Aplasia can be loosely transliterated from the Greek as "not molding." This definition doesn’t make much sense without additional details, about what is not molding or not being made. What aplasia really means in the medical sense is being born without an organ or some other body tissue, or being born without some process that creates needed elements in the body, like red blood cells. Very often these deficiencies are incredibly severe; lacking the brain or heart is fatal. At other times, aplasia of certain sorts can be addressed with medical intervention or there is no need to compensate for the missing element.
Two distinctions about aplasia must be made. It’s very different to be born without than it is to possess something and lose it through degeneration. Any form of wasting disease that causes non-functioning or gradual deterioration of an organ is usually called atrophy.
Even the non-medical community may be more familiar with terms like hyper- and hypoplasia. These respectively correspond to too much or too little of something being made or formed. Hypoplastic left heart is failure of the left ventricle in the heart to fully form, so that it is very undersized and usually non-functional. In contrast, cardiac aplasia would mean no heart at all, which is not survivable.
While aplasia sounds always fatal, this isn’t the case. There are many instances where babies are born with an aplastic organ and where they can lead relatively healthy lives. Children might be missing a kidney or spleen, for example. In the former case, a single healthy kidney can sustain life, and in the latter, using prophylactic antibiotics to prevent serious infections is generally the most effective treatment. It is more serious if the missing organ or structure performs a function that cannot be fully replaced, but even in these instances, doctors are sometimes able to use other parts of the body to create what is missing.
It’s important to note that this term doesn’t just relate to absent organs, but to other structures of the body. Some children don’t have all the lymph nodes that are expected, others have failure for hair cells to grow on a specific area or for skin to fully cover the body. Yet other children are born without the ability to produce body substances such as sperm cells or red blood cells. Each missing type gets its own treatment, and some conditions, which are not fully correctable, such as inability to produce sperm, aren’t likely to threaten life.
The causes of aplasia aren’t always known. Sometimes the manifest condition has clear links to inheritance. Other times, while the missing element arises from genetic errors, there’s no suggestion of family history or likelihood of recurrence in siblings or descendants of the person with the condition.