Intermediate filaments (IFs) represent the third major element involved in EC cytoskeletal structure. Despite greater diversity than the highly conserved components of either actin microfilaments or microtubules, IF proteins share a common dimer structure containing two parallel a-helices which combine to form apolar fibrils that associate with an array of IF-binding proteins while connecting to the nuclear envelope, peripheral cell junctions, and other cytoskeletal components. IF proteins are expressed in a highly cell-specific manner. Vimentin is the primary IF protein found in EC and other cells of mesenchymal origin. These data suggest that potential roles for IF in EC cytoskeletal structure, and more specifically barrier function, are likely to be subtle and subject to compensation by biologic redundancy.
IFs were defined on the basis of their 7- to 12-nm filament structure, which distinguished them from microfilament and microtubules. Helical polymers of IF proteins are made up of conserved central rod domains flanked by variable C- and N-terminal domains. In vivo studies of vimentin assembly resulted in characterization of three distinct, progressive structural assembly intermediates (particles, squiggles, and long filaments) that exist in a state of equilibrium between subunits and longer polymers. IF distribution in spread, interphase cells is predominantly arranged in long filaments extending from the nucleus to the cell surface, with particles and squiggles apparent at the extreme periphery of the cell. Evidence shows that the assembly of IFs is a complex process, which most likely is highly regulated by signaling cascades associated with cell motility.
Vimentin IFs are very dynamic structures undergoing constant assembly/disassembly, as well as anterograde and retrograde movements. Microtubule-based movement of IF is thought to be critical for assembly and maintenance of the vimentin IF network . Most likely, the dynamic properties of vimentin IF network in the vascular endothelial cell is important in regulation of cell shape and resistance to hemodynamic stress that accompanies blood flow.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.