Omentum Performs Unique Immunological Functions

Jun 2, 2017 by News Staff / Source

The omentum is an important immune organ that serves as a first line of defense against toxins and infection, according to a review paper published online this week in the journal Trends in Immunology.

             Structure of milky spots in the omentum. (A) schematic of whole omentum in mice; the omentum is a thin strip of fat (yellow) that is covered by a layer of mesothelial cells (green); milky spots (MSs, blue) are located just beneath the mesothelial layer embedded between adipocytes; the omentum is well vascularized with an extensive network of capillaries (not shown), which connect to a large central blood vessel (red), running between the adipocytes; blood vessels also connect to the MSs and form a glomerulus-like knot of blood vessels in the lymphoid clusters; a large central lymphatic vessel (light blue) runs down the center of the omentum and branches from this vessel lead to MSs as well as to areas of the omentum that lack MSs; the human omentum is much larger and resembles an apron hanging in front of the abdomen, but also contains MSs. (B) structure of MS; MSs are loose collections of leukocytes embedded between adipocytes (yellow) just beneath the mesothelial layer (green); B cells (blue) form a central cluster, whereas macrophages (brown) and dendritic cells (light green) tend to accumulate around the outside of the MS and are also found individually throughout the omentum; T cells (green circles) and ILCs (orange) can be intermixed with the B cells or may cluster around blood vessels (not shown); cells and antigens are passively collected from the peritoneal cavity through fenestrations in the mesothelial layer by fluid flow or may be actively carried by phagocytic cells like macrophages; cells can also enter the MSs from the blood through HEVs (not shown). Image credit: Selene Meza-Perez & Troy D. Randall, doi: 10.1016/j.it.2017.03.002.

              The omentum is the fat organ connected to the spleen, stomach, colon, and pancreas. In humans it reaches an area of 1,500 cm² and resembles an apron.
Sometimes called the ‘policeman of the abdomen,’ the omentum is known to secrete hormones related to obesity.
The omentum’s immune functions come from milky spots — groups of small, white filters located among the fat cells.
These cell clusters were first described in 1874 in rabbits by the French anatomist Ranvier, who gave them their name because of their whitish appearance amidst the yellow fat.
Recent studies have shown that they aid the omentum by collecting information about the health of the abdominal cavity.
While the size and shape of the omentum varies, milky spots speckle the entire tissue and serve as a filter for surrounding fluid.
“The fluid around the abdominal organs doesn’t just sit there, it circulates through the milky spots,” said Dr. Troy Randall, a clinical immunologist at the University of Alabama at Birmingham and corresponding author of the paper.
           “Milky spots collect cells, antigens, and bacteria before deciding what’s going to happen immunologically.”
The milky spots’ analyses cause the omentum to respond immunologically by releasing inflammatory molecules, tolerating the presence of an antigen, or beginning the process of fibrosis.
Humans develop milky spots in their omentum during early development, before bacteria even appears, indicating its role as a primary immune organ.
Unfortunately, even protective organs make mistakes.
“In concerns to tumor cells, the omentum makes the wrong decision. It decides to provide tolerance instead of immunity,” Dr. Randall said.
           “While tumors of the omentum are uncommon, the circulating fluids bring back cancer cells into the milky spots, where they get trapped like grass in a pool filter, promoting metastasis.”
“It is a breeding ground for aggressive tumors such as in ovarian and gastrointestinal cancer.”
Scientists hope to target these sites of tumor growth with therapies that can control abdominal tumors and assist anti-tumor immunity.
“If we can figure this out, then we can start really making inroads on cancer treatments because, in most cases, you don’t even catch ovarian cancer until it metastasizes,” Dr. Randall said.
“Understanding how cancer changes the immune system will lead us directly to ways to intervene and, hopefully, start to turn things around.”