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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]