Metaplasia and dysplasia

Definitions & Key takeaways

Metaplasia is the term used to describe the transformation of one mature type of cell into another mature type of cell. Dysplasia is a term used to describe an increased amount of immature cell types, often abnormal. Both metaplasia and dysplasia typically result from chronic environmental stressors. Metaplasia is considered benign, whereas it indicates a precancerous state.

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Introduction0:00–0:33

Do you remember your first day in elementary school? Everything and everyone was new and nothing was impossible.
But as you went through your education, you got further and further differentiated from your original classmates. That’s analogous to what happens to a cell that undergoes cellular differentiation.
An undifferentiated stem cell can become pretty much any tissue, influenced by both genes and the environment. Now sometimes, environmental stresses can alter that developmental path.

Metaplasia0:33–2:43

In metaplasia what happens is that a mature, differentiated cell type is replaced by another mature, differentiated cell type.
Often, this happens because there’s an environmental stressor, that the new cell type is better suited to handle. One example, is switching from breathing clean air to inhaling tobacco smoke each day.
Our airways are lined with columnar respiratory epithelial cells, which generally work well with air breathing, but not so well when faced with an irritant, such as tobacco smoke.
In response to the toxins in the smoke, already differentiated, mature columnar respiratory epithelial cells are replaced by stem cells undergoing differentiation into sandbag-shaped squamous epithelial cells, which become stratified - meaning that they form layers on top of another.
This replacement of already differentiated, mature cells into another type of cell is known as metaplasia. Another example is our esophagus, which is lined with a nonkeratinizing squamous epithelium.
These cells are adapted to withstand the passage of food going down to our stomach. However, in case of gastroesophageal reflux disease, acid from the stomach makes its way up into the esophagus on a regular basis.
Esophagus cells are not well-suited for chronic contact with acid and can get damaged. Normally, when there’s occasional damage, stem cells differentiate into new squamous epithelial cells to replace the damaged ones.
But when there’s regular exposure to acid, stem cells begin to adapt by differentiating into nonciliated, mucin producing columnar epithelial cells.
These cells are far better suited to withstand the acid - after all, they’re the same types of cells that are found in the small intestine.
This is an example of metaplasia, and the condition is known as Barrett’s esophagus. Now, metaplasia is technically reversible - so if the gastroesophageal reflux disease is treated, stem cells will begin to divide into regular esophagus epithelial cells again.
On the other hand, if the problem persists, the cells can become dysplastic.In dysplasia, the tissue develops a large number of immature cells that have weird shapes, which you don’t typically expect in normal, healthy tissue.

Dysplasia2:43–4:41

There are four major pathological microscopic changes. The first is cellular pleomorphism meaning that the cells are of uneven size - which is a strange finding, since usually tissue contain cells that are of roughly the same size.
Next there’s nuclear pleomorphism or variation in size and shape of the nucleus. There’s also hyperchromatism - which means that the cells have abnormal nuclear morphology due to abundant nuclear chromatin, resulting in darkly staining nuclei.
And finally, there can be an increased number of cells with mitotic figures - meaning that a lot of cells are undergoing mitosis, or cell division.
All of these are signs of unhealthy cells that may be cancerous. Let’s take the example of the cervix, which is normally lined with glandular epithelium.
However, infection with certain strains of human papillomavirus, or HPV, can cause the cells to become dysplastic. They can resemble immature squamous, epithelial cells, of various sizes and shapes, and they multiply at a faster rate than normal cells would.
Uncontrolled multiplication is generally bad news, therefore, dysplasia is considered a precancerous state. As they multiply further, we begin to speak of carcinoma in situ - which is a localized cancer.
Sometimes these immature and abnormal cells start to multiply so much that they break through the basement membrane. The basement membrane is a layer of fibrous tissue that separates epithelial tissue from underlying tissues.
So if the cells break through that barrier and continue multiplying, they can begin exerting pressure on local tissues, and break off clumps of cells that can go off to other tissues to spread the problem.
At this point it’s considered an invasive, cervical carcinoma. Alright, as a quick recap.

Review4:41–5:07

Metaplasia and dysplasia typically result from chronic environmental stressors. Metaplasia is the replacement of one mature type of cells into another mature type of cells, while dysplasia is an increased amount of immature cell types, which may sometimes show unusual properties as well.
Metaplasia is generally benign, while dysplasia is considered a precancerous state, that can develop into carcinoma.