INSL3

Where it comes from

Made in

Leydig cells of the testis; in women, theca cells of the ovary at a far lower level

Sent out by

Steady, in proportion to Leydig cell number; stimulated by LH over weeks rather than minutes

Acts on

• The gubernaculum (the cord that guides testicular descent)
• Germ cells
• Bone

What it does

• Makes the gubernaculum swell so the testes descend into the scrotum; mice without the gene have undescended testes ¹
• A constant readout of how many Leydig cells a man has, unlike testosterone, which swings through the day ²
• Men carrying a faulty RXFP2 receptor have lower bone mass, which is the evidence that it acts on bone ³

In medicine

• Measured in research as a Leydig cell marker in ageing, hypogonadism and after chemotherapy ²
• Mutations of INSL3 or RXFP2 are a rare cause of undescended testes, found in a small minority of cases ⁴

Beyond the body’s own use

• Nobody is given INSL3. No preparation of it exists, and undescended testes are treated by surgery, with hCG the only hormonal option
• The interest is as a test rather than a treatment: because the level follows Leydig cell number and not the hour of the day, a single blood sample is being studied as a measure of testicular capacity in ageing men ²