A stem cell found in the interstitial compartment of the neonatal testis; it is capable of self-renewal as well as differentiation into steroidogenic cells (adult Leydig cells). Intermediate stages of development include progenitor Leydig cells and immature Leydig cells. [ http://orcid.org/0000-0002-0819-0473 http://www.ncbi.nlm.nih.gov/pubmed/21181888 ]