The kidney-bean-sized gland at the base of your brain that runs your hormones
Tucked into a protective bony hollow at the base of the brain, the pituitary gland is about 1 cm across and weighs at most a gram. Yet with the hypothalamus above it, it steers growth, stress, metabolism, reproduction, water balance and labour, and its two main halves grow from entirely different embryonic tissue.
The gland rests in a dip of the sphenoid bone called the sella turcica, roofed over by a fold of the brain's tough outer membrane. Its height runs from 5.3 to 7.0 mm and its volume from 200 to 440 cubic millimetres. Surprisingly, its shape varies: in 46% of people it is flat on top, in 31.2% it bulges upward and in 22.8% it dips down. A small intermediate lobe between the front and back halves is almost missing in humans but distinct in many animals, and in rats and mice it has been studied closely.
The front lobe begins in the fourth week of pregnancy as Rathke's pouch, a pocket of tissue pushing up from the roof of the developing mouth, which later pinches off. The back lobe grows downward as an extension of the brain itself. Between weeks 12 and 16 the front lobe starts making hormones, including growth hormone.
That dual origin explains how the two halves work. The front lobe is a true gland, with separate cell types for each major hormone. The hypothalamus signals it through a private plumbing arrangement, the hypophyseal portal system, sending releasing hormones through one capillary bed to a second inside the pituitary. In response it releases ACTH for stress, growth hormone, FSH and LH for reproduction, TSH for metabolic rate and prolactin for milk production. Growth hormone, for example, is switched on by one hypothalamic signal and suppressed by another called somatostatin.
The back lobe makes nothing of its own. Nerve cells whose bodies sit in the hypothalamus run their fibres down the pituitary stalk and release two hormones stored there: antidiuretic hormone, which controls how much water the kidneys reclaim, and oxytocin. Oxytocin drives one of the body's rare positive feedback loops, since contractions of the womb trigger more oxytocin, which in turn strengthens the contractions throughout labour.
Source: Pituitary gland
