Form walls of the tubules. They are nondividing, respond to FSH by releasing ABP to stimulate the germ cells to migrate toward the lumen. As germ cells migrate they will divide and differentiate. Inhibin will be secreted to regulate spermatogensis vis inhibition of FSH. Forms the blood testis barrier through tight junctions. sustentacular cells protect sperm from blood, b/c blood can see sperm as foreign bc of different chromosomes numbers and proteins