RESUMEN
Electron microscopy was employed for ultrastructural observation of Marc-145 cells infected with porcine reproductive and respiratory syndrome virus (PRRSV) SC1 strain and studied the virus morphogenesis in infected cells. The results demonstrated that PRRSV was spherical and enveloped. The virion is 45-65 nm in diameter and its nucleocapsid was approximately 25-30 nm. PRRSV entered Marc-145 cells by endocytosis, and replicated in the cytoplasm. The mature viruses were released from infected cells by budding or exocytosis. The main ultrastructural changes of the infected cells were as follows: increased number of cytoplasmic vacuoles, dilated endoplasmic reticulum, mitochondria underwent hyperplasia with its ridges swollen, sloughed, and eventually vacuolated. Typical apoptosis was also observed in the infected Marc-145 cells, which included microvilli sloughing off the cell, appearance of apoptotic bodies and cell fragmentation.