mesosome

Mesosome refers to irregular, membranous invaginations of the bacterial plasma membrane, seen in transmission electron micrographs as vesicle-like bodies, lamellae, or tubular networks that appear to be embedded within the cytoplasm. They were first described in the mid-20th century and quickly became a subject of debate about bacterial cell organization. In early accounts, mesosomes were proposed to participate in various essential processes, including cell wall synthesis, cellular respiration, and the initiation of DNA replication or cell division. This led to widespread speculation about them as functional organelles in bacteria.

However, the mesosome concept has been challenged for decades. Many scientists argued that mesosomes are artifacts produced by chemical fixation and dehydration used in electron microscopy, or by sectioning. With the adoption of gentler fixation techniques and, more recently, cryo-electron microscopy, mesosomes are not consistently observed in living or rapidly preserved cells, and no universal, well-supported role for mesosomes has emerged. Consequently, the term has fallen out of favor in modern cell biology, and mesosomes are generally described as historical artifacts rather than bona fide organelles. In some contexts, the term is still used to denote true membrane invaginations or specific membrane-associated structures seen under certain conditions, but their existence and function vary among species and are not considered universal.