Exocrine gland

Classification

Structure

Exocrine glands contain a glandular portion and a duct portion, the structures of which can be used to classify the gland.

• The duct portion may be branched (called compound) or unbranched (called simple).
• The glandular portion may be tubular or acinar, or may be a mix of the two (called tubuloacinar). If the glandular portion branches, then the gland is called a branched gland.

Method of secretion

Depending on how their products are secreted, exocrine glands are categorized as merocrine, apocrine, or holocrine.

• Merocrine – the cells of the gland excrete their substances by exocytosis into a duct; for example, pancreatic acinar cells, eccrine sweat glands, salivary glands, goblet cells, intestinal glands, tear glands, etc.
• Apocrine – the apical portion of the cytoplasm in the cell membrane, which contains the excretion, buds off. Examples are sweat glands of arm pits, pubic region, skin around anus, lips and nipples; mammary glands, etc.
• Holocrine – the entire cell disintegrates to excrete its substance; for example, sebaceous glands of the skin and nose, meibomian gland, zeis gland, etc.

Product secreted

• Serous cells secrete proteins, often enzymes. Examples include gastric chief cells and Paneth cells
• Mucous cells secrete mucus. Examples include Brunner's glands, esophageal glands, and pyloric glands
• Seromucous glands (mixed) secrete both protein and mucus. Examples include the salivary glands: although the parotid gland (saliva secretion 25%) is predominantly serous, the sublingual gland (saliva secretion 5%) mainly mucous gland, and the submandibular gland (saliva secretion 70%) is a mixed, mainly serous gland.
• Sebaceous glands secrete sebum, a lipid product. These glands are also known as oil glands, e.g. Fordyce spots and Meibomian glands.