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MIT 20 441J - Nerve synthesis in vivo

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Nerve synthesis in vivo (regeneration)*1. Anatomy and function of a peripheral nerve. INervous System: CNS and PNSFocus of interest: nerve fibers and axons1. Anatomy and function of a peripheral nerve. II1. Anatomy and function of a peripheral nerve. III1. Anatomy and function of a peripheral nerve. IVCylindrical symmetry of peripheral nerve structure2. Experimental parameters for study of regenerationThe injured myelin sheath regenerates spontaneouslyNeuroma formation. The endoneurium does not regenerate. Contraction and scar formation.2. Experimental parameters (cont.)C. Assays of outcome (cont.) 2A. Synthesis of myelinated axons2B. Synthesis of nerve BM3. Evidence (?) for synthesis of an endoneuriumEvidence (?) for synthesis of endoneurium (cont.)5. Synthesis of a nerve trunk (including kinetics)Kinetics of induced nerve regenerationContractile cell zone surrounds regenerating nerve6. Comparative regenerative activity of various devices (Table 6.1, pp. 147-8)Tube wall composition and permeabilityTube fillingsEffect of degradation rate of tube filling based on a porous ECM analog (NRT)HistomorphometrySummary of results*Nerve synthesis in vivo (regeneration)*1. Anatomy and function of a peripheral nerve.2. Experimental parameters for study of induced regeneration.3. Synthesis of myelinated axons and BM (nerve fibers)4. Evidence (?) of synthesis of an endoneurium.5. Synthesis of a nerve trunk (including summary of kinetics of synthesis).6. Comparative regenerative activity of various reactants._______*Tissue and Organ Regeneration in Adults, Yannas, Springer, 2001, Ch. 6.1. Anatomy and function of a peripheral nerve. INervous system =central nervous system (CNS) +peripheral nervous system (PNS)Image: public domain (by Wikipedia User: Persion Poet Gal)Nervous System: CNS and PNSChamberlain, Yannas, et al., 1998CNS PNSLandstrom, Aria. “Nerve Regeneration Induced by Collagen-GAG Matrix in Collagen Tubes.” MS Thesis, MIT, 1994.Nerve fibers comprise axons wrapped in a myelin sheath, itself surrounded by BM (diam. 10-30 μm in rat sciatic nerve).Axonsare extensions (long processes) of neurons located in spinal cord. They comprise endoplasmic reticulum and microtubules. Focus of interest: nerve fibers and axons1. Anatomy and function of a peripheral nerve. IIMyelinated axons (diam. 1-15 μm) are wrapped in a myelin sheath; nonmyelinated axons also exist. They are the elementary units for conduction of electric signals in the body. Myelin formed by wrapping a Schwann cell membrane many times around axon perimeter. No ECM inside nerve fibers.Myelin sheathis a wrapping of Schwann cell membranes around certain axons.1. Anatomy and function of a peripheral nerve. IIINonmyelinated axons (diam. <1 μm) function in small pain nerves. Although surrounded by Schwann cells, they lack myelin sheath; Schwann cells are around them but have retained their cytoplasm. Basement membrane(tubular) encases the myelin sheath. Structure similar to that of skin BM.1. Anatomy and function of a peripheral nerve. IVNerve fibers are embedded in endoneurium: a delicate packing of loose vascular supporting tissue that is rich in collagen fibers. Definitely ECM!Many nerve fibers with their associated endoneurium are packed in a collagenous layer, the perineurium. This forms a fascicle.Multifascicular nerves encased in a collagenous layer, the epineurium.Cylindrical symmetry of peripheral nerve structureSummary of nerve trunk structureproceeding radially from the center: [axon ⎯ myelin sheath ⎯ BM] ⎯endoneurium ⎯ perineurium ⎯epineurium. [ …. ] = “nerve fiber”Cross section of rat sciatic nerve(“nerve trunk”).Several thousand nerve fibers.Noncircular cross section.Jenqa, C. B., and R. E. Coggeshall. Brain Research 326, no. 1 (1985): 27-40. Courtesy of Elsevier, Inc., http://www.sciencedirect.com. Used with permission.(idealized)nerve trunknerve fiberChamberlain, L. J. “Long Term Functional and Morphological Evaluation of Peripheral Nerves Regenerated Through Degradeable Collagen Implants.” MIT M.S. Thesis, 1994.Rat sciatic nerve cross sectionIndividual axonChamberlain, L. J. “Long Term Functional and Morphological Evaluation of Peripheral Nerves Regenerated Through Degradeable Collagen Implants.” MIT M.S. Thesis, 1994.Longitudinal view of nerve fiberMyelination of a nerve fiber during development or during induced regenerationFigure by MIT OpenCourseWare.2. Experimental parameters for study of regenerationA. Anatomically well-defined defect– Designate experimental volume– Delete nonregenerative tissue(s)– Anatomical bounds– Containment of exudateB. Timescale of observations– Short-term (<20 wk) and long-term (>20 wk) assaysRegenerative similarity of tissues in skin and nerves. Identify epithelial tissue, BM and stroma.Skin Peripheral nervesRegenerative TissuesEpidermisBasement membraneMyelin sheath Basement membrane (perineurium, in part only)Nonregenerative TissuesDermis Endoneurial stromaThe injured myelin sheath regenerates spontaneouslyRegeneratedmyelin Injured myelin. Axoplasm Myelin sheath Endoneurium intact. healingFigure by MIT OpenCourseWare.Neuroma formation. The endoneurium does not regenerate. Contraction and scar formation.Transected nerve.Both myelin and endoneurium areseverely injured.Neuroma formsat each stump by contraction and scar formation.healingFigure by MIT OpenCourseWare.Intact nerve fiberwith myelin sheath (left, black margin) and associatedSchwann cell (right).Endoneurium outside.Spontaneouslyhealed nerve fiberfilled with scar (Büngner bands, Bb)Healing following transectionHistology photo of nerve fiber removed due to copyright restrictions.See Figure 2.5 (top) in [TORA].Bradley, J. L., et al. J Anat 192, no. 4 (1998): 529-538.Copyright © 2002 John Wiley & Sons., Inc. . Reprinted with permission of John Wiley & Sons., Inc.2. Experimental parameters (cont.)C. Assays of outcome– Correction for experimental gap length. – Correction for animal species.– Critical axon elongation, Lc. – Shift length, ΔL. Characterization of devices.– Long-term: fidelity of regeneration.C. Assays of outcome (cont.)Use corrected values of frequency of reinnervation (%N) across tubulated gaps. This correction allows comparison of %N data from studies with different gap lengths and different species. Critical axon elongation, Lc, the gap length above which %N drops below 50% (or the gap length where the odds of rein-nervation are even). Data from several


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