A gland is one or more cells that produce and secrete a specific product. The product is always a water-based fluid (aqueous) and usually contains proteins (the product is referred to as a secretion). Secretion is considered an active process. Glandular cells obtain substances needed from blood and transform them (chemically) into a product that’s discharged from the cell.
Glands are classified into two groups based on characteristics:
Where they release their product- glands can be endocrine (secrete internally) or exocrine (secrete externally).
The number of cells they contain- glands can be unicellular (one-celled) or multicellular (more than one cell).
Endocrine glands are also called ductless glands because eventually, they lose their ducts. They produce hormones (chemical messengers ) and secrete them by exocytosis into the extracellular space. After entering the extracellular space, they enter the blood or lymphatic fluid and travel to specific organs. Each hormone makes its target organ respond in a specific way. For instance, hormones produced by intestinal cells cause the pancreas to release enzymes that aid in digestion.
Not all endocrine glands have the the same structure, so a single description cannot be used. Typically, they are compact multicellular organs but there are individual hormone producing cells as well (specifically in digestive tract mucosa and the brain). Hormones secreted by endocrine glands vary. For instance, one gland might secrete an amino acid while another secretes glycoproteins or steroids. Important– not all endocrine glands are epithelium.
Exocrine glands secrete their products onto the skin or into body cavities. Unicellular exocrine glands do this directly by exocytosis, while multicellular glands transport their product through a duct on the epithelial surface. Products secreted by exocrine glands include sweat, oil,...