Glandular epithelium can be classified in several ways: 1) By the presence or absence of duct systems into exocrine or endocrine glands. 2) Exocrine glands can be multicellular or unicellular based on cell number, and can secrete products through merocrine, holocrine, or apocrine secretion modes. 3) Exocrine glands can also be classified by the type of secretory product into serous, mucous, or mixed seromucous glands.