A sac-like structure that is found in the mature gland[GO]. [ http://amigo.geneontology.org/amigo/term/GO:0060749 ]
Synonyms: lactiferous alveolus mammary alveolus lactiferous gland alveolus alveolus of lobe of mammary gland alveolus of lobe of breast lobe of breast alveolus alveolus of lactiferous gland lobe of mammary gland alveolus alveolus of mammary gland