The Paneth cells are cells belonging to the small intestine. They are found, specifically, in the crypts of Lieberkühn, some tubular glands that are in the epithelium of intestinal lining, submerged in the lamina propria.
The small intestine is responsible for the digestion of food and the absorption of the end products of the entire digestive process. It has three well-defined regions: the duodenum, the jejunum, and the ileum.
Human Paneth cells (Source: en: Jpogi via Wikimedia Commons)
If a transverse section of this tube is observed, four known layers can be seen, from the inside out, as the mucosa, lamina propria, submucosa, external muscularis and serous; each with defined characteristics and functions.
The mucosa (innermost layer) has adaptations that allow it to increase the surface area, these adaptations consist of abundant folds and villi that, consequently, increase the number of cells capable of absorbing nutrients.
These folds and villi are distributed in the three layers that make up the intestinal mucosa called (from the inside out) the epithelium, the lamina propria and the muscularis mucosa. The epithelium covers the villi, the lamina propria represents connective tissue and the muscularis mucosa is the muscular layer that allows shortening of the villi.
Layers of the small intestine (Source: Boumphreyfr via Wikimedia Commons)
The main function of Paneth cells, located in glands present in the lamina propria, is to secrete antibacterial substances such as lysozyme, which is why they participate in the innate defense system.
characteristics
Paneth cells were described by G. Schwalbe and J. Paneth as pyramid-shaped epithelial "columnar" cells, located at the bottom of the crypts of Lieberkühn, which are tubular-like intestinal glands.
Representative diagram of the lumen of the small intestine with its folds and villi. The crypts or glands are observed, the place where the Paneth cells are found (Source: White Whale via Wikimedia Commons)
They share these enclosures with four other types of cells: surface absorption cells, goblet cells, regenerative cells, and SNED cells, or cells of the diffuse neuroendocrine system.
In addition to the small intestine, Paneth cells can sometimes be found outside the gastrointestinal tract, such as in the stomach and colon, where they respond to changes triggered by inflammation of the mucosa.
These are secretory cells, with long life spans (more than 20 days). It has been determined that they are also present in the small intestine of primates, rodents, pigs and horses, that is, in a large number of mammalian animals.
Embryonic origin
Paneth cells originate from multipotent stem cells, that is, they give rise to different cell lines (enterocytes, goblet cells, and enteroendocrine cells). These stem cells are found at the interface between the villi and the crypts of Lieberkühn.
During their development and maturation from stem cells, Paneth cells migrate to the bottom of the gland and fill with the cytosolic granules that characterize them.
In humans, these cells first appear in the colon and small intestine after 13 weeks' gestation. Only after week 17 are they confined to the small intestine.
In newborns, the expression of Paneth cells is very low, but it increases considerably with age thanks to the action of some soluble factors such as epidermal growth factor.
Features
Paneth cells, as determined from numerous immunohistochemical studies, are capable of secreting large amounts of what are known in the literature as "antimicrobial proteins or peptides."
This ability of Paneth cells introduces them into the framework of the innate immune response system of the small intestine, as their secretion products have important implications for the health of humans and other mammals.
The small intestine can be considered as under constant threat because it has a large surface area and its multiple villi and crypts represent potential sites for invasion by microorganisms that can be pathogenic.
In turn, given that the half-life of the cells in the epithelial lining is very short (between 2 and 5 days only), the new cells that populate the epithelium deserve constant protection, protection provided by the antimicrobial factors secreted from the crypts. by Lieberkühn.
The importance of Paneth cells in innate immunity is more significant if it is considered, in addition, that the lumen of the small intestine is a site rich in a large amount of nutrients that arrive with food, but that can be contaminated with bacteria and other microorganisms.
Antimicrobial molecules of Paneth cells
As will be seen later, Paneth cells are characterized by the cytosolic presence of large secretory granules, which are responsible for the release of the soluble antimicrobial factors that these cells produce.
Some of these endogenous antimicrobial molecules are identical to those found in the granules of certain leukocytes and macrophages. However, it has been determined that lysozyme is perhaps the molecule produced in greater abundance.
Structure of the protein Lysozyme, an antimicrobial protein produced by Paneth cells (Source: SciabaPDBsum staff at the European Bioinformatics Institute via Wikimedia Commons)
The secretory granules of Paneth cells also produce other molecules known as "defensins" and a secretory phospholipase A2, which is a potent microbicidal agent against Gram positive bacteria.
Like the other classes of antimicrobial molecules and peptides, the function of these molecules is to disrupt the membrane integrity of microbes, thereby achieving their lysis.
It is important to note that the production and release of the internal content of the secretory granules is a fairly controlled process, both from the intrinsic point of view of the cells that produce them, and from the microenvironmental point of view.
Histology
Paneth cells are cells specialized in secretion (some authors describe them as "professional secretors") and in Lieberkühn crypts there are an average of 5 to 15 of these cells.
They have a characteristic pyramidal shape and their cytosol contains a well-developed Golgi complex, a prominent endoplasmic reticulum, and large numbers of mitochondria.
Histologically, they are distinguished by the presence of secretory granules of considerable size in their apical portion, which are rich in basic peptides and proteins, some of which may be modified with glycans.
These granules are released into the luminal region of the glands in response to different stimuli, such as acetyl cholinergic agonists, bacterial surface products, and certain Toll-like receptor agonists.
In addition to lysozyme, Paneth cells also synthesize and secrete other enzymes known as “defensins” through cytosolic granules, which perform similar functions to the first.
References
- Bevins, CL (2004). The Paneth cell and the innate immune response. Current Opinion in Gastroenterology, 20 (6), 572–580.
- Bevins, CL, & Salzman, NH (2011). Paneth cells, antimicrobial peptides and maintenance of intestinal homeostasis. Nature Reviews Microbiology, 9 (5), 356–368.
- Clevers, HC, & Bevins, CL (2013). Paneth Cells: Masters of the Small Intestinal Crypts. Annual Review of Physiology, 75 (1), 289–311.
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- Ouellette, AJ (2010). Paneth cells and innate mucosal immunity. Current Opinion in Gastroenterology, 26 (6), 547–553.
- Porter, EM, Bevins, CL, Ghosh, D., & Ganz, T. (2002). The multifaceted Paneth cell. Cellular and Molecular Life Sciences, 59 (1), 156–170.