This discovery posits DHH as the only known factor to induce fetal Leydig cell differentiation. DHH is one of three mammalian homologs of a protein called Hedgehog, which was originally identified in the Drosophila fruit fly, where it regulates body pattern formation. Although all three mammalian Hedgehog homologs have important signaling functions during development, this work by Dr. Capel and colleagues demonstrates a primary role for DHH in establishing secondary male sex characteristics.
To study the potential role of DHH in fetal Leydig cell differentiation, Dr. Capel and colleagues first examined the expression pattern of Dhh within developing mouse embryos. They found that DHH and its receptor, Patched 1, were specifically expressed in XY (male) embryos in a temporally and spatially restricted pattern that corresponds to the correct time and place for fetal Leydig cell differentiation. Encouraged by these findings, Dr. Capel and colleagues analyzed mice that were genetically engineered to lack DHH. In male DHH-deficient mice, the Leydig cell precursors appeared intact, but the mice mice display defects in the differentiation of the precursors into Leydig cells, thereby establishing an integral role of DHH in fetal Leydig cell differentiation.
Dr. Capel and colleagues did note, though, that although all fetal Leydig cells express the DHH receptor, Patched 1, not all Patched 1-expressing cells become fetal Leydig cells. This key observation points to the existence of other signaling pathways that may act in conjunction with DHH/Patched 1 to fine-tune the specification of fetal Leydig cell fate. Regardless, the work has begun to answer a long-standing question in developmental biology: What signal(s) induce the testosterone-producing fetal Leydig cell lineage?
Ultimately, the full answer to this question will enable researchers to better identify and treat the human reproductive disorders that result when this sex-determination pathway goes astray.
Genes & Development