Define Exocrine gland.

Hint: Exocrine are duct glands that secrete chemical messengers. These glands are the opposite of endocrine glands (ductless glands). The most common example of an exocrine gland is the pancreas.

Complete answer: Exocrine glands are glands that release chemical messengers in the form of hormones to their target site through ducts. Whereas, endocrine glands are ductless glands that release secretions directly into the bloodstream. The endocrine glands include the major endocrine system; pituitary gland, thyroid gland, pancreas, ovaries, testis, parathyroid glands, etc. The exocrine glands are characterized by the presence of ducts. These glands release their secretions onto an epithelial surface through ducts. Examples of these glands include sweat, salivary, mammary, lacrimal, sebaceous, ceruminous, prostate and mucous glands. The exocrine glands contain the duct part and the glandular portion. The duct portion of the gland can be divided into branched or unbranched based on its structure. The glandular portion may be tubular or acinar or maybe a combination of the two which is known as tubuloacinar. The exocrine glands are classified based on the process in which their secretions are released as apocrine glands, holocrine glands and merocrine glands. In merocrine secretion, the cells release their secretions by exocytosis; for example, pancreatic acinar cells. In apocrine secretion, a portion of the cell membrane that contains the excretion buds off. While, in holocrine secretion, the entire cell collapses to secrete its substance; for example, sebaceous glands of the skin and nose.

Note: The serous cells secrete proteins, most commonly enzymes, for example, the gastric chief cells. Mucous cells secrete mucus. For example, Brunner's glands, esophageal glands, and pyloric glands. Mixed glands secrete both protein and mucus. For example, the salivary glands. Sebaceous glands secrete sebum, which is a lipid product. These glands are also known as oil glands.