[Ultrastructural observation of rat thymus tissue with coronavirus infection]

To investigate the ultrastructure of rat thymus tissues with rat coronavirus (RCV) infection for clarifying the mechanism responsible for the morphological changes of the cells infected by RCV.

Routine electron microscopy was performed for observing RCV-infected rat thymus tissues.

Following RCV infection, endoplasmic reticulum (ER) pools of different dimensions were observed in the cytoplasm of the thymic epithelial reticular cells, merging subsequently with each other into larger ER lakes filled with particles of mature RCV, or viral inclusion bodies. After germination on the ER membrane, the viruses entered the matrix of the ER lake to mature and were eventually excreted to the extracellular space. The RCV particles were spherical in shape with a diameter of 100-130 nm and two distinct membranes, the outer one being the envelope and the inner one the nuclear capsid to enclose the viroplasm. Between the envelop and nuclear capsid was a electron-lucent middle layer comprising one to two thin membranous structures. Large quantity of short spike-like projections starting from the nucleus capsid penetrated the middle layer and the envelop to reach the glycoprotein coat and formed a corona-like structure. Mature RCV particles were distributed around the ER pools, cytoplasm, and intercellular space, and the RCVs in the endosome/lysosome were devoid of the envelop and nuclear capsid.

The ER lakes are involved in the maturation of the viruses, and the envelop and nuclear capsid of the virus entering the cells from extracellular space are removed and degraded in the endosome/lysosome. Replications of virus occurs in plasma of the thymic epithelial reticular cells, and no RCV can be detected in the thymocytes.