Gland

A gland is an organ or tissue composed of specialized secretory cells that produce chemical substances for use within the body or for elimination. Glands can be unicellular, such as goblet cells, or multicellular with organized secretory units. They are typically classified by where their secretions go: exocrine glands release through ducts onto an epithelial surface or into a body cavity; endocrine glands release hormones directly into the bloodstream and lack ducts.

Exocrine glands include the salivary, sweat, lacrimal, and many digestive glands. They secrete enzymes, mucus, or electrolytes and often contain duct systems that canalize secretions to a target site. Endocrine glands—such as the pituitary, thyroid, parathyroid, adrenal glands, and pancreatic islets—secrete hormones that regulate metabolism, growth, development, and homeostasis. Some organs exhibit both exocrine and endocrine functions, such as the pancreas and gonads.

Structure and secretion mechanisms vary: glands have secretory units (acini or tubules) made of secretory cells, supported by connective tissue and ducts. Secretions are released by different processes, including merocrine (exocytosis), apocrine (part of the cell membrane is shed with the secretion), and holocrine (whole cells disintegrate).

Regulation of secretion involves neural input and hormonal feedback. Endocrine secretions are often governed by feedback loops, while exocrine activity responds to local stimuli, autonomic signals, or reflexes.

Clinical notes: gland dysfunction can cause hyper- or hyposecretion, atrophy, or tumors such as adenomas or adenocarcinomas. Diagnosis and treatment depend on the gland involved and the type of secretion produced.