An illustration of the polarized tissue explant model for studying PRRSV replication kinetics. Nasal mucosa explants were placed over a 3% agarose layer with the mucosal side facing up in a 6-well plate. The lateral edges of the explants were sealed with 4% agarose and cell-free PRRSV particles were added to the apical surfaces. PRRSV susceptible cells (PAM) were seeded on the bottom of the plate to examine virus particle penetration through the agarose layer. PAM: Porcine alveolar macrophages.