Square array of closely spaced intramembrane particles, 4-6 nm in size, that form supramolecular aggregates found in the plasma membrane of astrocytes, skeletal muscle and epithelial cells. They have been shown to contain aquaporins (water channels). [ http://www.ncbi.nlm.nih.gov/pubmed/22718347 http://www.neurolex.org/wiki/sao1747012216 ]