Hormones originate from endocrine glands or nerves.  They enter the blood and cause cells in target tissues containing specific receptors to produce new products or new hormones.  The original hormones and the products of their action are necessary for successful reproduction.  Protein hormones act via plasma membrane receptors and exert effects in the cytoplasm of the cell.  Steroid hormones act through nuclear receptors and cause transcription and translation that results in the production of new proteins.  Both types of hormones cause changes in the function of the target cells.