Ciliated cells are cells that are covered in tiny hair-like projections known as cilia. There tend to be two main types of this cell, namely motile and non-motile, sometimes also known as “primary.” In most cases this distinction has to do with how the cell uses its cilia. Motile cells use the protrusions to move through space or to move other substances, like dust and debris, off of their outer membrane. Primary cells, on the other hand, more frequently use their cilia to interpret the outside environment or to pick up signals from the nearby area. Both types of cells occur in many places in nature as well as in human and animal bodies. They play many roles, and problems with movement or sensory perception can cause a number of chronic issues.
Types and Basic Appearance
The most obvious thing about ciliated cells is their appearance: they are typically somewhat oblong in shape and the surface is covered with what often looks like a small fringe or short fur. Cilia are made of microtubules, which are thick spirals of tubulin that cover then entire outer cell membrane. The place where they “root” or fix to the cell wall is usually dense in various proteins and often serves as a conductor port for relaying signals to and from the nucleus, or cell operating center.
There are typically two main types of ciliated cell. Some use their protrusions for movement. The cilia in these cases will move and sway in synch, causing the cell itself to move or causing various substances to move across its surface. These are known as motile cells, and are some of the most commonly recognized. In some cases the protrusions also act almost like little antennae, though, helping a cell that either doesn’t move or has some other source of locomotion sense its environment. In these cases the cilia relay information about the chemical composition, temperature, or general characteristics about the surroundings back to the cell, which can guide anything from reproduction to programmed death. These are known as non-motile or primary.
Protists, which are considered by many biologists to be the most complex single-celled organisms, are examples of ciliated cells that occur in nature. These are very important to aquatic environments and exist in many rivers, streams, and lakes, as well as oceanic environments. Protists vary in their ability to tolerate pollution and as a result are often used as an indicator species to quickly measure the health of a body of water.
As common as they are in water, these sorts of cells are relatively rare in plants. One possible evolutionary explanation might have to do with how much water they need to perform optimally. Cilia dramatically increase the surface area of a cell, which increases the chance for evaporation and the need for water. This can stress a plant and put a strain on its resources, particularly when its roots are still developing. Cycads, a type of tropical tree, are one of the few plant genera that have these sorts of cells anywhere in them.
In the Body
These sorts of cells are very common in human and animal bodies and are found in most organs and arteries. In the brain, for example, they help circulate cerebrospinal fluids, and in children they may contribute to normal brain development as well. In female mammals they are often responsible for moving the ova to the uterus, and in the respiratory system of most animals they are essential when it comes to eliminating dust and germs from nasal passages.
In fact, the cilia-bearing cells in the respiratory system are the primary cells affected by the common cold and most flu viruses. These viruses work by killing the cell completely or by paralyzing the cilia temporarily, which can cause mucus and bacteria to back up or accumulate. In extreme cases this can lead to secondary infections like sinusitis or bronchitis.
People who have defective cilia or who don’t have the right number of these sorts of cells often have a range of serious health problems. A lot depends on where the cells are located. Cilia that don’t work in the female fallopian tubes can often cause ectopic pregnancy, for instance, and defective cells in the kidneys can lead to polycystic kidney disease. Chronic conditions like nephronophthisis and Alstrom syndrome are also possibilities. Sometimes these problems are a result of genetic mutations, but they can also be caused by injury or disease.