Anterior pituitary

The anterior pituitary is the glandular part of the pituitary gland, containing biological cells that synthesize and release hormones. Signals from the hypothalamus reach it through the hypophyseal portal system: blood passes through one network of capillaries, carries releasing or inhibiting hormones down connecting blood vessels, and enters a second network beside the hormone-secreting biological cells. This provides a direct chemical route for control by the brain.^[ref]

Its products include growth hormone, prolactin and thyroid-stimulating hormone. For example, prolactin promotes milk production, whereas oxytocin released by the posterior pituitary promotes milk ejection. Making milk and moving it outward are distinct controlled processes. Similarly, thyroid-stimulating hormone stimulates release of thyroid hormones, which participate in negative feedback on the control pathway. The anterior pituitary therefore both produces hormones and relays upstream signals; it is not simply a storage site for hormones made in the hypothalamus.^[ref]

A gonadotroph is a hormone-producing biological cell of the anterior pituitary that synthesizes the gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone. These products act on the gonads. Its upstream stimulus, gonadotropin-releasing hormone (GnRH), is made by hypothalamic neurons and delivered through the hypophyseal portal system. The incoming signal and the gonadotroph's secreted products are distinct hormones.^[ref]

A gonadotroph responds to a time-varying input, rather than merely counting the total amount of GnRH delivered. In a 1985 experiment, investigators surgically separated the pituitary from hypothalamic control in sheep and supplied equal-sized GnRH pulses hourly or every two hours. The less frequent schedule produced larger individual LH responses. This demonstrates that pulse timing can change the gland's output; it does not imply that doubling the interval doubles every response in every animal.^[ref]

For an experimental interpretation, distinguish pulse amplitude, interval and daily dose. Keeping the amount per pulse fixed while halving pulse frequency also halves daily input. Measuring the gland's LH output tests a physiological response of its secretory cells, but does not by itself identify every intracellular step of the response.

The hypophyseal portal system is the vascular route carrying regulatory hormones from the hypothalamus directly to the anterior pituitary. Hypothalamic neurons release their chemical signals into a first network of capillaries. Hypophyseal portal veins then carry that blood to a second capillary network alongside the secretory biological cells of the anterior pituitary. This is a portal system: two capillary beds connected in sequence.^[ref]

The route makes a small local secretion an effective input to another gland. In an ideal steady mass balance, let a hormone enter blood plasma at molar rate $r$, with plasma volumetric flow rate $Q$. Ignoring removal and assuming complete mixing, the downstream molar concentration isThis is a delivery model, not a universal physiological constant: changing secretion, flow, extraction or mixing changes the resulting signal. It explains the benefit of reaching the target before whole-body dilution.

A 1982 experiment by Iain J. Clarke and James T. Cummins sampled this upstream communication directly in conscious ovariectomized sheep, using a procedure that maintained pituitary function. They measured gonadotropin-releasing hormone (GnRH) in portal blood plasma and luteinizing hormone (LH) in peripheral jugular blood plasma. The secretory pulses were synchronous: each observed LH episode was linked to a pulsatile GnRH discharge through the portal network.^[ref]

Sampling the two locations matters. The portal measurement detects the hypothalamic message before it reaches the anterior pituitary; the peripheral measurement detects the pituitary's output. A single downstream hormone trace would not reveal both sides of that timing relationship.

The median eminence is a region at the base of the hypothalamus where neurosecretory terminals release regulatory hormones near the first capillary bed of the hypophyseal portal system. It is an interface between hypothalamic neurons and the vascular route to the anterior pituitary. A GnRH terminal here is a site of release; the neuron's soma need not occupy the same location.^[ref]

Access to the capillaries also depends on neighboring glial cells, including tanycytes. In a 2004 study, female-rat tissue containing the median eminence was incubated for 30 minutes with or without added arginine, a substrate for nitric oxide production. Immunoelectron microscopy identified GnRH-containing terminals. In treated preparations, the investigators observed structural changes bringing terminals into direct contact with the space beside the capillaries. Complementary cell-culture experiments implicated signals from endothelium in glial remodeling.^[ref]

The comparison is illuminating because the route is more than a fixed pipe connecting two glands. The arrangement of neurons, glial cells and blood vessels can alter access to the release surface. A microscopic structural observation and a measurement of circulating hormone nevertheless remain different kinds of evidence: proximity alone does not quantify how much GnRH enters the blood.

A hypophyseal portal vein is a vein connecting the first and second capillary networks of the hypophyseal portal system. The main route carries blood from the median eminence region of the hypothalamus, down the pituitary stalk, to the anterior pituitary. It delivers hypothalamic hormones before their dilution in the wider circulation.^[ref]

The word “portal” describes the route: blood leaving one capillary bed reaches another capillary bed before returning to the heart. Here the first bed receives a regulatory message and the second delivers it to responding biological cells. A hypophyseal portal vein is therefore an interconnecting segment, rather than the entire hypophyseal portal system.

A useful example is a pulse of gonadotropin-releasing hormone. In conscious sheep, simultaneous sampling of portal and peripheral blood plasma linked portal GnRH pulses with peripheral luteinizing hormone episodes. Sampling the hypophyseal portal vein route accesses the incoming hypothalamic signal; peripheral sampling detects the resulting pituitary output. That distinction is why the sampling location is part of the experiment, not just a convenient place to draw blood.^[ref]