Mesosomes were once described as convoluted infoldings of the plasma membrane within prokaryotic cells, particularly bacteria. These structures were initially identified using early electron microscopy techniques in the mid-20th century. Their appearance suggested they were genuine cellular components, prompting significant scientific interest in their potential roles in bacterial physiology.
Proposed Functions of Mesosomes
Early observations led scientists to propose several important roles for mesosomes within bacterial cells. One prominent theory suggested their involvement in cell division, where they were thought to anchor the newly forming septum, ensuring proper separation of daughter cells.
Another proposed function centered on DNA replication and segregation. It was believed that the bacterial chromosome attached to the mesosome, aiding in the orderly distribution of genetic material to daughter cells during replication.
Mesosomes were also hypothesized to play a part in cellular respiration. Their folded structure, resembling the cristae found in eukaryotic mitochondria, led some scientists to suggest they served as a site for respiratory enzymes. This analogy implied that mesosomes might increase the surface area for metabolic reactions, enhancing energy production in bacteria.
The Artifact Hypothesis
The prevailing scientific view today is that mesosomes are not true biological structures. Instead, they are widely considered to be artifacts, meaning they are formations that result from the preparation process of specimens for electron microscopy. This understanding emerged as scientists refined their techniques for visualizing cellular components.
When cells are prepared using older chemical fixatives, the harsh chemicals can damage and distort the delicate plasma membrane. This damage causes the membrane to invaginate and fold irregularly, creating the characteristic sac-like or vesicular appearances previously identified as mesosomes.
Evidence and Modern Techniques
The shift in understanding regarding mesosomes is largely attributed to advancements in electron microscopy preparation techniques. Historically, chemical fixation involved immersing bacterial cells in solutions containing aldehydes like glutaraldehyde and osmium tetroxide. While these chemicals cross-linked and stabilized cellular components, they also introduced significant alterations to the cell’s natural state.
Modern approaches, such as cryofixation, provide a much more accurate representation of cellular ultrastructure. Techniques like rapid freezing, freeze-fracture, and freeze-etching involve quickly freezing bacterial samples to extremely low temperatures, often below -150 degrees Celsius, within milliseconds. This rapid freezing vitrifies the water inside the cell, preventing the formation of ice crystals that could disrupt cellular structures.
When bacteria are prepared using these cryofixation methods, mesosomes are consistently absent. The smooth, continuous appearance of the plasma membrane in these preparations stands in stark contrast to the convoluted membranes seen with chemical fixation. This direct comparison provides evidence that mesosomes are indeed preparation artifacts. The journey of understanding mesosomes exemplifies how scientific knowledge evolves and self-corrects as technology and methodologies improve.