The essential differences between the anterior and posterior pituitary start as each part forms in an embryo. The posterior pituitary forms from neurons in the evolving hypothalamus forming at the base of the human brain, while the anterior pituitary forms from skin tissue that originates in the roof of the mouth. The anterior pituitary becomes an endocrine gland producing and secreting hormones for the body and connects to the posterior pituitary when fully formed. Meanwhile, the posterior pituitary remains connected to the hypothalamus, functioning as a repository for hormones produced by the hypothalamus and receiving messages from it that regulate when hormones are to be released to and through the anterior pituitary. Though the anterior and posterior pituitary organs work closely together for hormone regulation, they are essentially two different organs in physiology and function.
The medical term for the pituitary is the hypophysis. It is made up of the adenohypophysis, or the anterior lobe, and the neurohypophysis, or posterior part. Both are controlled directly by neurons and axon messenger pathways from the hypothalamus, and share a local vascular network with the hypothalamus. This network of arteries, veins, and capillaries connects to the body's bloodstream, and transports hormones to other endocrine organs, such as the thyroid, ovaries, adrenal, and other glands. These glands also send feedback signals to the posterior pituitary that adjust hormone releases relative to the body's needs.
Hormones developed in the anterior pituitary regulate physiological processes related to growth, reproduction, and metabolism. For example, it secretes hormones when it gets a signal from the hypothalamus to deliver growth hormones to bones; or when oxytocin is released from another part of the body, which stimulates the anterior pituitary to release hormones to stimulate milk gland contractions in breast-feeding mothers. In the anterior pituitary, adrenocorticotropic hormone (ACTH) is sent to the adrenal glands to stimulate the production of glucocorticoids to respond to stress reactions or fear. Endorphin releases produce a sense of well-being to counterbalance pain or danger signals that trigger adrenaline releases.
The posterior pituitary, unlike the anterior, is not a gland; this organ is a collection of vascular network and neural pathways from the hypothalamus to the anterior pituitary. It produces no hormones; rather, it stores them for the hypothalamus, which can secrete them when signaled to work in conjunction with other anterior pituitary hormones. From the storage in the posterior, vasopressin hormones that regulate water retention with the kidneys or that can elevate blood pressure can be released by hypothalamic signal.
Under high-powered electron microscopes, the physiological differences between the anterior and posterior pituitary cells are readily apparent. The cells of the anterior are clearly endocrine cells, and the cells of the posterior are clearly neural cells. The controls of the anterior and posterior pituitary also show that the anterior is controlled by bloodborne release factors from the hypothalamus, while nerve signals control the posterior pituitary.