Cytological effects of triiodothyronine on dorsal root regeneration in adult rat

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Because several light microscopic studies have indicated that thyroid hormones stimulate nerve regeneration, we studied the cytologic effects of triiodothyronine (T 3) on dorsal root regeneration in the adult Long-Evans rat. Under pentobarbital anesthesia a lumbar laminectomy was made and the central processes of two dorsal roots were crushed 5 mm from the spinal ganglion. The experimental group of animals received a daily i.p. injection of T 3 in a dose of 15 μg/kg dissolved in 0.01 n NaOH. The control animals received an equivalent amount of vehicle. Four control and four experimental animals were killed at 1, 2, 3, or 4 postoperative (p.o.) weeks and the crushed roots with their ganglia were removed and processed for electron microscopy. Cross sections of roots were cut 5 mm distal to the site of crush and studied with the electron microscope. Dorsal roots of both control and T 3-treated animals at the first p.o. week revealed Schwann cells to be primarily responsible for the fragmentation and resorption of myelin. The Schwann cells of the T 3-treated animals showed greater elaboration of smooth endoplasmic reticulum. By the second p.o. week early evidence of regeneration of myelinated fibers was seen with a distinct decrease in Schwann cells associated with myelin debris. By the third and fourth p.o. week there were greater numbers of regenerating myelinated fibers. It also appeared that the microtubular content of regenerating neurites and Schwann cell processess appeared to be greater in the T 3 compared with the control animals. In the T 3-treated group even small-diameter axons tended to have thick collars of myelin sheath which suggested enhanced maturation. Several Schwann cells in the T 3-treated animals contained enlarged myelinated axons with neurofibrillary tangles.