- Email: [email protected]
What Regulates Neonatal Gonocyte Migration?
What Regulates Neonatal Gonocyte Migration? GONOCYTES (also known as prespermatogonia or prospermatogonia) proliferate and migrate from the center to the periphery of the testicular cords between 1 and 4 days of life in rodents and between 8 and 12 weeks in humans.1 Once established at the tubular basement membrane as type A spermatogonia, the majority enter the differentiation pathway and a smaller proportion become established as spermatogonial stem cells (SCCs). Failure of migration leads to elimination of gonocytes by apoptosis. Deficient gonocyte migration and transformation and resultant germ cell depletion, particularly loss of adult dark spermatogonia (a subset of which includes SCCs), occur postnatally in boys with cryptorchidism. Notably the available evidence suggests that this early defect in germ cell development is also present, albeit to a lesser degree, in contralateral scrotal testes and may increase the long-term risk of oligospermia or azospermia in men with a history of cryptorchidism. The fact that gonocyte migration and transformation coincides with “mini-puberty,” or the physiological activation of the hypothalamicpituitary-gonadal (HPG) axis that is characterized by a surge in serum gonadotropins and testosterone levels at age 1 to 3 months, suggests a possible causal relationship.2 In this issue of The Journal Li et al (page 1361) address the long held hypothesis that the testosterone surge during mini-puberty regulates gonocyte migration.3 They conducted studies of testicular morphology in the androgen receptor knockout (ARKO) mouse, a strain with global inactivation of the androgen receptor (AR). These investigators used confocal imaging to estimate germ cell and Sertoli cell number per tubule, and intratubular position and proliferation of germ cells at intervals between embryonic day 17 and postnatal day (P) 10, and observed that all parameters were normal in ARKO males at all time points. They conclude that androgens do not regulate migration and subsequent transformation of gonocytes into SCCs. These observations are consistent with prior studies of androgen insensitive ARKO and tfm (testicular feminization) mice demonstrating intact
0022-5347/15/1934-1087/0 THE JOURNAL OF UROLOGY® © 2015 by AMERICAN UROLOGICAL ASSOCIATION EDUCATION
AND
RESEARCH, INC.
gonocyte migration despite loss of AR signaling.4,5 However, a significant decrease in neonatal testis weight in ARKO and tfm mice was reported in these studies, and stereological methods revealed reductions in Sertoli cell number per testis at P1 and P5 and germ cell number by P5 but a normal germ cell-to-Sertoli cell ratio. Interestingly hpg mice (lacking gonadotropinreleasing hormone), and to a lesser extent mice with deletion of the follicle-stimulating hormone receptor, show similar phenotypes.4e6 In hpg mice gonocyte migration and transformation occur but Sertoli cell number is decreased at P1 and P5, while germ cell number and testicular volume are normal at P1 but reduced at P5.6 These data suggest that, at least in the neonatal mouse, androgens and gonadotropins are not required for gonocyte migration and transformation, yet androgen action is critical for proliferation of Sertoli cells in the perinatal testis. Since neither Sertoli cells nor germ cells express AR in the neonatal period, the effect of androgens is likely mediated by AR expressing peritubular myoid cells.1 The methods used by Li et al3 and others7 reporting that neonatal testis histology is unaffected in cases of complete androgen insensitivity syndrome may be less likely to identify these global developmental defects. The Sertoli cell has not been studied extensively in cryptorchidism, but is a key player in early testicular development. Sertoli cell proliferation occurs prenatally and postnatally, and is a major determinant of short and long-term adequacy of spermatogenesis. Moreover, neonatal gonocyte migration requires paracrine signaling and physical interaction between Sertoli cells and germ cells, as revealed by the novel studies of Orth et al.8 Using neonatal germ cell-Sertoli cell cocultures, these investigators demonstrated that gonocyte migration occurs in the absence of extrinsic hormones or Leydig cells, suggesting that the process is completely independent of hormonal signaling. A current list of factors that have been directly shown to influence gonocyte migration includes c-kit/stem cell factor, various adhesion molecules, thyroid hormone, plateletderived growth factor receptor beta and Notch.1,8,9
http://dx.doi.org/10.1016/j.juro.2015.01.042 Vol. 193, 1087-1088, April 2015 Printed in U.S.A.
www.jurology.com
j
1087
1088
WHAT REGULATES NEONATAL GONOCYTE MIGRATION?
Animal models have helped to elucidate the role of specific signals in early postnatal testis development. However, no studies to date have directly addressed the role, if any, of the postnatal gonadotropin and testosterone surge in gonocyte migration and transformation in the human male. Of concern is the likelihood that species specific differences exist in the physiology of mini-puberty. In mice and rats a testosterone surge occurs immediately following birth, and levels return to baseline (equivalent to female levels) by hour 4 of life,10 with no reported evidence of a secondary increase before P5 or documentation of increased gonadotropin levels in the neonatal period. Therefore, a delay exists between its occurrence and the commencement of gonocyte activity in the testis. A similar transient testosterone surge occurs in humans immediately following birth.10 However, true minipuberty occurs later and is characterized by a robust and more prolonged activation of the HPG axis that commences before and continues during the process of gonocyte migration and transformation.11 Hormone levels fail to increase in infants with complete androgen insensitivity syndrome,
suggesting that mini-puberty in humans is the result of HPG axis activation in response to neonatal androgen withdrawal.11 No physiologically equivalent secondary surge in circulating testosterone and gonadotropins has been reported in rats or mice, and consequently the possibility exists that species specific differences limit the relevance of rodent studies. In any event hormone levels during mini-puberty are reportedly normal in many boys with cryptorchidism, and data showing a direct correlation between decreased hormone levels and defective germ cell migration do not exist. Therefore, the etiology of the early germ cell pathology in cryptorchidism remains undefined. Further studies are needed to elucidate the role of hormones and other factors in the key events that establish the spermatogenic potential of the neonatal testis, and to apply this knowledge to optimize the care of boys with cryptorchidism at long-term risk for hypospermatogenesis. Julia Spencer Barthold Division of Urology Alfred I. duPont Hospital for Children Wilmington, Delaware
REFERENCES 1. Culty M: Gonocytes, the forgotten cells of the germ cell lineage. Birth Defects Res C Embryo Today 2009; 87: 1. 2. Hadziselimovic F, Thommen L, Girard J et al: The significance of postnatal gonadotropin surge for testicular development in normal and cryptorchid testes. J Urol 1986; 136: 274. 3. Li R, Vannitamby A, Meijer J et al: Postnatal germ cell development during mini-puberty in the mouse does not require androgen receptor: implications for managing cryptorchidism. J Urol 2015; 193: 1361. 4. Johnston H, Baker PJ, Abel M et al: Regulation of Sertoli cell number and activity by folliclestimulating hormone and androgen during
postnatal development in the mouse. Endocrinology 2004; 145: 318. 5. O’Shaughnessy PJ, Monteiro A and Abel M: Testicular development in mice lacking receptors for follicle stimulating hormone and androgen. PLoS One 2012; 7: e35136.
8. Orth JM, Jester WF, Li LH et al: Gonocyte-Sertoli cell interactions during development of the neonatal rodent testis. Curr Top Dev Biol 2000; 50: 103. 9. Garcia TX and Hofmann MC: NOTCH signaling in Sertoli cells regulates gonocyte fate. Cell Cycle 2013; 12: 2538.
6. Baker PJ and O’Shaughnessy PJ: Role of gonadotrophins in regulating numbers of Leydig and Sertoli cells during fetal and postnatal development in mice. Reproduction 2001; 122: 227.
10. Corbier P, Edwards DA and Roffi J: The neonatal testosterone surge: a comparative study. Arch Int Physiol Biochim Biophys 1992; 100: 127.
7. Cheikhelard A, Morel Y, Thibaud E et al: Longterm followup and comparison between genotype and phenotype in 29 cases of complete androgen insensitivity syndrome. J Urol 2008; 180: 1496.
11. Quigley CA: The postnatal gonadotropin and sex steroid surgedinsights from the androgen insensitivity syndrome. J Clin Endocrinol Metab 2002; 87: 24.
0022-5347/15/1934-1087/0 THE JOURNAL OF UROLOGY® © 2015 by AMERICAN UROLOGICAL ASSOCIATION EDUCATION
AND
RESEARCH, INC.
gonocyte migration despite loss of AR signaling.4,5 However, a significant decrease in neonatal testis weight in ARKO and tfm mice was reported in these studies, and stereological methods revealed reductions in Sertoli cell number per testis at P1 and P5 and germ cell number by P5 but a normal germ cell-to-Sertoli cell ratio. Interestingly hpg mice (lacking gonadotropinreleasing hormone), and to a lesser extent mice with deletion of the follicle-stimulating hormone receptor, show similar phenotypes.4e6 In hpg mice gonocyte migration and transformation occur but Sertoli cell number is decreased at P1 and P5, while germ cell number and testicular volume are normal at P1 but reduced at P5.6 These data suggest that, at least in the neonatal mouse, androgens and gonadotropins are not required for gonocyte migration and transformation, yet androgen action is critical for proliferation of Sertoli cells in the perinatal testis. Since neither Sertoli cells nor germ cells express AR in the neonatal period, the effect of androgens is likely mediated by AR expressing peritubular myoid cells.1 The methods used by Li et al3 and others7 reporting that neonatal testis histology is unaffected in cases of complete androgen insensitivity syndrome may be less likely to identify these global developmental defects. The Sertoli cell has not been studied extensively in cryptorchidism, but is a key player in early testicular development. Sertoli cell proliferation occurs prenatally and postnatally, and is a major determinant of short and long-term adequacy of spermatogenesis. Moreover, neonatal gonocyte migration requires paracrine signaling and physical interaction between Sertoli cells and germ cells, as revealed by the novel studies of Orth et al.8 Using neonatal germ cell-Sertoli cell cocultures, these investigators demonstrated that gonocyte migration occurs in the absence of extrinsic hormones or Leydig cells, suggesting that the process is completely independent of hormonal signaling. A current list of factors that have been directly shown to influence gonocyte migration includes c-kit/stem cell factor, various adhesion molecules, thyroid hormone, plateletderived growth factor receptor beta and Notch.1,8,9
http://dx.doi.org/10.1016/j.juro.2015.01.042 Vol. 193, 1087-1088, April 2015 Printed in U.S.A.
www.jurology.com
j
1087
1088
WHAT REGULATES NEONATAL GONOCYTE MIGRATION?
Animal models have helped to elucidate the role of specific signals in early postnatal testis development. However, no studies to date have directly addressed the role, if any, of the postnatal gonadotropin and testosterone surge in gonocyte migration and transformation in the human male. Of concern is the likelihood that species specific differences exist in the physiology of mini-puberty. In mice and rats a testosterone surge occurs immediately following birth, and levels return to baseline (equivalent to female levels) by hour 4 of life,10 with no reported evidence of a secondary increase before P5 or documentation of increased gonadotropin levels in the neonatal period. Therefore, a delay exists between its occurrence and the commencement of gonocyte activity in the testis. A similar transient testosterone surge occurs in humans immediately following birth.10 However, true minipuberty occurs later and is characterized by a robust and more prolonged activation of the HPG axis that commences before and continues during the process of gonocyte migration and transformation.11 Hormone levels fail to increase in infants with complete androgen insensitivity syndrome,
suggesting that mini-puberty in humans is the result of HPG axis activation in response to neonatal androgen withdrawal.11 No physiologically equivalent secondary surge in circulating testosterone and gonadotropins has been reported in rats or mice, and consequently the possibility exists that species specific differences limit the relevance of rodent studies. In any event hormone levels during mini-puberty are reportedly normal in many boys with cryptorchidism, and data showing a direct correlation between decreased hormone levels and defective germ cell migration do not exist. Therefore, the etiology of the early germ cell pathology in cryptorchidism remains undefined. Further studies are needed to elucidate the role of hormones and other factors in the key events that establish the spermatogenic potential of the neonatal testis, and to apply this knowledge to optimize the care of boys with cryptorchidism at long-term risk for hypospermatogenesis. Julia Spencer Barthold Division of Urology Alfred I. duPont Hospital for Children Wilmington, Delaware
REFERENCES 1. Culty M: Gonocytes, the forgotten cells of the germ cell lineage. Birth Defects Res C Embryo Today 2009; 87: 1. 2. Hadziselimovic F, Thommen L, Girard J et al: The significance of postnatal gonadotropin surge for testicular development in normal and cryptorchid testes. J Urol 1986; 136: 274. 3. Li R, Vannitamby A, Meijer J et al: Postnatal germ cell development during mini-puberty in the mouse does not require androgen receptor: implications for managing cryptorchidism. J Urol 2015; 193: 1361. 4. Johnston H, Baker PJ, Abel M et al: Regulation of Sertoli cell number and activity by folliclestimulating hormone and androgen during
postnatal development in the mouse. Endocrinology 2004; 145: 318. 5. O’Shaughnessy PJ, Monteiro A and Abel M: Testicular development in mice lacking receptors for follicle stimulating hormone and androgen. PLoS One 2012; 7: e35136.
8. Orth JM, Jester WF, Li LH et al: Gonocyte-Sertoli cell interactions during development of the neonatal rodent testis. Curr Top Dev Biol 2000; 50: 103. 9. Garcia TX and Hofmann MC: NOTCH signaling in Sertoli cells regulates gonocyte fate. Cell Cycle 2013; 12: 2538.
6. Baker PJ and O’Shaughnessy PJ: Role of gonadotrophins in regulating numbers of Leydig and Sertoli cells during fetal and postnatal development in mice. Reproduction 2001; 122: 227.
10. Corbier P, Edwards DA and Roffi J: The neonatal testosterone surge: a comparative study. Arch Int Physiol Biochim Biophys 1992; 100: 127.
7. Cheikhelard A, Morel Y, Thibaud E et al: Longterm followup and comparison between genotype and phenotype in 29 cases of complete androgen insensitivity syndrome. J Urol 2008; 180: 1496.
11. Quigley CA: The postnatal gonadotropin and sex steroid surgedinsights from the androgen insensitivity syndrome. J Clin Endocrinol Metab 2002; 87: 24.