Cellular protrusions are microscopic extensions that project outward from the surface of cells. Formed by rearrangements of the cell’s internal scaffolding, the cytoskeleton, these structures allow cells to interact with their surroundings. Protrusions facilitate communication, movement, and absorption, supporting many fundamental life processes.
Common Types of Cellular Protrusions
Cells extend various types of protrusions, each with a distinct shape and purpose. Lamellipodia are broad, sheet-like extensions that resemble a fan or a flat foot, found at the leading edge of moving cells. They are composed of a dense, branched network of actin filaments, which push the cell membrane forward. These structures enable a cell to “crawl” along surfaces, similar to how an amoeba moves.
Filopodia are slender, finger-like projections, much thinner than lamellipodia. They typically measure 0.1 to 0.3 micrometers in diameter and extend several micrometers in length. Filopodia contain bundled actin filaments that allow them to grow and retract, sensing chemical and physical cues in the cell’s surroundings.
Microvilli are small, stable, finger-like projections that resemble a brush border on the surface of certain cells. They are found in large numbers, particularly lining organs like the small intestine. Unlike lamellipodia and filopodia, microvilli have a stable internal structure of actin filaments. Their primary role involves increasing the cell membrane’s surface area.
The Function of Protrusions in the Body
The specific forms of cellular protrusions enable diverse bodily functions. Lamellipodia and filopodia facilitate cell movement, a process observed in wound healing when cells migrate to repair damaged tissue. Immune cells also use these crawling extensions to navigate towards sites of infection, engulfing pathogens and clearing debris. This directed movement is also important during embryonic development, guiding cells to their correct locations to form organs and tissues.
The extensive surface area provided by microvilli is important for absorption in the digestive system. In the small intestine, millions of microvilli lining epithelial cells increase the available area for nutrient uptake. These structures also contain digestive enzymes on their surface, which break down complex food molecules into simpler forms that can be absorbed into the bloodstream. This dual function enhances the efficiency of nutrient absorption.
Connection to Human Disease
When the formation or function of cellular protrusions becomes unregulated, it can contribute to disease progression. In cancer, malignant cells often exploit these mechanisms, forming specialized invasive protrusions called invadopodia. These structures allow cancer cells to degrade surrounding tissue and spread throughout the body, a process known as metastasis. Cancer cells can extend long protrusions to pull themselves through tissue barriers like the basement membrane.
Defects in other types of protrusions, such as cilia, are linked to a group of genetic disorders known as ciliopathies. Cilia are hair-like projections that act as cellular antennae, sensing signals and facilitating fluid movement. When cilia do not form or function correctly, it can lead to problems affecting multiple organs, including the kidneys, retina, and brain. Examples of ciliopathies include polycystic kidney disease and Bardet-Biedl syndrome.