Metaplasia

Metaplasia is a medical condition in which one type of cell turns into another type of cell. This change sometimes occurs as part of normal growth. Other times, the cell changes to adapt to something in its environment. These environmental causes can include stressors such as chemicals and physical stress or irritation. In most cases, the original cell type returns if the environmental stressor is removed.

Background

The word “metaplasia” comes from the Greek word metaplase. This word, in turn, comes from the Greek metaplassein, which means “change into a new form.” The first known use of the word was in 1883.

Cells are the most basic structure of life. All types of cells have similar components and capabilities, such as producing food and reproducing. However, cells can vary greatly in how they look and function. For example, the cells in human skin do not look or act like red blood cells because they have different jobs.

Once cells reach maturity, they look and perform like other cells of the same type. However, in some cases, something affects this process and leads to alterations in the cells. The three main types of changes as identified by biologists are metaplasia (cells that change from one type to another), dysplasia (normal cells that become abnormal), and neoplasia (the growth of new, abnormal cell tissue). Dysplasia and neoplasia are associated with cancer, while metaplasia generally is not. In some cases, cells can progress from metaplasia to dysplasia to neoplasia.

Overview

Metaplasia is an adaptive reaction that occurs when cells are no longer able to function in their environment. While this can occur as part of the normal growth process, it more commonly occurs when the original cells are exposed to a source of stress that they are not designed to withstand. The source of stress can be a chemical irritant, such as stomach acid or smoke, or a physical irritant, such as a bladder or kidney stone. The original cells are replaced by different cells that are better adapted for the new environment.

One of the most common examples is the change that occurs in the lungs in response to chronic exposure to cigarette smoke. The normal respiratory laryngeal epithelial cells are unable to cope with the damage inflicted by the smoke, so they are replaced with squamous epithelial cells. These squamous cells are better able to exist under the stress created by repeated exposure to smoke.

Smoke is only one potential cause of metaplasia. Others include exposure to radiation, chemotherapy, and certain infections such as human papillomavirus. Metaplasia is also associated with the presence of some diseases, including myelogenous leukemia, polycythemia Vera, primary myelofibrosis, and some disorders of the bone marrow. The physical irritation caused by kidney or bladder stones can cause those organs to respond by generating new types of cells. In addition, some people are believed to have a genetic tendency to metaplasia in some parts of the body, such as the uterus, or womb.

Another condition associated with metaplasia is a gastro-intestinal disorder known as Barrett’s esophagus. In this condition, the esophagus becomes irritated because the contents of the stomach back up into it, which is known as reflux. This causes intestinal metaplasia as the squamous mucus cells that normally line the esophagus are replaced by intestinal cells that are better able to withstand the stomach acid and other substances that are part of the reflux. The cause of Barrett’s esophagus is still not known, but the presence of an infection known as H. pylori as well as consumption of large amounts of salt and alcohol and the use of nonsteroidal anti-inflammatory drugs (NSAIDs) for the relief of pain are thought to play a role.

The symptoms of metaplasia vary depending on the type of cells that have been affected. It is also possible that metaplasia will have no symptoms. For example, metaplasia resulting from smoking can trigger wheezing and frequent respiratory infections. This is because the body no longer has the protective mucus barrier present in the original respiratory epithelial cells. This barrier is not in the squamous cells that replace the epithelial cells. On the other hand, intestinal metaplasia is only detected by examination via an endoscopy. Other symptoms that can be indicators of cells affected by metaplasia include anemia (lack of sufficient red blood cells), fatigue, bleeding, bone and joint pain, sudden weight loss, and decreased resistance to infection.

Metaplasia is a reversible condition. When the source of the stress is removed, the new adaptive cells are replaced by the original cells. However, if the condition continues for too long, dysplasia may occur and new abnormal cells can form. This can then lead to neoplasia, where abnormal tissue develops. For instance, the intestinal metaplasia associated with Barrett’s esophagus can progress to dysplasia and ultimately lead to stomach cancer.

While metaplasia remains an acceptable term to describe the process of one type of cell being replaced by another, some scientists prefer to use the term transdifferentiation. Also known as lineage reprogramming, transdifferentiation refers to a cell changing from one specific or differentiated state to another. Some researchers consider transdifferentiation to be a type of metaplasia instead of using the terms interchangeably. Others use transdifferentiation only when the cell change is prompted intentionally through gene therapy.

When used in human biology, metaplasia generally refers to cells that have reacted or adapted to some form of stress. In animal biology, the term is also used to refer to cell change related to regeneration. For example, if a salamander’s eye is damaged, the iris cells from the colored portion of the eye can change to lens cells and allow the animal to regain its sight.

Bibliography

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