Abstract
Summary Thin and semi-thin sections and freeze fracture replicas of unfixed, propane jet frozen cells of the methanogenic bacterium Methanosarcina mazei were examined in normal and medium-voltage (200 KV) transmission electron microscopes. A typical unit membrane surrounds the cytoplasm. Exterior to that is a single dark layer attached to the membrane by regular bridges. The “wall” is really a loose matrix of galactose derivatives polymerized into fibrils parallel to the plasma membrane. Cells 2 to 3 months old contain crystals, fibrillar inclusions, lipid-like inclusions, and cylindrical intrusions of the plasma membrane. We are calling these cylinders “exotubes”. On the cytoplasmic surface of their membrane, they are covered by regular hexagonal arrays. The plasma membrane faces of freeze fractured cells resemble those seen in other bacteria, exhibiting a protoplasmic fracture (PF) face with numerous intramembrane particles and an exoplasmic fracture (EF) face with some particle-free areas, many depressions, and only a few intramembrane particles.
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