Some endocrine cells exist as single cells within a tissue. Many hormones of the digestive tract, for example, are secreted by isolated endocrine cells in the wall of the stomach and small intestine.
Many hormones, however, are secreted by aggregations of endocrine cells that form secretory organs called endocrine glands. The name "endocrine" reflects the fact that these glands do not have ducts that lead to the outside of the body; rather, they secrete their products directly into the extracellular fluid. Vertebrates have nine major endocrine glands, which collectively make up the endocrine system (Figure 42.2). A single endocrine gland may secrete several different hormones.
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