Cryo-SEM: frozen hydrated root hairs
One of the most delicate structures borne on higher plants is the root hair. This is simply a projection of the outer tangential wall of the epidermis of the root, and has a mechanical function as well as a physiological one.
Because root hairs are over 90% water, conventional preparation methods, such as air or critical point drying, always result in considerable collapse, distortion and shrinkage of these structures. Low-temperature SEM (cryo-SEM), on the other hand, provides good preservation of structure without any collapse.
Root hairs illustrated here still have a thin layer of water (ice) on their surface, verifying their true frozen hydrated nature. If the ice proves to obscure important detail, it can be removed during the cryo-SEM process by careful sublimation (carefully raising the specimen temperature for a limited time).
Bar: 50 µm (inset: 25 µm)
Cryo-SEM: starch granules in maize
Cryo-SEM prepared specimen: rapidly frozen in slushy nitrogen, fractured at -140ºC, sublmated at -90ºC for 3 minutes and sputter coated with platinium.
Image courtesy of Tongji University, Shanghai.