Testosterone is an example of a chemical signal that affects the very cells that synthesize it, the neighboring cells in the testis, along with distant cells outside the gonads. Thus, testosterone is an example of

The endocrine system, or hormonal system, is made up of glands that produce hormones. Hormones are chemical messengers that travel through the bloodstream to organs, skin, muscles, and other tissues. They tell the body what to do and when to do it.