Diffusion tensor imaging (DTI) will provide data on tissue anisotropy, and regional distribution of motor proteins (dynein, kinesin, myosin II, myosin V) will be assessed histologically. Stiffness and residual stress are expected to vary spatially, temporally, and as a function of direction. Such variations would be critically important in brain morphogenesis.
Specific aims : In the neonatal ferret brain (1) Measure local residual stress in different locations and directions;(2) Image shear wave propagation in different directions to estimate local stiffness and anisotropy for small deformations. (3) Measure force-displacement relationships during indentation of tissue;use inverse modeling of local deformation to develop constitutive relationships for large strain. (4) Perform DTI [and histological] studies during folding to characterize anatomical, microstructural, and cellular changes. Compare spatial, directional, and developmental variations in diffusion and mechanical properties. Significance: This is the first step toward development of a rigorous biomechanical model of cortical folding, including growth. Such models are needed to understand the causal pathways of pathologies (e.g., lissencephaly, polymicrogyria) responsible for mental disability and disease (retardation, seizure).
Disturbances of cortical folding in brain development have serious and lasting consequences, but the mechanism is not well understood. This project is the first step toward development of a rigorous biomechanical model of cortical folding, including growth. Such models are needed to understand the causal pathways of pathologies (e.g., lissencephaly, polymicrogyria) responsible for mental disability and disease (such as retardation, seizure).
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