Which of the following is NOT a type of exocrine gland?

• A. Merocrine
• B. Apocrine
• C. Holocrine
• D. Endocrine

Answer: (D)

Glands are classified by how they release their secretions. Exocrine glands secrete onto surfaces through ducts, while endocrine glands are ductless and release hormones directly into the bloodstream. Among exocrine glands, secretions can be released by different modes: merocrine involves exocytosis with the cell preserved; apocrine releases a portion of the cell’s apical cytoplasm; holocrine results in the whole cell disintegrating to release its product. Examples help: sweat and salivary glands are typically merocrine, mammary glands are often cited for apocrine, and sebaceous glands for holocrine. The key point is that endocrine glands are not exocrine because they lack ducts and secrete hormones into the blood. Endocrine is the type that is not an exocrine gland.