How do exocrine glands differ histologically from endocrine glands?

• A. Exocrine glands have ducts and secretory units that release products onto surfaces; endocrine glands lack ducts and release hormones into interstitial fluid/blood
• B. Exocrine glands lack ducts; endocrine glands have ducts to surfaces
• C. Exocrine glands secrete hormones into blood; endocrine glands secrete onto surfaces
• D. Exocrine glands have secretions stored in ducts only; endocrine glands do not secrete

Answer: (D)

The main idea is that the way a gland releases its product is defined by the presence or absence of ducts and where the product goes. Exocrine glands make secretions that travel through ducts to a surface or luminal space, such as the skin, gut, or a mucosal surface. Their secretory units (acini or tubules) drain into these ducts, so the product acts locally on a nearby surface.

Endocrine glands, in contrast, do not have ducts. They release hormones into the surrounding interstitial fluid, from which these hormones quickly enter the bloodstream to be carried to distant target organs. The gland’s tissue is highly vascular to facilitate this uptake into blood.

A helpful example is the pancreas, which has an exocrine portion that secretes digestive enzymes into ducts leading to the duodenum, and an endocrine portion (islets of Langerhans) that releases hormones like insulin into the bloodstream.

The statement about secretions being stored in ducts isn’t accurate, because ducts are pathways, not storage sites. Secretions are produced and stored in secretory cells or granules and released through ducts, while endocrine products are released directly into the blood.