Practice question

In mammalian testis seminiferous tubules, spermatogonial stem cells reside on basement membrane, maintained by glial-derived neurotrophic factor GDNF secreted by Sertoli nurse cells activating Ret and Gfra1 receptors plus FGF2 and CSF1 from interstitial Leydig cells. Functional transplantation demonstrates single SSC can repopulate experimentally emptied testis and reinitiate complete spermatogenesis generating functional spermatozoa continuously. Although defined culture conditions with growth factors may reveal in vitro plasticity toward pluripotency, physiological lineage tracing confirms essentially exclusive sperm production throughout adult life, establishing unipotent classification despite remarkable self-renewal capacity.