What is aplasia, and where can it occur?


Aplasia is a condition in which an organ, limb, or other body part does not develop. In most cases, aplasia is obvious at birth. However, certain types of aplasia may sometimes not be apparent until later in life.

In this article, we look at the causes and symptoms of different types of aplasia. We also explain how it differs from similar conditions, such as hypoplasia, atrophy, agenesis, and dysplasia.

As aplasia can affect any organ or tissue, there are many types of this condition. Some examples include:

Acquired pure red cell aplasia (PRCA)

In the bone marrow, red blood cells begin as cells called erythroblasts. These later develop into mature red blood cells.

People with PRCA do not develop erythroblasts. As a result, they may have aplastic anemia, which is a condition in which the bone marrow does not create the blood cells that the body needs.

According to the National Center for Advancing Translational Sciences, symptoms of PRCA include:

There are various causes of this condition, including autoimmune diseases, thymomas, tumors, and viral infections. However, in some cases, the cause is unknown.

Aplasia cutis congenita

Aplasia cutis congenita is a rare condition that causes newborns to have skin missing from parts of their body. In some cases, the underlying structures beneath the skin, such as bone, may also not be present.

This congenital condition most commonly affects the scalp. However, it can affect any part of the body. The affected areas will have a thin transparent membrane over the top. In some cases, it is possible to see the baby’s internal organs through the membrane.

Aplasia cutis congenita may be due to a mutation of certain genes.

Radial aplasia

The radius is a bone that connects the humerus bone in the upper arm to the wrist. People with radial aplasia are born without the radius bone.

Without the radial bone, the forearm appears shorter than it should. In addition, the hand and wrist turn inward toward the thumb side of the forearm.

Although there are many theories as to what causes radial aplasia, more research is necessary to determine the exact cause of this condition.

Germ cell aplasia

The seminiferous tubules within the testes are where spermatogenesis, or sperm creation, takes place. The tubules contain two types of cells: spermatogenic cells and Sertoli cells.

Spermatogenic cells help with the process of spermatogenesis. One of the main functions of Sertoli cells is to provide nutrition to the sperm.

People with germ cell aplasia have Sertoli cells but no spermatogenic cells. Medical professionals may also refer to this type of aplasia as Sertoli cell-only syndrome or Del Castillo syndrome.

This type of aplasia does not produce any physical symptoms. The main sign that a person has germ cell aplasia is infertility.

For this reason, a person may not realize that they are experiencing this condition until they attempt to conceive via penile-vaginal intercourse.

If a doctor suspects germ cell aplasia, they will likely perform a biopsy using tissue from the person’s testes.

Thymic aplasia

The thymus is a gland that plays an important role in the immune system. Although it only functions up until puberty, the thymus helps young T cells mature and specialize.

T cells recognize and attack harmful microbes and cells, such as bacteria, viruses, and cancers. However, each T cell fights off only one type of microbe or cell.

Thymic aplasia occurs alongside DiGeorge syndrome. In people with this genetic condition, a small segment of a chromosome is missing. A baby without a thymus is at high risk of developing life threatening infections.

Aplasia of the lung

In rare cases, a baby may be born without one of their lungs.

A 2015 case study details the examination of a baby born without a lung. The authors of the study note that shortly after birth, the baby was having breathing difficulties and required ventilator care.

This type of aplasia can be difficult to identify in an antenatal scan. However, the authors of the case study highlight that at the time of writing, the baby was stable and reaching all expected growth milestones.

Agenesis, aplasia, and hypoplasia are very similar terms that all refer to varying stages of incomplete development. They all result in either incomplete or undersized organs or tissues.

Agenesis

Some people may use the terms agenesis and aplasia interchangeably.

However, researchers have used agenesis to describe the complete absence of an organ and aplasia to refer to the failure of an organ to develop past the earliest stage.

Hypoplasia

Hypoplasia refers to the underdevelopment or incomplete development of body parts.

In some situations, conditions associated with aplasia may also fall under the category of hypoplasia. For example, a person with radial aplasia may not be missing the whole radius bone and instead have a radius bone that is shorter than usual.

Atrophy

Atrophy is another related term. It refers to the partial or complete wasting of a cell, organ, or tissue following normal, matured growth.

Atrophy usually appears as a reduction in size or functionality. It differs from hypoplasia, in which the reduction in size is due to a cell, organ, or tissue not reaching normal maturity.

According to the National Cancer Institute, dysplasia is where the cells of tissues or organs develop abnormally. It is different than aplasia, where the organ or tissue does not develop past the earliest stage,

Dysplasia can occur in any part of the body and can affect both children and adults.

A doctor can diagnose dysplasia in a developing fetus before birth, and it can cause developmental problems as the child grows.

In adults, dysplasia typically refers to the abnormal growth of tissues or cells. These cells can be precancerous and create tumors if they continue to grow.

There are many types of dysplasia, and the causes of each type are not yet known.

Sometimes, medical professionals may use the same term to describe aplasia or dysplasia that affects a specific area of the body.

For example, they may refer to both radial aplasia and radial dysplasia as radial club hand. They might sometimes also use this term to refer to hypoplasia of the radius.

Doctors classify radial club hand into four different types, depending on the specific cause.

This categorization can lead to confusion as to whether it is appropriate to use the terms aplasia and dysplasia interchangeably.

However, people should not see the two terms as equivalent. Although they may produce similar effects in the same area of the body and cause conditions with a shared name, aplasia and dysplasia are different.

Aplasia occurs when an organ, limb, or body part does not develop during fetal development.

Many types of aplasia will be obvious at birth, but some may not become apparent until later in life.



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