According to the sources, each seminiferous tubule is lined on its inside by two types of cells: male germ cells and Sertoli cells . The primary function of Sertoli cells is to provide nutrition to the dividing germ cells during the process of spermatogenesis . Other options are incorrect because progesterone is a steroid hormone produced by the ovary, adrenaline is secreted by the adrenal medulla (not the cortex), and cholecystokinin (CCK) is a peptide hormone of the gastrointestinal tract .