Atresia

Atresia refers to a situation where there is underdevelopment of a structure with very rudimentary remnant tissues. This contrasts with an agenesis meaning there is no development of the structure as all. The term atresia is often used with hollow structures such as a bronchus or intestine. 

For example:

• respiratory tract
  • bronchial atresia
  • choanal atresia
  • tracheal atresia
• alimentary tract 
  • esophageal atresia
  • duodenal atresia
  • ileal atresia
  • jejunal atresia
  • jejuno-ileal atresia
  • anal atresia
• bile ducts
  • biliary atresia
• genito-urinary tract
  • vaginal atresia
    • lower vaginal atresia
  • urethral atresia

Some authors occassionally overlap the two terms (i.e atresia and agenesis).