<Li> ETS Related Molecule or ERM transcription factor ERM transcription factor--needed for maintenance of the spermatogonial stem cell in the adult testis . </Li> <Li> transferrin--a blood plasma protein for iron ion delivery </Li> <P> The occluding junctions of Sertoli cells form the blood - testis barrier, a structure that partitions the interstitial blood compartment of the testis from the adluminal compartment of the seminiferous tubules . Because of the apical progression of the spermatogonia (sperm stem cells), the occluding junctions must be dynamically reformed and broken to allow the immunoidentical spermatogonia to cross through the blood - testis barrier so they can become immunologically unique . Sertoli cells control the entry and exit of nutrients, hormones and other chemicals into the tubules of the testis as well as make the adluminal compartment an immune - privileged site . </P> <P> The cell is also responsible for establishing and maintaining the spermatogonial stem cell niche, which ensures the renewal of stem cells and the differentiation of spermatogonia into mature germ cells that progress stepwise through the long process of spermatogenesis, ending in the release of spermatozoa in a process known as spermiation . Sertoli cells bind to spermatogonial cells via N - cadherins and galactosyltransferase (via carbohydrate residues). </P>

Where are the leydig and sertoli cells located