What Is Aplasia?

Aplasia is a medical term meaning that some part of the body is absent after it should already have developed. It comes from the roots “a,” (meaning without) and the Greek “plasia” (meaning formation).

Aplasia isn’t a single medical condition, and it can refer to a lack of formation of different parts of the body. Usually this means problems occur before birth, but not always. 

Aplasia is closely related to some other terms, particularly “agenesis,” “hypoplasia,” and “dysplasia.” Depending on the specifics of your condition, your healthcare provider might use these terms relatively interchangeably, but technically speaking they can be defined a little differently. 

This article will discuss different causes and examples of aplasia, as well as some of these other related terms.

Types of Aplasia

Many different parts of the body can be affected by aplasia. Symptoms and severity will depend on the exact part of the body affected. Most types are very rare. 

Most types of aplasia are present from birth. That makes sense when you think about it because most body parts are formed during pregnancy. So, some problems during this developmental phase can lead to aplasia.

Scientists aren’t sure what causes all forms of aplasia, and in some cases, no cause can be found. But some potential causes are:

• Mother’s exposure to a toxin during pregnancy
• Problems with the placenta during pregnancy
• Infection during pregnancy
• Exposure to certain medications
• Genetic problems

Some of these people have aplasia or other kinds of malformations in addition to other symptoms. 

The following are only some of the possible examples of different types of aplasia.

Aplasia Cutis Congenita

Aplasia cutis congenita is a form of aplasia that affects the development of the skin. Infants born with the condition have part of their skin that is absent at birth. This can lead them to be prone to excess bleeding, infection, and electrolyte imbalances. 

Radial Aplasia

In radial aplasia, one of the bones in the forearm (the radius), doesn’t form. Without the radius, the other bone in the forearm (the ulna), often bends abnormally. This causes the forearm to appear deformed.

Radial aplasia, a complete absence of the radius bone, is also sometimes described as a type IV radial anomaly. Radial anomalies, sometimes called radius deficiencies, include a larger group, such as those in which the radius might be present but shorter than usual. 

Other parts of the arm are sometimes affected in someone with radial aplasia, like the bones of the wrist or the thumb bone. 

Thymic Aplasia

Thymic aplasia is a condition in which the thymus doesn’t form. Because the thymus is an important organ for your immune system, this means that these people are more prone to certain kinds of severe infections. People who have at least part of their thymus have less serious issues than people who have more severe thymic aplasia. 

DiGeorge syndrome is one of the most common causes of thymic aplasia. In addition to immune problems, it can cause developmental delays and other issues. 

Aplasia of the Lung

In some people, one of their lungs is totally absent, except for a very rudimentary, ineffective lung structure. In lung aplasia, the tube leading to the lung (bronchus) begins to form, but none of the other surrounding tissues develop. 

Aplasia of the lung is one of a spectrum of congenital malformations that can affect the lungs. These people may be more prone to certain infections or lung cancer. However, many of these people do quite well with only one lung. 

Germ Cell Aplasia (Sertoli Cell-Only Syndrome)

Germ cell aplasia affects the “germ cells,” the type of cells your body uses to reproduce. Specifically, it affects the production of sperm in cisgender males. 

In germ cell aplasia, the testes don’t produce sperm normally. They might produce no sperm at all, or only in very low numbers. Not surprisingly, this causes problems with fertility.

Pure Red Cell Aplasia

Normally, your body forms new red blood cells throughout your life. In pure red cell aplasia, your body stops producing new red blood cells normally. (The word "pure" just means that other types of blood cells are not affected.) Your body may not produce any red blood cells, or it may not produce as many as normal.

This leads to anemia, a condition in which the blood doesn’t have enough functioning red blood cells to carry oxygen through your body. This might cause symptoms like fatigue and dizziness. 

Some people have trouble producing enough red blood cells from birth (congenital pure red cell aplasia). More commonly, pure blood cell aplasia is “acquired” and happens later in life. That might be from various medical conditions or from exposure to certain drugs or toxins. 

Medical Terms Related to Aplasia

Agenesis

Agenesis refers to the complete failure of an organ to develop. In many cases, agenesis means basically the same thing as aplasia. 

However, aplasia is sometimes used to indicate an organ in which some very early structures formed, but it can’t function at all. In agenesis, the organ didn’t even start to develop. In terms of clinical features, the two are often identical.

Hypoplasia

Hypoplasia refers to the incomplete development of the organ. (It comes from the root “hypo,” meaning less.) The organ might be smaller than normal, but it may have all the same key features as a normal organ. The ability of the organ to function will depend on the severity of the hypoplasia, but often it can at least function a little. 

In general, hypoplasia leads to less severe symptoms compared to aplasia. But surgery or other interventions might still be needed.

Congenital Malformations (Congenital Anomalies; Congenital Deficiencies) 

Congenital malformations refer to any situation in which some part of the body doesn’t form normally. By definition, these are always present at birth. 

For example, aplasia of the lung, kidneys, or radius are all examples of congenital malformations.  

Atrophy

Atrophy is a different medical term. Unlike most kinds of aplasia, atrophy is not present from birth. Instead, it describes a wasting away of part of the body due to aging, lack of use, or illness. 

Many people are most familiar with muscle atrophy, in which the muscles can decrease in size due to lack of use and aging. However, atrophy can happen to other organs and components of the body as well. 

Aplasia vs. Dysplasia: What’s the Difference?

Dysplasia is another related, but sometimes confusing, medical term. It comes from the roots “dys” (bad) and “plasia” (formation). It means, roughly, some sort of abnormal development or growth in some part of the body. 

Dysplasia can also mean different things in different contexts, which can also be confusing. Sometimes it refers to a kind of abnormal development that happened before birth. For example, someone with dysplasia of the radius might have a shorter radius than normal. 

This contrasts with someone with radial aplasia, in which the radius is absent. However, medical professionals might use the term “radial deficiencies” to refer to both radial dysplasias and radial aplasias.

Summary

Aplasia is a medical term that means that part of the body hasn’t formed. It isn’t a single medical condition, but rather a word that can be used in a number of different medical situations. Most types are present from birth, such as aplasia cutis congenita, radial aplasia, aplasia of the lung, thymic aplasia, and germ cell aplasia. Pure red cell aplasia is one type you might get later.

A Word From Verywell

It can be terrifying to learn that your new baby has health problems. In assessing something like an aplasia, it will take time to get a full sense of the medical picture. Nobody will be able to give you complete answers right away. Try to be patient as you and your medical team work to find the best treatments in your situation.