Mesosomes were once understood as internal, convoluted membranous structures found within the cytoplasm of prokaryotic cells, particularly in Gram-positive bacteria. First observed in the 1950s using early electron microscopy, scientists initially viewed them as major cellular components. They proposed that these structures were responsible for various fundamental life processes and were genuine features of a healthy, living bacterial cell. This article explores the initial interpretations, the significant roles once assigned to the mesosome, and why modern cell biology has largely rejected it as a true biological entity.
Early Observations and Proposed Structure
The initial visualization of mesosomes began in 1953, and the structures were formally named in 1960. They appeared as deep, localized infoldings of the bacterial plasma membrane extending inward toward the cell’s center. Scientists interpreted this membrane invagination as a mechanism to significantly increase the inner membrane’s surface area without altering the overall cell size. This increased surface area was hypothesized to enhance the cell’s capacity for membrane-associated biological reactions.
The morphology of these structures was not uniform, appearing in micrographs in several distinct configurations: simple spherical sacs, branching tubular networks, or complex stacks of flattened sheets known as lamellae. The primary technique used for these observations was chemical fixation, treating bacterial samples with heavy-metal compounds like osmium tetroxide. This preparation method was the standard for high-resolution imaging at the time and formed the basis for their initial acceptance as genuine structures.
Assigned Roles in Cellular Processes
Mesosomes were hypothesized to perform dual roles, supporting both cell division and energy generation. One major hypothesis positioned the mesosome as the coordinating center for the bacterial cell cycle. It was suggested that the bacterial chromosome would attach to the mesosome membrane, facilitating the orderly separation of genetic material during binary fission.
The mesosome was believed to anchor the replicating chromosome to the membrane during DNA replication. The subsequent outward growth of the cell membrane would then physically pull the two newly formed chromosomes apart, ensuring each daughter cell received a complete copy of the genetic material. Furthermore, the mesosome was linked to the formation of the septum, the cross-wall that physically divides the parent cell.
In a separate hypothesis, the mesosome was proposed to be the site of energy metabolism, drawing a direct parallel to the cristae of mitochondria in eukaryotic cells. Mesosomes were theorized to house essential respiratory enzyme systems. These enzymes would facilitate oxidative phosphorylation, using the infolded membrane to create the necessary electrochemical gradients for generating adenosine triphosphate (ATP), the cell’s primary energy currency.
Current Understanding: The Artifact Hypothesis
The long-held view of mesosomes as true organelles faced significant challenges by the late 1970s, leading to a consensus that they are not present in healthy, living cells. This rejection is based on the artifact hypothesis, which posits that the structures are merely an illusion created by the harsh sample preparation methods used for electron microscopy. Chemical fixation, which used fixatives like osmium tetroxide (OsO4), was found to severely damage the delicate bacterial plasma membrane.
Studies demonstrated that the chemical treatment caused a rapid disruption of the membrane’s permeability, leading to the leakage of intracellular contents and a drastic change in internal pressure. This damage mechanically forced the membrane to fold and collapse inward, resulting in the characteristic mesosomal appearance. The size and complexity of the observed mesosomes were directly proportional to the extent of the damage caused by the fixative.
The definitive evidence against the mesosome’s existence came from alternative sample preparation methods that avoided chemical disruption. Techniques such as cryofixation and freeze-substitution involve ultra-rapid freezing of the cell, preserving the structure in a state much closer to its natural, living condition. Electron micrographs of bacteria prepared using these methods consistently showed a smooth, continuous plasma membrane without the complex, convoluted mesosomal invaginations.
While the physical structure of the mesosome has been dismissed, the biological functions once assigned to it are performed by the bacterial plasma membrane itself. The cell membrane remains the site for respiratory enzymes, oxidative phosphorylation, and anchoring the bacterial chromosome for segregation. The mesosome, therefore, represents a historical misinterpretation where a preparation-induced structural anomaly was mistaken for a functional cellular component.