What Is Fat Necrosis?
Put as simply as possible, fat necrosis is..."necrosis of fat." Fat necrosis can be thought of as a "special case" under the general heading of necrosis, and it's always considered separately because the peculiar features of fat cells cause things to happen that aren't common in other types of tissues.
There are two modes by which fat necrosis occurs: enzymatic and traumatic injury. Enzymatic fat necrosis results from acute pancreatitis, which is an abdominal emergency. Traumatic fat necrosis is not enzyme mediated but is secondary to trauma to fatty tissue.
In this image, you see at left a mesentery that has become gorged with necrotic fat, in a gross specimen; at right is a sample of the tissue prepared for microscopy. Some relatively normal white fat is visible in the latter at upper left; and necrotic fat in the middle of the mass. As with any necrotic condition, inflammation will occur in fat necrosis, reflected by the large numbers of inflammatory cells around the lesion.
In pancreatitis, leakage of active lipase and other enzymes (especially proteases) occurs into the adipose tissue that surrounds the pancreas or into the peritoneal cavity, formed by the fat laden mesentery. The activated pancreatic enzymes (especially lipases) destroy fat cell membranes and split the triglyceride esters contained within fat cell. This process of enzymatic breakdown releases fatty acids. Fatty acids will combine with calcium (a ubiquitous component of the intercellular fluid) to produce grossly visible chalky white areas. This is the process of "fat saponification (from Latin, sapon = soap) and the chalky materials are soaps, sodium and/or potassium salts of long-chain fatty acids. (This is exactly the same process, chemically speaking, by which commercial soap is made.)
Microscopically, necrotic fat cells are distinguishable as pale shadowy outlines whose cytoplasm is filled with an amorphous, faintly basophilic material. That's the soap produced by the process. Grossly, fat necrosis appears as firm, minute, yellow-white deposits in the adipose tissue that surrounds the pancreas and is located in the mesenteries.
Traumatic fat necrosis is a consequence of mechanical cell injury. As might be expected. it is seen most often in the adipose tissue of breasts, thigh or other locations that are subject to impact. Physical injury of the adipocytes releases triglycerides whose subsequent hydrolysis by serum lipases to create fatty acids results in the same saponifcation with formation of chalky material.