Brush Border – Function, Structure and Diseases
The brush border is a specialized cell structure found in the small intestine, formed by densely packed microvilli that greatly increase the surface area for nutrient absorption.
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The brush border is a specialized cell structure found in the small intestine, formed by densely packed microvilli that greatly increase the surface area for nutrient absorption.
What is the Brush Border?
The brush border (also called the microvillous border) is a characteristic surface structure found on certain epithelial cells. It consists of thousands of tiny, finger-like cell projections known as microvilli. Under the microscope, these tightly packed projections resemble the bristles of a brush, which is the origin of the term. The brush border is primarily located in the small intestine and the kidneys, where it plays a key role in the absorption of nutrients and the filtration of substances from fluids.
Structure and Composition
Each individual microvillus is a narrow, cylindrical extension of the cell membrane supported by an internal scaffold of actin filaments (protein fibers). The microvilli are so densely arranged that they form a uniform, brush-like surface. On their outer side, the microvilli are coated by the glycocalyx, a layer of sugar molecules that anchors important enzymes and transport proteins.
Functions of the Brush Border
Expansion of the Absorptive Surface Area
The primary function of the brush border is to dramatically increase the surface area of the cell. In the small intestine, together with the intestinal villi and the circular folds (plicae circulares), the brush border expands the total absorptive surface area to approximately 200–300 square meters. This enables the efficient uptake of nutrients such as carbohydrates, proteins, fats, vitamins, and minerals from digested food.
Digestive Enzymes of the Brush Border
Numerous digestive enzymes are embedded in the membrane of the microvilli, including:
- Lactase: breaks down milk sugar (lactose) into glucose and galactose
- Sucrase-isomaltase: digests table sugar (sucrose)
- Maltase: splits malt sugar (maltose)
- Aminopeptidases: cleave small protein fragments (peptides)
- Alkaline phosphatase: involved in fat and mineral absorption
These enzymes enable so-called membrane digestion, in which nutrients are broken down into their smallest components directly at the cell surface and immediately absorbed into the cell.
Transport Function
In addition to enzymes, the brush border contains numerous transport proteins that actively or passively move nutrients across the cell membrane into the cell interior. These include, for example, glucose transporters (SGLT1) as well as carriers for amino acids, fatty acids, and various vitamins and minerals.
Brush Border in the Kidney
A well-developed brush border is also found in the proximal tubule of the kidney. Here it is responsible for the reabsorption of important substances from the primary urine, including glucose, amino acids, phosphate, and sodium. In this way, the brush border prevents the loss of valuable compounds through urine.
Clinical Significance and Diseases
Damage to or loss of the brush border has significant clinical consequences, as both digestive enzymes and transport proteins are reduced or absent:
- Lactase deficiency (lactose intolerance): The most common enzyme defect of the brush border, leading to bloating, diarrhea, and abdominal cramps after consuming dairy products.
- Celiac disease (gluten-sensitive enteropathy): An immune-mediated condition in which the cereal protein gluten triggers inflammation of the small intestinal lining. This destroys the villi and thereby the brush border, resulting in pronounced malabsorption of nutrients.
- Microvillus inclusion disease: A rare congenital disorder in which a genetic defect prevents proper formation of the brush border. Affected infants suffer from severe diarrhea and are barely able to absorb nutrients.
- Infectious gastroenteritis: Certain viruses (e.g., rotaviruses) and bacteria can damage the brush border and temporarily reduce enzyme activity, leading to diarrhea and transient lactose intolerance.
Diagnosis
Diseases of the brush border are typically diagnosed using the following methods:
- Small intestinal biopsy: A tissue sample from the small intestine is examined under a light or electron microscope for changes in microvillus and villous structure.
- Enzyme activity measurements: Direct assessment of brush border enzyme activity (e.g., lactase, sucrase) in biopsy material.
- H2 breath test: Indirect detection of enzyme deficiency, for example in lactose intolerance, by measuring hydrogen in exhaled air following ingestion of the relevant sugar.
- Blood tests: For example, detection of antibodies in celiac disease (anti-transglutaminase antibodies).
References
- Kasper, D. L. et al.: Harrison's Principles of Internal Medicine. 21st Edition. McGraw-Hill Education, 2022.
- Johnson, L. R. (ed.): Physiology of the Gastrointestinal Tract. 6th Edition. Academic Press, 2018.
- World Gastroenterology Organisation (WGO): Global Guidelines – Celiac Disease. WGO, 2017. Available at: https://www.worldgastroenterology.org
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