The tiny pituitary gland, at the base of the brain, secretes hormones that stimulate other glands to produce their own hormones.
What is Pituitary Gland?
The pea-sized pituitary gland, located on the inferior aspect of the brain, is called the “master gland” because it regulates many key processes. It has two lobes: the posterior lobe, which stores and releases oxytocin and antidiuretic hormone produced by the hypothalamus, and the anterior lobe, which produces at least six hormones.
- Growth hormone (GH), or somatotropin
- Thyroid-stimulating hormone (TSH), or thyrotropin
- Follicle-stimulating hormone (FSH)
- Luteinizing hormone (LH)
The pituitary gland consists of two anatomically and functionally different parts: an anterior lobe and a posterior lobe. The anterior lobe forms the bulk of the pituitary, and consists of glandular tissue that manufactures hormones. The posterior pituitary is really part of the brain and is derived from hypothalamic tissue. It does not make hormones itself, but stores and releases hormones produced by the hypothalamus.
Anatomy of the Pituitary Gland:
The two lobes link to the hypothalamus differently. The anterior lobe is linked by a system of interconnected blood vessels called a portal system. In a portal system, blood from arteries and veins connects directly rather than traveling through the heart first. This system allows hormones from the hypothalamus to be delivered to the anterior pituitary rapidly. The posterior lobe is linked to the hypothalamus by a nerve bundle, the hormone-producing neurons of which originate in the hypothalamus. The axons of these neurons extend into the posterior lobe and carry their hormones there for storage. Nerve signals from these neurons prompt release of their hormones “on demand.”
The pituitary gland consists of two lobes and a stalk, or infundibulum, which connects the lobes to the hypothalamus. Traveling through the stalk are blood vessels and nerve fibers that transport hormones from the hypothalamus.
Anterior Lobe Hormones
Seven hormones are produced in the anterior pituitary. Four of these, known as tropic hormones, target other glands, prompting them to release their hormones. They are thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), follicle-stimulating hormone (FSH), and luteinizing hormone (LH). The others—growth hormone (GH), prolactin, and melanocyte-stimulating hormone (MSH)—act directly on target organs.
The release of hormones from the anterior pituitary is regulated by the hypothalamus, which secretes releasing or inhibiting hormones. Although different hormones from the hypothalamus reach the anterior lobe, secretory cells recognize those directed at them and secrete or release their specific hormones accordingly. The hormones are secreted into capillaries that drain into veins and into the general circulation to reach their target organs.
Posterior Lobe Hormones:
Two hormones—oxytocin and antidiuretic hormone (ADH)—are stored in the posterior lobe of the pituitary gland. These hormones are not made in the gland but by the cell bodies of neurons located in two different areas of the hypothalamus. After production, the hormones are packaged in tiny sacs and transported down the axons (nerve fibers) of the neurons to the axon terminals, where they are stored until needed. Nerve impulses from the same hypothalamic neurons where they were produced trigger the release of the hormones into capillaries.
From the capillaries, they pass into veins for distribution to their target cells. Oxytocin and ADH are almost identical in structure: each is made of nine amino acids, only two of which differ between them. However, each has a different effect. Oxytocin stimulates smooth muscle to contract, especially that of the uterus, cervix, and breast. ADH influences the balance of water in the body.