A hormone is a chemical messenger released by biological cells that regulates target biological cells bearing suitable receptors. In endocrine signaling, hormones travel to distant targets through the bloodstream. The extract-transfer experiment under second messenger isolates part of this signal transduction from intact biological cells. [ref]
https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors. Clark, Douglas and Choi, Biology 2e, 9.1: hormonal signaling
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