Paneth cell

Paneth cells, along with goblet cells, enterocytes, and enteroendocrine cells, represent the principal cell types of the epithelium of the small intestine. (A few may also be found sporadically in the cecum and appendix.) They are identified microscopically by their location just below the intestinal stem cells in the intestinal glands and the large eosinophilic refractile granules that occupy most of their cytoplasm. These granules consist of several anti-microbial compounds and other compounds that are known to be important in immunity and host-defense. When exposed to bacteria or bacterial antigens, Paneth cells secrete some of these compounds into the lumen of the intestinal gland, thereby contributing to maintenance of the gastrointestinal barrier.

Paneth cells are named after Joseph Paneth (1857–1890), Austrian physician.

Location
Paneth cells are found throughout the small intestine and the appendix at the base of the intestinal glands. Like the other epithelial cell lineages in the small intestine, paneth cells originate at the stem cell region near the bottom of the gland. However, unlike the other epithelial cell types, paneth cells migrate downward from the stem cell region and settle just adjacent to it. This close relationship to the stem cell region is thought to suggest that paneth cells are important in defending the gland stem cells from microbial damage, although their function is not entirely known.

Function
Small intestinal crypts house stem cells that serve to constantly replenish epithelial cells that die and are lost from the villi.

Protection of these stem cells is essential for long-term maintenance of the intestinal epithelium, and the location of Paneth cells adjacent to stem cells suggests that they play a critical role in defending epithelial cell renewal.

Sensing microbiota
Paneth cells sense bacteria via MyD88-dependent toll-like receptor (TLR) activation which then triggers antimicrobial action.

Defensins
The principal defense molecules secreted by Paneth cells are alpha-defensins, also known as cryptidins. These peptides have hydrophobic and positively-charged domains that can interact with phospholipids in cell membranes. This structure allows defensins to insert into membranes, where they interact with one another to form pores that disrupt membrane function, leading to cell lysis. Due to the higher concentration of negatively-charged phospholipids in bacterial than vertebrate cell membranes, defensins preferentially bind to and disrupt bacterial cells, sparing the cells they are functioning to protect.

Paneth cells are stimulated to secrete defensins when exposed to bacteria (both Gram positive and  negative types) or such bacterial products as lipopolysaccharide, muramyl dipeptide and lipid A.

Other secretions
In addition to defensins, Paneth cells secrete lysozyme, tumor necrosis factor-alpha , and phospholipase A2. Lysozyme and phospholipase A2 both have clear antimicrobial activity. This battery of secretory molecules gives Paneth cells a potent arsenal against a broad spectrum of agents, including bacteria, fungi and even some enveloped viruses.