Mesosomelike

Mesosomelike is an adjective used in some biological writings to describe membrane-bound intracytoplasmic structures in certain prokaryotes that resemble the historically described mesosomes. Mesosomes are invaginations or vesicular extensions of the bacterial cytoplasmic membrane that were once thought to function as sites of respiration, secretion, DNA replication, or cell division. The term mesosomelike thus refers to structures that appear similar in electron micrographs, but whose reality and function are uncertain.

Historical context and controversy: In the 1950s and 1960s, transmission electron microscopy of bacteria produced reports of mesosomes. As fixation and preparation methods improved, many scientists demonstrated that observed mesosomes could be artifacts of chemical fixation, dehydration, or staining. Consequently, mesosomes fell out of favor as a standard feature of bacterial cell biology, and the idea that all bacteria harbor functional mesosomes largely declined.

Contemporary perspective: The consensus is that mesosomes are not universal functional organelles in bacteria; many observed instances are artifacts. Nevertheless, some bacteria and archaea possess real internal membrane systems that are functionally important, such as photosynthetic or respiratory membranes, magnetosome membranes, or intracytoplasmic membranes in certain lineages. These structures are not generally called mesosomes today, but in older literature they might have been described as mesosomelike.

Usage: In modern texts, mesosomelike is encountered mainly in historical discussions, in reviews noting methodological artifacts, or in descriptions of electron micrographs where internal membranes are observed but not conclusively characterized.