<prompt> Erythropoietin is secreted by:Coical interstitial cellsCells in macula densaGranular cellsMesangial cells  <response>: JG apparatus Cells in macula densa Granular cells JG cells Mesangial cells Senses Na Cl- in tubular fluids initiate tubuloglomerular feedback. Synthesis secrete renin Cells that can alternate the vessel diameter. Type I Coical interstitial cells : synthesize erythropoietin 