The pituitary is a small bean-shaped, reddish-gray organ located in the saddle-shaped depression (sella turcica) in the floor of the skull (the sphenoid bone) and attached to the base of the brain by a stalk . The gland is comprised of two segments, the anterior (front) and posterior (back) lobes. The posterior lobe makes up 25% while the anterior lobe accounts for the remaining 75% of the gland. Even though both lobes together are only about the size of a small acorn, they represent a major player in the endocrine system. Because it releases the major hormones involved in growth, we will be concerned only with the anterior section of the gland. The major function of the anterior pituitary is tropic. This suffix is used to point out hormones that affect other glands and organs. The anterior pituitary produces six vital tropic hormones:
Thyroid-stimulating hormone (TSH)
Adrenocorticotropic hormone (ACTH)
Follicle-stimulating hormone (FSH)
Luteinizing hormone (LH)
Lactotropic Hormone (LTH)
Growth Hormone (GH)
The most important of these hormones in terms of growth, is Growth Hormone (GH). Before delving into the world of GH, some background is needed on the role of a hormone.
Hormones are substances released into the bloodstream from a gland like the pituitary, or an organ that affect activity in cells at another site . Hormones are proteins made up of folded amino acid chains, typically between 50 and 100 amino acids long . The folds in the chains cause the hormone to take on a tiny three-dimensional shape specific to that particular hormone. Because of this specific shape, a hormone can attach to receptor proteins that are present on the membrane surface of target cells (see fig 1). The shape of the protein molecule on the cell will exactly match that of the hormone binding with it . Cells are equipped with this mechanism so as to prevent hormones from binding with any cell it comes in contact with. For a hormone to function properly, it must bind with the correct cells. The shape of the proteins on a hormone is incredibly important as they serve as a means of identifying its hormone's target cell (see fig 2). When a protein hormone attaches to its receptor, the receptor acts by changing shape . This change in shape allows the cell to interact with other proteins that float around on the surface of a cell, one of these being a G protein (see fig 3). G proteins are named after their ability to bind to GTP (guanosine triphosphate), and have profound effects on the function of a cell . Once the G protein has attached to the receptor, it splits into two parts. One of the parts remains bound to the hormone receptor, while the other part is released and binds with yet another protein . The third protein is an enzyme called adenylate cyclase, which responds to the binding of the G protein fragment by transforming a molecule of ATP (adenosine triphosphate) into a smaller molecule called cyclic AMP (CAMP) (see fig 4) . It is this molecule that finally acts as the powerful second messenger (also known as a somatomedin) in the cell that carries out the instructions of the hormone . Because thousands of CAMP molecules can be created...