+A **sac** is a closed, pouch-like anatomical structure, often containing fluid or air. These simple enclosures serve diverse biological roles, from protecting developing [Embryo](/wiki/Embryo) to holding vital substances within a larger [Organ](/wiki/Organ).
+## See also
+- [Cyst](/wiki/Cyst)
+- [Vesicle](/wiki/Vesicle)
+- [Pouch](/wiki/Pouch)
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