Pre-pupertal Spga and Sertoli cells express complementary ligands and receptors known as stem cell and niche markers. (A) Functional link associated with their gene product with a putative role in the spermatogonia stem cell niche. (B) Expression of vascular epithelial growth factor C (vegfc 1) and its receptor (kdr), of endothelin 1 (end1) and its receptor (ednrb), and of brain derived neurotrophic factor (bdnf) and its receptor (ntrk) in pre-pubertal (9 dpp) and pubertal (22 dpp) spermatogonia and Sertoli cells (G9, G22. S9, S22, respectively) was quantified by real time RT-PCR. Values were normalized to the housekeeping gene ps9; the same pattern was observed when normalizing with the expression of another housekeeping gene, ubc (not shown). Shown are the mean values (± SEM) from the 3 distinct cell preparations, for which genome-wide analysis by hybridization to a high density array is shown in Figure 2.