Paneth cells are specialized cells found in the mucosa of the intestinal tract, particularly in microscopic structures called glands or crypts of Lieberkühn. They contain several important substances in their cytoplasm, including enzymes, minerals, and granules. The granules are located in the apex of the cells and appear large, refractile, and reddish or eosinophilic. In gastroenterology, these cells play an important role in the generation of immune responses against bacteria that are introduced through the oral route.
In the intestinal crypts of Lieberkühn, there are stem cells that constantly renew the intestinal mucosal epithelial cells. These epithelial cells are essential in the preservation of the functions of the gut. Microscopic studies of the intestinal mucosa have shown that Paneth cells are located adjacent to these stem cells, indicating that the former have a critical role in the renewal of epithelial cells and maintenance of the integrity of the gut.
The Paneth cells protect the stem cells by secreting defense molecules called cryptidins or alpha-defensins. These are proteins that can interact with the phospholipid membrane of bacteria, leading to the creation of pores. The pores lead to the spillage of important ions and other substances from the bacterial cell to the external environment, eventually resulting in the death of the bacteria. A unique characteristic of alpha-defensins released by these cells is that they have positively charged peptide chains that preferentially bind to the highly negatively charged cell membranes of bacterial cells. The result is that alpha-defensins damage bacterial cells, but spare the human cells adjacent to the cells.
In addition to defensins, Paneth cells also release the powerful enzymes lysozyme and phospholipase A2 (PLA2). PLA2 catalyzes the breakdown of phospholipid membranes through the release of fatty acids such as arachidonic acid. Lysozyme catalyzes the hydrolysis of peptidoglycan chains. Both Gram-positive and Gram-negative bacteria have peptidoglycan in their cell walls, but Gram-positive bacteria are more prone to the action of lysozyme because their cell walls have higher amounts of peptidoglycan.
Stimulation of Paneth cell secretion is brought about by contact with either Gram-positive or Gram-negative bacteria. Bacterial products such as lipid A, lipopolysaccharide, and muramyl dipeptide also induce secretion of the cell's antimicrobial agents. Research has shown that a molecule called myeloid differentiation primary response protein-88-dependent toll-like receptor (MyD88-dependent TLR) needs to be activated in order to trigger the antimicrobial action of Paneth cells. Although the secretions of these cells are known for their antibacterial properties, they are also effective against some fungi and enveloped viruses.