Same address, two very different tenants
The pituitary gland sits in a tiny bony pocket beneath the brain. Although it looks like a single bean-sized lump, it has two halves that develop from different tissues and work in completely different ways. The front part is the anterior pituitary, built from gland tissue. The back part is the posterior pituitary, which is actually an extension of the brain itself. Getting this split clear is the single most useful thing you can learn about the pituitary — almost every other fact follows from it.
The front: a private blood bridge
The hypothalamus controls the anterior pituitary by chemistry, not by nerves. It uses a clever piece of plumbing called the hypophyseal portal system — a small private network of blood vessels that runs straight from the hypothalamus down to the anterior pituitary. A “portal” system is one where blood passes through two capillary beds in a row instead of returning to the heart in between.
Why bother with a private bridge? Because it lets the hypothalamus deliver tiny amounts of releasing and inhibiting factors at high concentration straight to the front pituitary, without diluting them in the whole body's blood. The neurons doing this are small — the parvocellular neurons — and they drip their orders into the top of the portal vessels. This tight delivery is sometimes called pituitary neurovascular coupling. We will follow these chemical orders in the next guide.
The back: a direct nerve cable
The posterior pituitary works in a totally different way: by wire, not by chemistry. Large hypothalamic neurons called magnocellular neurons make two hormones — oxytocin and vasopressin — package them, and send them down their long fibers all the way into the back of the pituitary. There the hormones wait in the nerve endings until an electrical signal tells them to spill into the bloodstream. This is neurosecretion in its purest form: the back of the pituitary is really just the tips of brain cells.
- Front pituitary: controlled by blood (portal vessels carry hypothalamic factors) → makes its own six hormones.
- Back pituitary: controlled by nerves (axons from magnocellular neurons) → only stores and releases two hormones made upstream.
- Damage the stalk and both fail — but in different ways, because one path is vascular and the other neural.