Author's School

School of Engineering & Applied Science

Author's Department/Program

Biomedical Engineering

Language

English (en)

Date of Award

5-24-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Larry Taber

Abstract

In structures with obvious mechanical function, like the heart and bone, the relationship of mechanical forces to growth and development has been well studied. In contrast, other than the problem of neurulation: formation of the neural tube), developmental mechanisms in the nervous system have received relatively little attention. The central aim of this research is to characterize the biophysical mechanisms that shape the early embryonic brain. Experiments were performed primarily in the chicken brain, which is morphologically similar to humans during early stages of development. Proposed mechanisms were tested using computational models to ensure that hypotheses are consistent with physical law. The brain initially forms as a straight epithelial tube in the embryo: approximately 3 weeks gestation in humans). We first investigated a potential role for mechanical feedback in regulating the development of this structure. We find that the neuroepithelium actively stiffens under decreased loading and softens under increased loading. Nuclear shapes are elongated in stiffer brains and circular in softer brains, consistent with changes in cytoskeletal contractility and wall stress. These results suggest a role for stress-based mechanical feedback in regulating epithelial development. We next investigated the more specific role of cytoskeletal contraction in forming the primary brain vesicles and rhombomeres that subdivide the primitive brain tube. We show that a combination of circumferential contraction in the boundary regions and isotropic contraction between boundaries can generate realistic vesicle morphologies, whereas longitudinal contraction between boundaries likely causes rhombomere formation. Models are used to show how regional variations in contraction may be a function of brain geometry and morphogenetic plasticity. As an extension of the previous study, we show that enhancing contractility in the embryonic chicken brain induces morphologies reminiscent of more primitive species such as frog and fish. In particular, brain cross sections that are relatively circular transform into diamonds, triangles, and narrow slits, shapes that are present in normal zebrafish and Xenopus brains at comparable stages of development. Models show that these shapes are likely produced by locally elevated cytoskeletal contraction, indicating a potential role for differential contractility in early brain development and evolution. In summary, results from this thesis should improve our understanding of the biophysical mechanisms that establish and regulate phenotype in the developing brain. The research begins to establish the framework necessary to connect early-stage mechanisms to interspecies differences in brain morphogenesis that occur during later development.

DOI

https://doi.org/10.7936/K7NG4NMD

Comments

Permanent URL: http://dx.doi.org/10.7936/K7NG4NMD

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