Hypoplasia (from Ancient Greek ὑπo- hypo-, "under" + πλάσις plasis, "formation"; adjective form hypoplastic) is underdevelopment or incomplete development of a tissue or organ. Although the term is not always used precisely, it properly refers to an inadequate or below-normal number of cells. Hypoplasia is similar to aplasia, but less severe. It is technically not the opposite of hyperplasia (too many cells). Hypoplasia is a congenital condition, while hyperplasia generally refers to excessive cell growth later in life. (Atrophy, the wasting away of already existing cells, is technically the direct opposite of both hyperplasia and hypertrophy.)
Hypoplasia can be present in any tissue or organ. It is descriptive of many medical conditions, including underdevelopment of organs such as:
Breasts during pubertyTestes in Klinefelter's syndromeOvaries in Fanconi anemia, gonadal dysgenesis, trisomy XThymus in DiGeorge syndromeLabia majora in popliteal pterygium syndromeCorpus callosum, connecting the two sides of the brain, in Agenesis of the corpus callosumCerebellum caused by mutation in the Reelin geneTooth caused by oral pathology, such as Turner's hypoplasiaChambers of the heart in hypoplastic left heart syndrome and hypoplastic right heart syndromeOptic nerve in optic nerve hypoplasiaSacrum in sacral agenesisFacial muscle in asymmetric crying faciesThumb from birthLungs, often as a result of oligohydramnios during gestation or the existence of congenital diaphragmatic herniaSmall bowel in coeliac diseaseFingers and ears in Harlequin type ichthyosisMandible in congenital hypothyroidism