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Hormones are chemical signalling molecules which are released from Endocrine glands; other tissues, such as the Kidney and heart muscle, apart from glands can produce hormones, however the effects of these hormones are usually more local than those of hormones produced by Endocrine glands. Endocrine tissues/glands which both males and females possess are the pineal gland, hypothalamus, pituitary gland, thyroid gland, parathyroid glands, thymus, adrenal glands and islets of Langerhans in the Pancreas; males also have Testes which produce Testosterone, while females have Ovaries which produce Oestrogen. Each of the Endocrine glands produces and secretes hormones; hormones are secreted from their glands into the blood stream and, generally, act on distant target cells which possess specific Receptor molecules to regulate cellular function. Receptors can be located either on the target cell Plasma membrane or inside the cell depending on the type of hormone [1].

There are three different types of hormones:

Hormones can be released by various mechanisms:

Hormone effects must be controlled in accordance with the individual's requirements; hormones can be controlled by these means:

A hormone is a substance produced by glands,cells or organs which transports a signal from one cell to another. They induce chemical changes at a cellular level and are necessary for development and growth along with many other features. Only small volumes of hormone are required to initiate a change in the cell's behaviour. Hormones are produced by all multicellular organisms, but are widely called phytohormones in plants. Examples of such phytohormones are auxins and ethylene

The word hormone derives from a Greek work meaning "to spur on".

Hormones can be categorised in many different ways, for example the way they reach the target cells (endocrine vs exocrine), or the ways they interact with the target cell to induce a change (steroid vs peptide hormones).


  1. Alberts. B, Bray. D, Hopkin. K, Johnson. A, Lewis. J, Raff. M, Roberts. K, Walter. P (2004),Essential Cell Biology, 2nd Edition, New York: Garland Science p541
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