For animals in the LJ, EP and MP groups, testicular development had been tracked by monitoring testicular volume and location (inguinal versus scrotal) and morning (08:30 hours) and nighttime (19:00 hours) circulating testosterone (T) concentrations on a weekly basis as reported previously (Simorangkiret al., 2012). in the fraction (10020%) of SOHLH1-positive germ cells co-expressing GFR1 and (ii) a significant increase in the proportion of SOHLH1-positive spermatogonia that co-expressed the tyrosine kinase receptor (cKIT). Spermatogonia exhibiting nuclear SOHLH1 staining were found to be cKIT positive, but not all Ranirestat cKIT-positive spermatogonia exhibited nuclear SOHLH1 staining. Taken together, these results suggest that, in the monkey, nuclear location of SOHLH1 is closely associated with spermatogonial Ranirestat differentiation. Keywords:spermatogenesis, SOHLH1, spermatogonial differentiation, rhesus monkey == Introduction == In the rodent, thespermatogenesis- andoogenesis-specific basichelix-loop-helix1and2(SOHLH1 and SOHLH2) transcription factors, which are expressed only in the gonads (Ballowet al., 2006), have been shown to be essential for spermatogenesis (Ballowet al., 2006;Haoet al., 2008;Toyodaet al., 2009;Barrioset al., 2012;Suzukiet al., 2012). Mice deficient in either SOHLH1 or SOHLH2 are infertile due to a block to spermatogonial differentiation (Ballowet al., 2006;Haoet al., 2008;Toyodaet al., 2009;Suzukiet al., 2012), and mutation ofSOHLH1in humans is associated with azoospermia (Choiet al., 2010). In light of the role for these transcription factors in spermatogenesis, we examined the association between immunoexpression of SOHLH1 and spermatogonial differentiation at the onset of puberty in the rhesus monkey (Macaca mulatta), a representative higher primate. The seminiferous cords of the pre-pubertal monkey testis contain only Sertoli cells and undifferentiated Type A spermatogonia (Marshall and Plant, 1996;Ramaswamyet al., 2000;Simorangkiret al., 2005,2012), and therefore the juvenile stage of primate development provides a stable baseline state for examining the cell biology of spermatogonial differentiation, which in the monkey is initiated by the increase in gonadotrophin secretion that is observed in association with the onset of puberty at 3.5 years of age (Plantet al., 2005). In order to relate the pattern of immunoexpression of SOHLH1 in the peripubertal monkey testis to spermatogonial differentiation, established markers of proliferating undifferentiated spermatogonia and differentiating spermatogonia were simultaneously examined using dual immunoflorescence. For this purpose, the expression of glial cell line-derived neurotrophic factor family receptor alpha 1 (GFR1) and the tyrosine kinase receptor (cKIT) was used as a marker for undifferentiated and differentiating spermatogonia, respectively (Mauduitet al., 1999,Aponteet al., 2005,Hermannet al., 2009,2010,Gasseiet al., 2010). In primates, two types of undifferentiated spermatogonia are generally recognized, namely, type A dark (Ad) and A pale (Ap) (Clermont, 1972;Luetjenset al., 2005;Simorangkiret al., 2005;Amann, 2008;Muciacciaet al., 2013). Ad spermatogonia are characterized by homogenous and dense chromatin as revealed by the nuclear stain, hematoxylin (Simorangkiret al., 2005) and express GFR1 (Hermannet al., 2009), while Ap spermatogonia exhibit a granular pattern of chromatin (Simorangkiret al., 2005) and may express GFR1 and/or cKIT (Hermannet al., 2009). Interestingly, we observed that the initiation of spermatogonial differentiation at the time of puberty in the monkey was associated with a translocation of SOHLH1 into the nucleus in association with the upregulation of cKIT. These findings providing the first description of the cellular localization of SOHLH1 in the primate testis are presented and their translational significance is discussed. == Materials and Methods == == Animals and testicular tissue == Testicular tissue from 15 male rhesus monkeys (M. mulatta) was used. The majority of these animals had been castrated at various ages for earlier studies (Simorangkiret al., 2005,2012). The monkeys had been maintained under a controlled photoperiod (lights on 07:0019:00 Ranirestat hours) and an ambient temperature of 21C. The original experiments were conducted in accordance with NIH Guidelines for the Care and Use of Experimental Animals, and were approved by the University of Pittsburgh Institutional Animal Committee on Use and Care. == Experimental design == At the time of castration, the 15 animals were classified into 5 developmental groups (n=3/group): mid-juvenile (MJ, 1719 months of age), late juvenile (LJ, 3336 months of age), early pubertal (EP, 3847 months of age), mid-pubertal (MP, 4551 months of age) and adult (AD, 71144 months of age). For animals in the LJ, EP and MP groups, testicular development had been tracked by monitoring testicular volume and location (inguinal versus scrotal) and morning (08:30 hours) and nighttime (19:00 hours) circulating testosterone (T) concentrations on a weekly basis as reported previously (Simorangkiret al., 2012). The initiation of the onset of puberty in the male rhesus monkey may be recognized by an increase in nighttime circulating T concentrations, which, in our laboratory, typically occurs at 36 months of age (Plant, DDPAC 1985). Animals classified as LJ had testicular volume of <5 ml and showed.