Many cells interact with their environment using projections, which are extensions that protrude from the main cell body. These highly organized structures can be thought of as a cell’s arms and legs, reaching out to move, sense, or communicate. Each type of projection is built for a specific purpose, allowing cells to perform diverse functions.
Projections for Movement
Many cells move using specialized projections. Cilia are numerous, short, hair-like structures that cover a cell’s surface and beat in a coordinated, whip-like fashion to create currents. In the human respiratory tract, millions of cilia work in unison to sweep mucus, trapped dust, and pathogens out of the airways. This action helps keep the lungs clear.
A different motile projection is the flagellum, which is significantly longer than a cilium and appears alone or in small numbers. The tail of a sperm cell is a classic example, acting as a propeller to drive the cell forward. Both cilia and flagella share a common architecture: a core composed of protein filaments called microtubules. This scaffolding provides the support and force required for their rhythmic beating.
Projections for Absorption and Sensing
Some cellular projections interact with the surroundings by increasing surface area or detecting signals. Microvilli are tiny, finger-like extensions that dramatically expand the cell’s surface area. This is evident in the cells lining the small intestine, where a vast surface area is needed for efficient nutrient absorption. The dense packing of microvilli creates a structure known as a brush border, maximizing this capacity.
Another projection is the stereocilium, which is structurally more like a microvillus than a cilium. These structures are specialized for sensory reception. In the inner ear, stereocilia on hair cells bend in response to sound waves. This bending opens ion channels, converting the physical vibration into an electrical signal for the brain. Unlike cilia and flagella, both microvilli and stereocilia are built around bundles of supportive actin filaments.
Projections for Communication
The nervous system relies on highly specialized projections for communication. Neurons, the primary cells of the nervous system, use two types of projections to form complex networks. Dendrites are elaborate, branching extensions that act as the neuron’s receivers. They collect incoming signals from other cells and convey them toward the neuron’s cell body.
The second projection, the axon, is a single, long, cable-like structure that functions as the neuron’s transmitter. The neuron generates an electrical impulse, or action potential, which travels down the axon. At its end, the axon branches out to connect with other cells, passing the signal along. This system of dendrites and axons forms the wiring for rapid communication throughout the body.
Impact on Human Health
When cellular projections do not function correctly, the consequences for human health can be serious. A condition called Primary Ciliary Dyskinesia (PCD) highlights the importance of motile projections. In PCD, defective cilia in the respiratory tract cannot clear mucus, leading to chronic respiratory infections and sinus problems. This condition can also cause infertility due to malfunctioning flagella on sperm cells.
Defects in sensory projections can also lead to permanent impairment. For instance, stereocilia of the inner ear can be damaged by loud noises, certain medications, or aging. Once lost, these projections do not regenerate, often resulting in irreversible hearing loss.
The nervous system is also affected by projection damage. In multiple sclerosis, the protective coating around axons is damaged, disrupting nerve signals and causing neurological symptoms. Peripheral neuropathy involves similar damage to axons in peripheral nerves, causing pain, numbness, and weakness.