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A. M. de Vos investigated how growth hormone binds its growth hormone receptor. With colleagues, de Vos determined the X-ray crystallography structure of a complex containing human growth hormone and the extracellular portion of its receptor. The 1992 structure, measured at $2.8\text{A˚}$, contained one hormone molecule and two receptor molecules.^[ref]

The growth hormone molecule contacted the two receptors through different binding surfaces, although the receptors used largely the same residues in each contact. Additional contact between the two receptors supported a model in which hormone binding brings a receptor pair together. This made the geometry of signal transduction concrete: a single bound hormone can organize more than one receptor molecule. The measured complex contained extracellular fragments, so the structure did not by itself record the complete activation process across a cell membrane.^[ref]

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]

Growth hormone is a protein hormone synthesized and released by the anterior pituitary. It regulates body growth and metabolism, acting on tissues directly and partly through insulin-like growth factor 1 produced by the liver and other tissues. Its release is controlled by signals from the hypothalamus, including inhibition by somatostatin.^[ref]

This makes “growth” a coordinated physiological response rather than merely a change in one chemical concentration. Growth hormone can initiate signaling in one tissue, while a second circulating signal influences other tissues. Negative feedback connects the resulting signal levels back to their control pathway.^[ref]

A revealing molecular experiment determined the structure of human growth hormone bound to the extracellular portions of the growth hormone receptor. The 2.8-angstrom X-ray crystallography structure contained one hormone molecule and two receptor portions. Two different surfaces of the hormone contacted the receptor partners. This provides a concrete example of a signaling molecule organizing a multi-part complex, rather than simply occupying one isolated pocket. The structure supported a model for receptor association; establishing the full sequence of activation in a living biological cell requires additional functional experiments.^[ref]

The growth hormone receptor is a membrane receptor that recognizes growth hormone and couples its binding to intracellular signaling through the associated tyrosine kinase JAK2. The receptor binds the extracellular hormone, while JAK2 supplies a catalytic signaling activity on the cytoplasmic side. Binding and kinase activation are related but distinct events.^[ref]

A 1992 crystal structure resolved one growth hormone molecule bound to two receptor extracellular domains at 2.8-angstrom resolution. The hormone contacted the two receptor molecules through different surfaces. This establishes an informative stoichiometry and geometry, but the isolated extracellular complex does not by itself show all movements or phosphorylation reactions in a living membrane.^[ref]

Later experiments tested the geometry inside receptor dimers. A 2014 study used Förster resonance energy transfer to monitor the relative positions of receptor motifs that bind JAK2, together with engineered receptors and molecular modeling. The resulting model linked receptor rearrangement to changes in how kinase and inhibitory pseudokinase regions interact.^[ref]

An important consequence is that measuring hormone occupancy alone cannot specify the signaling output. Two receptor configurations can have bound ligand yet position their intracellular partners differently. Experiments that separately measure binding, geometry and downstream kinase activity are therefore more informative than assuming that every occupied receptor is equally active.

Insulin-like growth factor 1 is a peptide hormone and growth mediator associated with growth hormone action. The liver supplies an important circulating source, and IGF-1 signalling helps coordinate growth of tissues including cartilage and bone. The regulatory pathway therefore connects a pituitary signal with growth-related responses in other tissues.^[ref]

The motivating experiment was to look for a measurable activity in blood serum. A 1957 report by W. D. Salmon Jr. and William Daughaday examined a hormonally controlled serum factor through sulfate incorporation by cartilage in vitro.^[ref] In his later account, William Daughaday described cartilage culture and sulfate uptake as the readout that motivated the hypothesis of a serum mediator of growth hormone action.^[ref]

This was a functional assay: it detected what a serum preparation did to a tissue. Identifying the responsible molecules required further biochemical work. In 1978, E. Rinderknecht and R. E. Humbel reported structural homology between serum-derived IGF-I and proinsulin, providing a molecular basis for the insulin-like designation.^[ref]

The historical progression connects three kinds of evidence: pituitary control, a tissue response to blood serum, and molecular identification of a protein. A serum bioactivity measurement and a measurement of one purified hormone answer different experimental questions.

W. D. Salmon Jr. coauthored the 1957 report A hormonally controlled serum factor which stimulates sulfate incorporation by cartilage in vitro by W. D. Salmon Jr. and William Daughaday. The work helped motivate the idea that some effects of growth hormone are mediated by a factor carried in blood serum, rather than being explained solely by direct growth hormone action on every responding tissue.^[ref]

The experimental distinction was between applying growth hormone directly to isolated cartilage and measuring the response to blood serum from an animal exposed to the hormone. Sulfate incorporation provided a measurable tissue response. In William Daughaday's retrospective account, this supported the circulating-mediator interpretation that developed into the somatomedin hypothesis and research on insulin-like growth factor 1. A tissue response establishes biological activity; it does not, on its own, establish the chemical identity or purity of a single molecule.^[ref]

William H. Daughaday was a researcher in endocrinology whose work helped connect growth hormone to circulating growth-promoting factors, including insulin-like growth factor 1. He became the founding chief of Washington University's Metabolism Division in 1951 and later led its diabetes and endocrine research centers. The division's institutional history also credits his work on corticosteroid-binding globulin and early measurements of growth hormone and prolactin.^[ref]

With W. D. Salmon Jr., Daughaday investigated why blood serum could stimulate sulfate incorporation in isolated cartilage. The resulting somatomedin hypothesis made an important conceptual separation: a hormone released by one organ can act through another circulating signal. Daughaday's 2000 retrospective describes the historical experiments behind that interpretation; later identification of insulin-like growth factor 1 gave a molecular identity to an important part of this activity.^[ref]

The distinction remains useful when interpreting experiments: observing a growth hormone-dependent response in a whole animal does not show that growth hormone acts directly on the responding tissue.