Figure 1.

Padlock assay. (A) Diagram illustrating the main experimental steps of the padlock assay carried out in deparaffinized sections from the human placenta. Formalin-fixed paraffin-embedded sections (4 µm thickness) from the SARS-CoV-2 positive and SARS-CoV 2-negative placentas were first deparaffinized then processed for the padlock assay. A padlock probe was designed with the 5′- and 3′-terminal bases complementary to a selected sequence of SARS-CoV-2 RNA. The padlock probe was ligated and circularized with its complementary RNA template. A short DNA primer was used for initiating rolling circle amplification (RCA). In this way, the target sequence was converted in a long DNA amplicon with several copies of the padlock probe. Finally, amplicons became detectable by hybridization with the fluorophore-labeled probe. Sections were then analyzed by a fluorescence microscope. (B) (top) A schematic model of the anatomy of human placental villi. (bottom) Enlarged detail illustrating the major cellular components of the villus.