The scanning electron microscope was used to study the morphological features of human red blood cells during hypotonic swelling. During exposure to hypotonic swelling the majority of cells assumed the shape of swollen cups with a single deep concavity. Approximately 10% of the cells, however, had deep pits or invaginations of membrane within the central concavity. This appeared to result from internal trapping of the membrane during the swelling process and was not reversible when the cells were returned to isotonicity.
Metz EN, Jensen WN, Larrimer NR, Balcerzak SP. Irreversible Changes in the Red Blood Cell Membrane During Hypotonic Swelling. Arch Intern Med. 1971;127(2):273-277. doi:10.1001/archinte.1971.00310140101013