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The gut microbiome: balancing the body
The gut microbiome: balancing the body

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2.4 Structural integrity of the gut wall

The human gut wall is made up of several layers of different tissue types, including muscle, nerves and blood vessels and an innermost mucosal layer. The cells of the mucosal layer of the small intestine secrete important enzymes for digestion of food and allow nutrients to pass through them into the bloodstream. They also secrete mucus to lubricate the contents in the gut tube. The inner layers of the gut wall are highly folded into finger-like projections known as villi, which increase the surface area of the gut (Figure 10).

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Figure 10 Schematic of the small intestine, showing the individual layers and highly folded surface of the internal layers of the gut. Blood vessels supply oxygen to the gut wall and transports nutrients to the rest of the body.
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A schematic illustration demonstrates the general structure of the small intestine composed of 4 histological layers. The cylindrical concentric layers are extended out to reveal the histological layers of the small intestine. The small intestine has longitudinal and circular muscle layers, present beneath the outermost serosa layer. The lumen of the small intestine is enclosed by submucosa and innermost mucosa. The blood vessels serving the small intestine supply blood to all the histological layers. The internal surface of the mucosa has circular folds (plicae circulares). There are numerous finger-like projections in the circular folds labelled ‘villi’.

Figure 10 Schematic of the small intestine, showing the individual layers and highly folded surface of the internal layers of the gut. ...

  • Question 9

    Why do you think a large surface area would be useful in the gut?

  • The large surface area created by the presence of villi and folds in the gut wall of the small intestine maximises the absorptive capacity of the gut (i.e. more of the nutrients obtained from the digestion of food substances, can be absorbed from the gut lumen).

The mucosal cells of the innermost layer are tightly bound to one another, to prevent pathogens in the food from crossing over into the tissue of the gut or into the bloodstream. These cells are held together by protein complexes known as tight junctions (Figure 11).

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Figure 11 Cells are connected to each other by three main types of junctions: tight junctions, anchoring junctions, and gap junctions
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This figure shows a schematic diagram which illustrates the three main types of cell junctions. The schematic shows three rectangular cells, with finger-like projections on the upper edge, lying in close proximity and above the basal membrane. From left to right, the cells are labelled cell 1, cell 2 and cell 3, and there are three types of junctions joining the cells together. Cell 1 is attached to cell 2 by an anchoring junction placed in the middle between both cells, holding the two cell membranes close together. This junction is labelled ‘anchoring junctions: important for structural support, particularly in tissues with a lot of movement/friction’. The cells are anchored to one another by a bundle of tubular cytoskeletal structures and extracellular matrix components. Cell 1 is also attached to cell 2 by a tight junction labelled ‘tight junctions: form a seal between two cells that prevents movement of water and other small molecules’. An intercellular barrier between two neighbouring adjacent cells has intercellular spaces between protein complexes and connects two cells tightly forming a fluid-tight seal. Cells 2 and 3 are attached by a gap junction placed near the basal membrane. This junction is labelled ‘gap junctions: cells are connected by channels that allow the passage of small substances between the cells’. The cell junction is formed by an intercellular bridge (fluid-filled protein channels) formed between adjacent cell membranes of neighbouring cells.

Figure 11 Cells are connected to each other by three main types of junctions: tight junctions, anchoring junctions, and gap junctions

A healthy gut microbiome helps to maintain the structural integrity of the gut wall in several ways. Firstly, it helps to regulate the proteins that are made by the human gut cells which are needed to secure the cells to each other via tight junctions. This helps to maintain the structural integrity of the gut wall and prevent it from becoming permeable to damaging substances. Additionally, the SCFAs produced by the microbiome modify the production of mucus by the gut wall, and affect how the immune cells located in the gut respond to pathogenic invaders (Sanders, 2021). These all have an important role in the health of the gut wall, and consequently the human host.

Dysbiosis is associated with increased permeability of the gut. That is, the tight junctions between the cells of the gut wall become weakened and substances that normally would not be able to cross the gut wall can enter the local tissue or bloodstream. The presence of such substances triggers an immune response, leading to significant inflammation of the gut. Several health conditions are associated with this increased permeability and inflammation, as you’ll learn later in this course.