Animal cells possess microscopic structures called cilia, which play varied roles in their biology. These hair-like protrusions perform diverse functions, ranging from orchestrating fluid movement to acting as cellular antennae.
What Are Cilia?
Cilia are slender, hair-like projections extending from the surface of many eukaryotic cells. These cellular appendages are built upon a core structure, the axoneme, which consists primarily of microtubules. Microtubules are cylindrical structures made of tubulin proteins, forming the internal skeleton of the cilium. Cilia are anchored to the cell body by a basal body, structurally similar to a centriole.
The axoneme is encased by an extension of the cell’s plasma membrane. This microtubule-based cytoskeleton allows cilia to either move or sense their environment. Their presence and arrangement on the cell surface are key to their biological functions.
Two Main Forms of Cilia
Animal cells feature two distinct types of cilia: motile cilia and primary, or non-motile, cilia. These forms are distinguished by their internal structure, specifically the arrangement of microtubules within their axoneme.
Motile cilia possess a “9+2” axoneme, with nine pairs of microtubule doublets arranged in a ring around two central, single microtubules. This arrangement, along with associated motor proteins called dyneins, enables these cilia to bend and generate movement. Dynein motor proteins facilitate sliding motions between adjacent microtubules, resulting in a coordinated beating or rotational motion.
In contrast, primary cilia are non-motile and exhibit a “9+0” axoneme structure, lacking the central pair of microtubules found in motile cilia. Most cells possess a single primary cilium.
Diverse Roles in Animal Physiology
Cilia play a wide array of roles in animal physiology. Motile cilia are involved in generating fluid flow and moving substances across cell surfaces. For example, in the human respiratory tract, motile cilia work in coordinated waves to sweep mucus and trapped particles away from the lungs, a process known as mucociliary clearance.
These motile cilia are also present in the female reproductive tract, in the fallopian tubes, where their synchronized beating helps transport egg cells from the ovary towards the uterus. Additionally, motile cilia in the brain’s ependymal cells contribute to the movement of cerebrospinal fluid.
Primary cilia function as sensory organelles or “cellular antennae,” detecting signals from the extracellular environment. In the kidneys, primary cilia on tubular epithelial cells sense fluid flow and play a part in maintaining kidney function. Defects in these cilia can lead to conditions like polycystic kidney disease.
Primary cilia are also important in the eye’s photoreceptor cells, where they form a specialized bridge connecting the inner and outer segments. This “connecting cilium” transports proteins and materials necessary for light detection. In brain development, primary cilia regulate cell signaling pathways that influence neurogenesis and neuronal maturation.
Distinguishing Cilia from Flagella
Cilia and flagella are both hair-like cellular appendages composed of microtubules. Cilia are shorter and more numerous on the cell surface, often appearing in hundreds or thousands. They exhibit a coordinated, oar-like or whip-like beating pattern, which can move substances across the cell surface or propel the cell itself.
Flagella, in contrast, are longer and less numerous, with a cell often possessing only one or a few. Their movement is characterized by a whip-like or propeller-like motion designed for cell propulsion. A prominent example of a flagellum in animal cells is the tail of a sperm cell, which propels it through fluid. While eukaryotic flagella and motile cilia share the “9+2” axoneme structure, their length, number, and movement patterns are distinct.