Many epithelia include gland cells. These may be endocrine or exocrine, depending on the fate of their secretions. Endocrine glands release secretions into the surrounding interstitial fluid. The secretions or hormones enter the circulatory system and are distributed around the body. Exocrine glands produce secretions onto epithelial surfaces, usually the linings of ducts.
Examples are sweat glands or salivary glands.
Perspiration on the surface of the skin is one example of merocrine secretion. Milk is an apocrine secretion. Sebaceous glands, associated with hairs, are holocrine glands. The cells are replaced by division of stem cells. Glands are organised in a number of different ways, and produce secretions of varying types.