Microvilli are microscopic, finger-like projections that extend from the surface of many cells throughout the human body. They are present in vast numbers, covering the outer surfaces of various cell types. Microvilli perform functions fundamental to numerous bodily processes. Their widespread presence highlights their significance in maintaining the body’s internal environment and facilitating everyday functions.
Understanding Microvilli
Microvilli are delicate extensions of a cell’s plasma membrane. Each microvillus is about 0.1 micrometers in diameter and can range from 0.1 to 2 micrometers in length. These protrusions are smaller than cilia, though they are thicker. The structural support for each microvillus comes from a dense bundle of cross-linked actin filaments, which are a type of protein.
These actin filaments are tightly bundled and held together by bundling proteins such as fimbrin, villin, and espin. Microvilli contain little to no cellular organelles within their structure. This architecture allows microvilli to increase the surface area of the cell membrane, much like the bristles on a brush. Dense collections of microvilli are often called a “brush border.”
Where Microvilli Reside
Microvilli are present in various locations within the human body, each instance serving a specific role related to the organ’s function. They are abundant in the small intestine, forming part of the intestinal lining. Here, microvilli create a “brush border” on the surface of epithelial cells, which are responsible for absorption.
Beyond the digestive tract, microvilli are found in the kidney tubules, specifically in the proximal convoluted tubules of the nephrons. In these structures, they contribute to reabsorption processes as blood is filtered. Specialized forms of microvilli, known as stereocilia, are present in the inner ear, playing a role in sensory reception. Microvilli can also be found on the surface of egg cells, aiding in sperm anchoring, and on white blood cells where they serve as anchoring points.
The Diverse Roles of Microvilli
The primary function of microvilli is to increase the surface area of cells, enhancing their ability to interact with their surroundings. In the small intestine, this increased surface area is important for nutrient absorption. The microvillar membrane contains enzymes, such as glycosidases, that break down complex nutrients like carbohydrates into simpler, more easily absorbed compounds. This extensive surface area, amplified approximately 25 times, allows for the efficient uptake of digested food, water, and electrolytes into the bloodstream.
In the kidneys, microvilli play a role in reabsorption and secretion processes. Their presence increases the surface area for the reabsorption of water, sodium, and other solutes back into the bloodstream from the filtered fluid. This reabsorption is flow-dependent, meaning changes in fluid movement within the tubules can influence sodium uptake.
Specialized microvilli are found on the hair cells within the inner ear, where they are involved in sensory reception for hearing and balance. These stereocilia respond to mechanical stimuli, such as sound vibrations or head movements. Their bending causes the opening of ion channels, converting mechanical energy into electrical signals transmitted to the brain, allowing for the perception of sound and maintenance of equilibrium. Microvilli also contribute to the sense of taste, as they are present on the surface of taste cells within taste buds, where they bind with taste molecules dissolved in saliva.
Microvilli and Human Health
When microvilli are damaged or dysfunctional, it can lead to various health problems. In the small intestine, conditions that damage the brush border can impair nutrient absorption. Celiac disease, an autoimmune disorder triggered by gluten, leads to the flattening and damage of intestinal microvilli, resulting in malabsorption of nutrients, abdominal cramping, and diarrhea.
Damage to the specialized microvilli in the inner ear can lead to sensorineural hearing loss. Dysfunction in these microscopic structures, even in seemingly unrelated parts of the body, can have broader implications for overall well-being.