The process of cell division, or mitosis, allows organisms to grow, repair tissues, and reproduce. At the final step of this process, a temporary structure known as the midbody forms. First described in 1891, the midbody acts as a transient bridge connecting two newly formed daughter cells. This structure is also a hub of proteins and cellular machinery that ensures the division process concludes with precision and has functions that extend long after the cells have parted ways.
How the Midbody is Formed
The formation of the midbody is a consequence of cytokinesis, the physical process of one cell splitting into two. As the parent cell prepares to divide, its outer membrane begins to pinch inward, creating a channel called the cleavage furrow. This furrow continues to constrict, squeezing the connection between the two emerging daughter cells into a narrow intercellular bridge. It is within this bridge that the midbody assembles.
This assembly process involves organizing leftover components from the mitotic spindle, the apparatus that pulls chromosomes apart. Bundles of microtubules, which formed the spindle, become tightly compacted within this bridge. These microtubules are arranged and cross-linked by specialized proteins, creating a dense core. The final structure measures about 1 micrometer in diameter and 3 to 5 micrometers in length.
The structure is organized into distinct zones, including a central dense region called the “bulge” or “Flemming body,” and flanking areas that connect to each daughter cell. Specific proteins migrate to these different zones as cytokinesis progresses. For instance, proteins like MKLP1 are found in the central bulge, while others position themselves at the sites where the final cut will be made. This organization ensures all necessary molecular players are in the right place.
The Midbody’s Role in Cell Division
During the final moments of cell division, the midbody’s primary job is to serve as the precise location for abscission. This is the regulated molecular process that severs the connection between the two daughter cells.
This function ensures that each daughter cell becomes a complete and independent entity with the correct complement of genetic material. The midbody’s integrity during this stage prevents errors like failed division, which can result in cells with double the normal amount of DNA. It is the final quality control checkpoint, confirming that the process of creating two new cells concludes successfully.
To accomplish this, the midbody attracts a specialized group of proteins known as the ESCRT (Endosomal Sorting Complex Required for Transport) machinery. This protein complex is directed to the narrowest parts of the intercellular bridge, on either side of the midbody’s central core. The ESCRT proteins then assemble into spiral-like filaments that constrict the membrane inward, leading to the final cut that liberates the two cells.
The Afterlife of a Midbody: Remnants and Their Surprising Roles
For a long time, the midbody was thought to be discarded cellular “junk” after its work in cell division was done. However, research has revealed its story does not end with abscission. After the intercellular bridge is severed, the core of the midbody is inherited by one of the two daughter cells as a structure called a midbody remnant (MBR). In some cases, it is released into the space outside the cells.
These remnants are not inactive debris but have biological functions long after cell division is complete. The MBR can persist within the cell for many hours and acts as a signaling platform. This platform influences the cell’s future behavior.
One role of the MBR is its influence on cell fate, particularly in the context of stem cells. The cell that inherits the MBR may be directed toward a different path than its sister cell. This affects whether it continues to divide or specializes into a specific cell type. MBRs have also been implicated in establishing cell polarity—the process by which a cell determines its orientation, which is important for tissue organization.
Midbodies and Human Health
Errors in the midbody’s formation or function can have significant consequences for human health. Failures during the final abscission step can lead to cells with an abnormal number of chromosomes, a condition known as aneuploidy. Aneuploidy is a hallmark of many types of cancer, as it can provide cancer cells with genetic advantages that fuel their uncontrolled growth.
The roles of midbody remnants are also being linked to disease. Because MBRs can influence cell fate and proliferation, alterations in how they are processed or the signals they send could contribute to the development of cancer. For example, if an MBR promotes cell division, its persistence could encourage tumor growth.
This understanding of the midbody and its remnants opens new possibilities for medicine. The proteins associated with the midbody, sometimes called the “Flemmingsome,” could serve as diagnostic markers for diseases characterized by abnormal cell division. These proteins also represent potential targets for new therapies aimed at halting the proliferation of cancer cells or correcting developmental disorders.