[What are hormones? The term hormone in retrospect and prospect]

Historically, the meaning of the term hormone has changed during the last decades. Morphological studies of secreting cells lead Feyrter to the concept of paracrine action of some hormones. While endocrine regulators are blood-borne, paracrine messengers reach their target cells through the diffusion in the intracellular space. Though it is rather difficult to draw a line between true hormones and hormone-like substances, valid definitions for endocrine and paracrine regulatory systems can be given. The term "hormonal control' should be restricted to endocrine systems. For effectors acting by paracrine mechanisms, the term paramone is proposed in this article.