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BACKGROUND: Olfactory ensheathing cell transplantation for treating spinal cord injury is an issue of concern, which mainly explores the changes of microenvironment after spinal cord injury. However, the effect of olfactory ensheathing cell transplantation on the ultrastructure of spinal cord after spinal cord injury is never reported. OBJECTIVE: To investigate the ultrastructure alterations of neurocytes, axons, myelin sheaths, synapses, and glial scar after spinal cord injury, and the effect of olfactory ensheathing cell transplantation on the protection and regeneration of nerve repair after spinal cord injury. METHODS: The study was approved by the Ethical Committee of Biomedicine of Medical Department of Xi'an Jiaotong University, approval No. 2018-2048. Twenty female Sprague-Dawley rats ware divided into three groups: blank control group (n=4, complete laminectomy of T10, partial laminectomy of T9and T11), DF12 group (n=8, cordotomy+injection of DF12 solution), and olfactory ensheathing cell transplantation group (n=8, cordotomy+olfactory ensheathing cell transplantation). The spinal cord was removed under anesthesia to observe the ultrastructure alterations of neurocytes under transmission electron microscope at 1, 7, 28 and 56 days after injury. RESULTS AND CONCLUSION: (1) Compared with the blank control group, the organelles in the neurons of the injured lesions were significantly reduced, and the obvious changes were found in the ultrastructure of axon, synapses and myelin sheath in the DF12 group. In the olfactory ensheathing cell transplantation group, the organelles in the neurons of the injured lesions were significantly increased with obvious nucleolus, the regeneration of axon, myelin sheath and synapses were significantly promoted, and the glial scar was significantly decreased. (2) The degree of reaction of the astrocytes and perlcytes in the olfactory ensheathing cell transplantation group was light. (3) These findings suggest that olfactory ensheathing cell transplantation can effectively protect the nerve tissues in the lesions after spinal cord injury, promote the regeneration of axon, myelin sheath and synapses, and inhibit the hyperplasia of astrocytes and pericytes, so that the post-injury microenvironment is available for the regeneration of neurons, axons and synapses. [ABSTRACT FROM AUTHOR]