Hormones

A hormone is a substance produced in the body that directs and regulates the events occuring inside of certain cells or organs. Hormones are classified by both how they travel in the body and their chemical structure. There are five classifications based on how they travel in the body; endocrine, paracrine, autocrine, intracrine, and juxtacrine. Endocrine hormones act at distance over the body, while paracrine hormones act within the tissue they are synthesized. Autocrine hormones act on the cell they are produced by; they are secreted and then attach to the receptors on the surface to trigger a reaction. Intracrine hormones also act on the cells they are produced by, but they act directly inside the cell rather than through the receptors on the outside of the cell. Juxtacrine hormones act on juxtaposed cells. They are chemically classified into four groups; amines, peptides/proteins, steroids, and eicosanoids... and are also classified as either water-soluble or lipid-soluble. Amines and peptide/protein hormones are water-soluble and target cells by acting on receptors on their membranes. Steroids and eicosanoids are lipid-soluble and thus can penetrate the cell membrane and act within the cell.