Receptors: Molecular mediators of hormone action

The appropriate proliferation and differentiation of cells during development and the maintenance of cellular homeostasis in the adult require a continuous flow of information to the cell. This is provided either by diffusible signaling molecules or by direct cell-cell and cell-matrix interactions. All cells utilize a wide variety of signaling molecules and signal transduction systems to communicate with one another, but within the vertebrate endocrine system, it is the secreted hormones that are classically associated with cellular signaling. Hormones are chemical messengers produced from the endocrine glands that act either locally or at a distance to regulate the activity of a target cell. As discussed in detail elsewhere within this volume, prominent groups of hormonal agents include peptide hormones; steroid, retinoid, and thyroid hormones; growth factors; cytokines; pheromones; and neurotransmitters or neuromodulators.