The omentum is one of the human body’s largest organs, but also arguably one of its least familiar – to scientists as well as the rest of us.
Now, however, it is coming under increased scrutiny as researchers strive to understand how it functions as an important part of the immune system but also, perversely, as a nursery for cancer cells.
The organ is effectively a large apron-like expanse of fatty tissue that encases the liver, intestine and stomach.
In a review published in the journal Trends in Immunology, Troy Randall and Selene Meza-Perez of the University of Alabama, in the US, look at the omentum’s role in resisting infection, and at how its toxin-fighting mechanism inadvertently serves to protect, rather than destroy, certain cancer cells.
Key to the organ’s activity are discrete white patches that cover its surface. Known as “milky spots”, they were recorded by anatomists as far back as 1874, although their function wasn’t deduced until rather more recently.
In effect, they act as monitors and alert systems for the abdominal cavity by sampling circulating fluid.
“Milky spots collect cells, antigens, and bacteria before deciding what’s going to happen immunologically,” explains Randall.
Information thus obtained by the spots then prompts a range of actions from the omentum itself, which include triggering inflammation and other immunological responses.
However, the omentum’s suite of possible responses also includes the option to tolerate particular types of antigen, taking no defensive actions.
For reasons that remain unclear, a tolerance response is sometimes prompted by certain types of cancer cells.
The organ itself is strongly resistant to tumours – omentum cancer is known, but rare – but the filtering action of the milky spots can suck cancer cells into its fatty layers. Instead of setting off alarm bells, the cells, especially those of prostate and ovarian cancer, are instead protected.
“The omentum makes the wrong decision,” Randall says. “It decides to provide tolerance instead of immunity.”
Randall likens the result to blades of grass getting caught up inside a swimming pool filter. Thus safely hidden from the body’s other defence mechanisms, the cells can begin metastasis.
Randall and Meza-Perez suggest that better understanding the mechanism by which the omentum shields cancer cells could lead to new therapeutic targets, enabling early intervention.