Polyp: Definition, Types, and Causes

Polyp Definition

In biology, polyp is a term with several meanings. It can refer to a stage in the development of certain species of invertebrates, or it can refer to a growth in the tissues of organs. Both are described below.

Types of Polyp

Polyps as a Life Cycle

Cnidarians, or jellyfish, anemones, and coral have special cells on the surface of their tentacles called nematocysts. These small explosive cells are used to deliver poison or stick to the prey. Once the prey is captured, it can be eaten. All cnidarians have some sort of nematocysts, and many exist in the polypoid form, although some do not.

Coral polyps in symbiosis with unicellular dinoflagellates

The above image shows a group of coral polyps. These small organisms start as a planula or small, free-swimming larvae. These larvae are formed when two coral gametes meet in the open sea. The larvae travel away from the parent, to establish new colonies. When a planula finds a hard surface with plenty of nutrients in the surrounding water, it can begin to establish a new colony by developing into a polyp. Polyps of most species have the same general layout. A polyp attaches to the hard surface with a special tissue on the bottom called the pedal disc. Extending away from the pedal disc, the stalk suspends the mouth and tentacles in the water column. The polyp uses these tentacles to filter food from the surrounding water and deliver it to the mouth. The polyps can reproduce asexually by budding apart. This is how large coral formations are made. Alternatively, when it is time to reproduce sexually, the corals release their gametes in the water column. When the gametes meet, new planula larvae will form and seek new places to colonize.

In the same class, Anthozoa, sea anemones also take a polyp shape for the majority of their lives. Although much bigger than individual coral polyps, the sea anemones operate in the same way. A pedal plate attaches the anemone to a hard surface, and they use their tentacles to catch food and deliver it to the mouth. Each tentacle has many nematocysts, which can sting and disable passing prey. Sea anemones reproduce in much the same way as coral polyps. They exist as polyps, release gametes which form new larvae capable of colonizing new areas. The polyp can also reproduce asexually, by budding off small pieces which can develop into full anemones.

Other cnidarians, like jellyfish, also have a polyp stage. The typical form of free-swimming jellyfish is known as the medusa stage. The polyps of jellyfish resemble those of coral and anemones, however they give rise to the medusa stage, instead of gametes. The medusas then develop and release gametes. The gametes, once fertilized, will develop into a planula larva which will grow into a polyp. The polyps of jellyfish, known as scyphistoma, develop small medusa and release them as they develop. Some jellyfish do not have a polyp stage, and the larvae develops directly into another medusa.

Polyps as a Medical Condition

A polyp, in the medical community, is an outgrowth of organ tissue. Like the polyps seen in invertebrates, some tissue polyps are attached to the mucous membrane by a thin stalk. Unlike invertebrate polyps, medical polyps are not individual organisms. Instead, they are the result of some tissue growing faster than the tissue around it. A common type of polyp, the colorectal polyp is seen in many people. Typically, these polyps are not cancerous, and do not have the potential to do harm. These small growths can be removed during a routine colonoscopy to decrease the likelihood they will develop into cancer. Large, abnormal polyps can be made of malignant cancerous tissue. These polyps will tend to grow and merge with surrounding tissues, creating a tumor. Cancerous or not, polyps are often detected by the additional bleeding they cause from the tissues they originate in. Their presence may be detected by a blood test showing higher levels of certain proteins, which indicate internal bleeding.

What Causes Polyps?

A wide variety of sources can give rise to polyps. Much like cancerous cells, polyps generally form when cell division becomes dysregulated. The cause of the dysregulation could be DNA damage, inflammation in the tissue, or any other stress on the cell that causes increased cell division. A number of genetic conditions can lead to an increase in polyps or an early-onset of polyps. Genetic mutations could also be caused by carcinogens in the environment. Carcinogens are molecules that can change DNA, sometimes leading to increased cell division and polyps. Other polyps, such as nasal polyps, can be caused by the repetitive inflammation caused by allergies. The same polyps could also be caused by inflammation due to drug use or exposure to toxic chemicals.

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