Pituitary gland · Grey Matter

The pituitary gland is a pea-sized gland hanging below the hypothalamus that carries the brain's orders to the rest of the endocrine system: on instruction from the hypothalamus it releases the hormones that set the activity of the thyroid, the adrenal glands and the gonads, and those that regulate growth, milk and wat

Pituitary gland. The pituitary gland is a pea-sized gland hanging below the hypothalamus that carries the brain's orders to the rest of the endocrine system: on instruction from the hypothalamus it releases the hormones that set the activity of the thyroid, the adrenal glands and the gonads, and those that regulate growth, milk and water balance.

It sits in the sella turcica, a saddle-shaped hollow in the sphenoid bone, attached to the hypothalamus by a stalk and lying just below the optic chiasm. It has two parts with different origins. The anterior pituitary (adenohypophysis) is true gland tissue: hypothalamic releasing and inhibiting hormones reach it through a small portal blood system, and its cells answer with six main hormones. The posterior pituitary (neurohypophysis) is an extension of the brain: it holds the axon endings of hypothalamic neurons and releases the oxytocin and vasopressin they make.

The anterior pituitary's hormones mostly command other glands. TSH drives the thyroid (which makes T4 and T3), ACTH the adrenal cortex (cortisol), LH and FSH the ovaries and testes; growth hormone and prolactin act more directly on tissues.

The chain has three levels with feedback at each. Hypothalamus, pituitary and peripheral gland form axes such as the HPA axis, and the final hormone switches off the levels above it.

Its position makes its tumours visible in the eyes. A pituitary adenoma growing upward presses on the crossing fibres of the chiasm and causes bitemporal hemianopia.

Questions: How does the hypothalamus give orders to the pituitary? It uses two channels, one for each half of the gland. For the anterior pituitary, hypothalamic neurons release releasing and inhibiting hormones (CRH, TRH, GnRH, GHRH, somatostatin, dopamine) into a small portal system of blood vessels that carries them straight down the stalk, and the pituitary cells answer with their own hormones. For the posterior pituitary there is no messenger at all: the gland is made of the axon endings of hypothalamic neurons, which release oxytocin and vasopressin directly into the blood when those neurons fire. What happens when the posterior pituitary stops releasing vasopressin? Without vasopressin the collecting ducts of the kidney cannot insert their aquaporin water channels, so water that should be reclaimed leaves in the urine. The result is central diabetes insipidus: large volumes of dilute urine, often several litres a day, and intense thirst that keeps the person drinking to match. Its causes are damage to the hypothalamus, the stalk or the posterior pituitary (surgery, trauma, tumours), and it is treated with a synthetic analogue of the hormone. Why does a pituitary tumour take away the outer half of vision in both eyes? The pituitary sits just below the optic chiasm, and a tumour growing upward out of the sella turcica presses on the chiasm's centre from below. The fibres crossing in that centre come from the nasal half of each retina, which, because the lens inverts the image, sees the outer (temporal) half of the visual field. Damaging them removes the right half-field of the right eye and the left half-field of the left eye, leaving a central band, the pattern called bitemporal hemianopia.