Functional Importance of Mini-Puberty in Spermatogenic Stem Cell Formation
Yuichi Shima- Division of Microscopic and Developmental Anatomy, Department of Anatomy, Kurume University School of Medicine, Fukuoka, Japan
Primordial germ cells nesting in the fetal testis give rise to gonocytes. The gonocytes then transform into spermatogenic stem cells (SSCs) during the neonatal period and thereafter serve as a lifetime source of spermatogenesis. Therefore, gonocyte to SSC transformation is quite an important process that supports fertility in males. During the gonocyte to SSC transformation, morphological and transcriptomic changes sequentially occur and gonocytes migrate from the center to the peripheral region of the seminiferous tubules. However, extrinsic signals which trigger the transcriptomic changes as well as the migration are not yet fully clarified. Recent studies have drawn attention to the temporal activation of the hypothalamic-pituitary-gonadal axis during the neonatal stage which occurs concurrently with SSC formation. This phenomenon is called mini-puberty, and recent studies on human cryptorchid patients as well as animal models partially support the hypothesis that mini-puberty plays pivotal roles in gonocyte-to-SSC transformation. Focusing on this point, here, we aimed to discuss the latest knowledge on the importance of mini-puberty in spermatogenesis in this review.
1 Introduction
Spermatozoa are the most specialized cell type in the male body and play indispensable roles in transferring genetic and epigenetic information from generation to generation. One of the features of male germ cell production is that spermatozoa are continuously produced and that spermatogenic stem cells (SSCs, also called spermatogonia) serve as the cellular source of spermatozoa through an individual’s lifetime. The process of SSC formation is unique and complex. Primordial germ cells (PGCs) migrate from the ectoderm to the gonadal primordium (called genital ridge) giving rise to gonocytes (Kluin and de Rooij, 1981) in the fetal testis. A part of the gonocytes then undergo apoptosis, whereas the remaining part migrates from the central part of the seminiferous tubules to the basement membrane. These migrated cells thereafter retain their stemness and serve as the source of all the germ cells. Morphological and transcriptomic changes during the gonocyte to SSC transformation have been extensively investigated (reviewed in Law and Oatley, 2020). However, extrinsic signals which trigger the gonocyte migration and SSC formation have not yet been identified.
Luteinizing hormone (LH) and follicle stimulating hormone (FSH) are secreted from the anterior pituitary and stimulate testicular Leydig and Sertoli cells, respectively. LH and FSH are collectively called gonadotropins, and gonadotropin synthesis and secretion are activated by gonadotropin releasing hormone from the hypothalamus. These hierarchical relationships between the endocrine organs are designated the hypothalamic-pituitary-gonadal (HPG) axis. In mammals, the HPG axis shows triphasic activation during fetal, neonatal, and pubertal periods. Among these, temporal activation of the HPG axis during the neonatal period is known as mini-puberty (Kuiri-Hänninen et al., 2014). Mini-puberty is concurrent with gonocyte to SSC transformation in the seminiferous tubules, and analyses of human cryptorchid patients strongly support the hypothesis that mini-puberty plays important roles in SSC formation. In contrast, studies using animal models demonstrate both positive and negative evidences for the functional importance of mini-puberty. In this review, we aimed to discuss the recent studies arguing the importance of mini-puberty in SSC formation.
2 Germline Development in Fetal and Neonatal Testis
2.1 Gonocyte Development in Fetal Testis
The most ancestral type of germ cells are the PGCs, the part of the epiblast cells which are destined to become a germline. In mice, PGCs experience several rounds of mitosis and then start to migrate from the primitive streak through the dorsal mesentery, finally settling in the gonads (Richardson and Lehmann, 2010). After sex determination, PGCs in the male gonad (testis) commit to male germ cell fate and form gonocytes (Kluin and de Rooij, 1981). Gonocytes are initially mitotically active but become quiescent at around E12.5–16.5 in mice (Western et al., 2008) and 20–25 weeks of gestation in humans (Hilscher and Engemann, 1992). Detailed and extensive analyses of mouse models revealed that a large part of the gonocytes (called differentiating spermatogonia) undergo differentiation and form the first wave of spermatogenesis. Meanwhile, a small part of gonocytes are destined to establish the SSC population which thereafter retains their stemness and provides the neurogenin3-positive spermatogenic progenitor pool that contributes to continuous sperm production (Yoshida et al., 2006).
2.2 SSC Formation in Neonatal Testis
After birth, heterogeneity of the male germline cells becomes apparent in terms of morphology and marker gene expression. Specifically, several marker proteins such as RET (Jain et al., 2004), PAX7 (Aloisio et al., 2014), GFRA1 (Oatley et al., 2007), and ID4 (Helsel et al., 2017) are expressed in a limited population, and these cells contribute to SSC establishment. In contrast, the SOHLH1 expressing cell population is programmed to differentiate and form the first wave of spermatogenesis (Ballow et al., 2006). These findings have been certified by recently performed single cell RNA-sequencing of the murine or human neonatal testis which enabled not only detailed clustering of germ cells but also clarified the trajectories of gonocyte to SSC transformation (Hermann et al., 2018; Law et al., 2019; Sohni et al., 2019; Tan et al., 2020). In parallel to the transcriptomic changes, gonocyte start to migrate from the luminal side to the basal side of the seminiferous tubule (McGuinness and Orth, 1992). The germ cells migrate and attach to the basement membrane and exhibit flattened morphology which is clearly distinct from the round-shaped gonocytes (Orwig et al., 2002). Although detailed molecular mechanisms regulating the migration is not yet clarified, platelet-derived growth factor and Notch signaling are thought to have some influence over the migration (Basciani et al., 2008; Garcia et al., 2013). Additionally, histological studies clarified that migrating gonocytes show increased contact with Sertoli cells (Clermont and Perey, 1957), suggesting an important role of Sertoli cells in regulating the migration of the gonocytes. Gonocyte-to-SSC transformation occurs soon after birth (2–6 days) in mice (Law and Oatley, 2020) and 3–6 months in humans (Hutson et al., 2013). A considerable number of gonocytes fail to migrate and undergo apoptosis (Orwig et al., 2002), and the unmigrated cells often serve as the source of germ cell tumor formation in cryptorchid testes in humans (Tien et al., 2020).
3 HPG Axis Activation During the Neonatal Stage
In humans, it has been reported that serum gonadotropin and testosterone levels show a transient surge during neonatal stages (Winter et al., 1976; Forest, 1979). Recent technical advances certified the above observations and clarified that serum gonadotropin levels peak up until 3 months and decrease to basal level at 6–9 months in humans (Kuiri-Hänninen et al., 2014). This is called mini-puberty, and the same phenomenon is also observed in mice during the first week after birth (Li et al., 2017). Several evidences support the notion that mini-puberty plays important roles for the subsequent development of male reproductive tissues such as penile growth, prostatic activity (Boas et al., 2006; Kuiri-Hänninen et al., 2011), and male-specific behavior (Lamminmaki et al., 2012).
4 Importance of Mini-Puberty on SSC Formation
4.1 Observation of Cryptorchid Patients
Cryptorchidism, also called undescended testis, is a pathological condition in which testes are not descended to the scrotum and stay at the inguinal or abdominal region. Recent technical advances on peripheral gonadotropin and testosterone measurements revealed that cryptorchidism is associated with lowered testosterone levels and is often accompanied by hypogonadotropic hypogonadism (Hadziselimović et al., 1986; Rodprasert et al., 2020). Moreover, histological observation of cryptorchid testes revealed that gonocyte migration and SSC formation was disturbed (Huff et al., 1991). These observations strongly support the hypothesis that mini-puberty plays important roles in gonocyte-to-SSC transformation (Hadziselimovic and Hoecht, 2008). Moreover, gonadotropin releasing hormone analogue treatment has reportedly improved fertility of cryptorchid patients, supporting the importance of mini-puberty in gonocyte-to-SSC transformation (Hadziselimovic and Hoecht, 2008), although there is an adverse opinion on this treatment from the viewpoint of efficacy and side effects (Thorsson et al., 2007).
4.2 Possible Molecular Link Between Mini-Puberty and SSC Formation
As noted above, mini-puberty was initially identified in humans, and accumulated data strongly suggests that mini-puberty plays pivotal roles in the development of reproductive functions in humans. As such, several animal models have been used to clarify the mechanistic connection between mini-puberty and SSC formation.
4.2.1 Androgen
Considering the functional importance of testosterone in male reproductive function, it was conceived that testosterone also regulates the gonocyte migration and SSC formation. To support this hypothesis, it was reported that gonocyte migration is partially inhibited in complete androgen insensitivity syndrome patients (Hadziselimovic and Huff, 2002). However, a more recent and extensive study exhibited contradictory data that gonocyte migration and SSC formation are normal in 30 androgen insensitivity syndrome patients (Su et al., 2014). In the case of mice, androgen receptor (Ar) knockout mice showed normal gonocyte migration from E17 to P10, denying any influence of the androgen signal on gonocyte to SSC transformation (Li et al., 2015). In summary, it seems likely that testosterone is not an essential factor in SSC formation.
4.2.2 FSH
It is widely accepted that FSH stimulate Sertoli cells to support spermatogenesis. However, both Fshb and Fshr knockout male mice showed reduced testis size but normal spermatogenesis, suggesting that FSH alone plays only a minor role in SSC formation (Kumar et al., 1997; Dierich et al., 1998; Abel et al., 2000). To support this hypothesis, men with the inactivating FSHR mutation showed reduced testis size and subfertility (Tapanainen et al., 1997). However, the in vitro culture of rat testis suggested the possibility that FSH regulates SSC formation in combination with follistatin (Meehan et al., 2000).
5 Discussion
As noted above, positive results were mainly provided from studies of cryptorchidism in humans. However, there is a considerable number of contradictory results even in studies on humans, preventing any concrete conclusion. Similarly, the results of animal studies are also confusing, and the consensus on the importance of mini-puberty has not yet been firmly achieved. In addition, we should also consider the interspecies differences in the role of the HPG axis on reproductive function. Taken together, the functional importance of mini-puberty is still under debate. In future studies, the impact of mini-puberty on SSC formation should be clarified at the molecular level, and such studies are expected to clarify the pathogenesis of male infertility and expand the possibilities of its treatment.
Author Contributions
YS: Conceptualization, Investigation, Writing, and Funding acquisition.
Funding
This work was supported by the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT) KAKENHI Grant number: 21H00235.
Conflict of Interest
The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Publisher’s Note
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Acknowledgments
We would like to thank Editage (www.editage.com) for English language editing.
Abbreviations
FSH, follicle stimulating hormone; HPG axis, hypothalamic-pituitary-gonadal axis; LH, luteinizing hormone; PGC, primordial germ cell; SSC, spermatogenic stem cell.
References
Abel, M. H., Wootton, A. N., Wilkins, V., Huhtaniemi, I., Knight, P. G., and Charlton, H. M. (2000). The Effect of a Null Mutation in the Follicle-Stimulating Hormone Receptor Gene on Mouse Reproduction1. Endocrinology 141, 1795–1803. doi:10.1210/endo.141.5.7456
Aloisio, G. M., Nakada, Y., Saatcioglu, H. D., Peña, C. G., Baker, M. D., Tarnawa, E. D., et al. (2014). PAX7 Expression Defines Germline Stem Cells in the Adult Testis. J. Clin. Invest. 124, 3929–3944. doi:10.1172/jci75943
Ballow, D., Meistrich, M. L., Matzuk, M., and Rajkovic, A. (2006). Sohlh1 Is Essential for Spermatogonial Differentiation. Develop. Biol. 294, 161–167. doi:10.1016/j.ydbio.2006.02.027
Basciani, S., De Luca, G., Dolci, S., Brama, M., Arizzi, M., Mariani, S., et al. (2008). Platelet-Derived Growth Factor Receptor β-Subtype Regulates Proliferation and Migration of Gonocytes. Endocrinology 149, 6226–6235. doi:10.1210/en.2008-0349
Boas, M., Boisen, K. A., Virtanen, H. E., Kaleva, M., Suomi, A.-M., Schmidt, I. M., et al. (2006). Postnatal Penile Length and Growth Rate Correlate to Serum Testosterone Levels: A Longitudinal Study of 1962 normal Boys. Eur. J. Endocrinol. 154, 125–129. doi:10.1530/eje.1.02066
Clermont, Y., and Perey, B. (1957). Quantitative Study of the Cell Population of the Seminiferous Tubules in Immature Rats. Am. J. Anat. 100, 241–267. doi:10.1002/aja.1001000205
Dierich, A., Sairam, M. R., Monaco, L., Fimia, G. M., Gansmuller, A., LeMeur, M., et al. (1998). Impairing Follicle-Stimulating Hormone (FSH) Signaling In Vivo : Targeted Disruption of the FSH Receptor Leads to Aberrant Gametogenesis and Hormonal Imbalance. Proc. Natl. Acad. Sci. U.S.A. 95, 13612–13617. doi:10.1073/pnas.95.23.13612
Forest, M. G. (1979). Pattern of the Response of Testosterone and its Precursors to Human Chorionic Gonadotropin Stimulation in Relation to Age in Infants and Children∗. J. Clin. Endocrinol. Metab. 49, 132–137. doi:10.1210/jcem-49-1-132
Garcia, T. X., DeFalco, T., Capel, B., and Hofmann, M.-C. (2013). Constitutive Activation of NOTCH1 Signaling in Sertoli Cells Causes Gonocyte Exit from Quiescence. Develop. Biol. 377, 188–201. doi:10.1016/j.ydbio.2013.01.031
Hadziselimovic, F., and Huff, D. (2002). “Gonadal Differentiation—Normal and Abnormal Testicular Development,” in Pediatric Gender Assignment. Advances in Experimental Medicine and Biology. Editors S. A. Zderic, D. A. Canning, M. C. Carr, and H. M. Snyder (Boston, MA: Springer), 511.
Hadziselimović, F., Thommen, L., Girard, J., and Herzog, B. (1986). The Significance of Postnatal Gonadotropin Surge for Testicular Development in normal and Cryptorchid Testes. J. Urol. 136, 274–276. doi:10.1016/s0022-5347(17)44839-7
Hadziselimovic, F., and Hoecht, B. (2008). Testicular Histology Related to Fertility Outcome and Postpubertal Hormone Status in Cryptorchidism. Klin. Padiatr. 220, 302–307. doi:10.1055/s-2007-993194
Helsel, A. R., Yang, Q. E., Oatley, M. J., Lord, T., Sablitzky, F., and Oatley, J. M. (2017). Id4 Levels Dictate the Stem Cell State in Mouse Spermatogonia. Development 144, 624–634. doi:10.1242/dev.146928
Hermann, B. P., Cheng, K., Singh, A., Roa-De La Cruz, L., Mutoji, K. N., Chen, I.-C., et al. (2018). The Mammalian Spermatogenesis Single-Cell Transcriptome, from Spermatogonial Stem Cells to Spermatids. Cel Rep. 25, 1650–1667. e8. doi:10.1016/j.celrep.2018.10.026
Hilscher, B., and Engemann, A. (1992). Histological and Morphometric Studies on the Kinetics of Germ Cells and Immature Sertoli Cells during Human Prespermatogenesis. Andrologia 24, 7–10. doi:10.1111/j.1439-0272.1992.tb02600.x
Huff, D. S., Hadžiselimovič, F., Snyder, H. M., Blyth, B., and Duckett, J. W. (1991). Early Postnatal Testicular Maldevelopment in Cryptorchidism. J. Urol. 146, 624–626. doi:10.1016/s0022-5347(17)37874-6
Hutson, J. M., Li, R., Southwell, B. R., Petersen, B. L., Thorup, J., and Cortes, D. (2013). Germ Cell Development in the Postnatal Testis: the Key to Prevent Malignancy in Cryptorchidism? Front. Endocrin. 3, 176. doi:10.3389/fendo.2012.00176
Jain, S., Naughton, C. K., Yang, M., Strickland, A., Vij, K., Encinas, M., et al. (2004). Mice Expressing a Dominant-Negative Ret Mutation Phenocopy Human Hirschsprung Disease and Delineate a Direct Role of Ret in Spermatogenesis. Development 131, 5503–5513. doi:10.1242/dev.01421
Kluin, P. M., and Rooij, D. G. (1981). A Comparison between the Morphology and Cell Kinetics of Gonocytes and Adult Type Undifferentiated Spermatogonia in the Mouse. Int. J. Androl. 4, 475–493. doi:10.1111/j.1365-2605.1981.tb00732.x
Kuiri-Hänninen, T., Seuri, R., Tyrväinen, E., Turpeinen, U., Hämäläinen, E., Stenman, U. H., et al. (2011). Increased Activity of the Hypothalamic-Pituitary-Testicular axis in Infancy Results in Increased Androgen Action in Premature Boys. J. Clin. Endocrinol. Metab. 96, 98–105. doi:10.1210/jc.2010-1359
Kuiri-Hänninen, T., Sankilampi, U., and Dunkel, L. (2014). Activation of the Hypothalamic-Pituitary-Gonadal axis in Infancy: Minipuberty. Horm. Res. Paediatr. 82, 73–80. doi:10.1159/000362414
Kumar, T. R., Wang, Y., Lu, N., and Matzuk, M. M. (1997). Follicle Stimulating Hormone Is Required for Ovarian Follicle Maturation but Not Male Fertility. Nat. Genet. 15, 201–204. doi:10.1038/ng0297-201
Lamminmäki, A., Hines, M., Kuiri-Hänninen, T., Kilpeläinen, L., Dunkel, L., and Sankilampi, U. (2012). Testosterone Measured in Infancy Predicts Subsequent Sex-Typed Behavior in Boys and in Girls. Horm. Behav. 61, 611–616. doi:10.1016/j.yhbeh.2012.02.013
Law, N. C., and Oatley, J. M. (2020). Developmental Underpinnings of Spermatogonial Stem Cell Establishment. Andrology 8, 852–861. doi:10.1111/andr.12810
Law, N. C., Oatley, M. J., and Oatley, J. M. (2019). Developmental Kinetics and Transcriptome Dynamics of Stem Cell Specification in the Spermatogenic Lineage. Nat. Commun. 10, 2787. doi:10.1038/s41467-019-10596-0
Li, R., Vannitamby, A., Meijer, J., Southwell, B., and Hutson, J. (2015). Postnatal Germ Cell Development during Mini-Puberty in the Mouse Does Not Require Androgen Receptor: Implications for Managing Cryptorchidism. J. Urol. 193, 1361–1367. doi:10.1016/j.juro.2014.10.024
Li, R., Vannitamby, A., Yue, S. S. K., Handelsman, D., and Hutson, J. (2017). Mouse Minipuberty Coincides with Gonocyte Transformation into Spermatogonial Stem Cells: a Model for Human Minipuberty. Reprod. Fertil. Dev. 29, 2430–2436. doi:10.1071/rd17100
McGuinness, M. P., and Orth, J. M. (1992). Reinitiation of Gonocyte Mitosis and Movement of Gonocytes to the Basement Membrane in Testes of Newborn Rats In Vivo and In Vitro. Anat. Rec. 233, 527–537. doi:10.1002/ar.1092330406
Meehan, T., Schlatt, S., O'Bryan, M. K., de Kretser, D. M., and Loveland, K. L. (2000). Regulation of Germ Cell and Sertoli Cell Development by Activin, Follistatin, and FSH. Develop. Biol. 220, 225–237. doi:10.1006/dbio.2000.9625
Oatley, J. M., Avarbock, M. R., and Brinster, R. L. (2007). Glial Cell Line-Derived Neurotrophic Factor Regulation of Genes Essential for Self-Renewal of Mouse Spermatogonial Stem Cells Is Dependent on Src Family Kinase Signaling. J. Biol. Chem. 282, 25842–25851. doi:10.1074/jbc.m703474200
Orwig, K. E., Ryu, B.-Y., Avarbock, M. R., and Brinster, R. L. (2002). Male Germ-Line Stem Cell Potential Is Predicted by Morphology of Cells in Neonatal Rat Testes. Proc. Natl. Acad. Sci. U.S.A. 99, 11706–11711. doi:10.1073/pnas.182412099
Richardson, B. E., and Lehmann, R. (2010). Mechanisms Guiding Primordial Germ Cell Migration: Strategies from Different Organisms. Nat. Rev. Mol. Cel Biol. 11, 37–49. doi:10.1038/nrm2815
Rodprasert, W., Virtanen, H. E., Mäkelä, J. A., and Toppari, J. (2020). Hypogonadism and Cryptorchidism. Front. Endocrinol. (Lausanne) 10, 906–927. doi:10.3389/fendo.2019.00906
Sohni, A., Tan, K., Song, H.-W., Burow, D., de Rooij, D. G., Laurent, L., et al. (2019). The Neonatal and Adult Human Testis Defined at the Single-Cell Level. Cel Rep. 26, 1501–1517. e4. doi:10.1016/j.celrep.2019.01.045
Su, S., Szarek, M., Vooght, A., Hutson, J., and Li, R. (2014). Gonocyte Transformation to Spermatogonial Stem Cells Occurs Earlier in Patients with Undervirilisation Syndromes. J. Pediatr. Surg. 49, 323–327. doi:10.1016/j.jpedsurg.2013.11.047
Tan, K., Song, H. W., and Wilkinson, M. F. (2020). Single-cell RNAseq Analysis of Testicular Germ and Somatic Cell Development during the Perinatal Period, Development. 147. dev183251. doi:10.1242/dev.183251
Tapanainen, J. S., Aittomäki, K., Min, J., Vaskivuo, T., and Huhtaniemi, I. T. (1997). Men Homozygous for an Inactivating Mutation of the Follicle-Stimulating Hormone (FSH) Receptor Gene Present Variable Suppression of Spermatogenesis and Fertility. Nat. Genet. 15, 205–206. doi:10.1038/ng0297-205
Thorsson, A. V., Christiansen, P., and Ritzén, M. (2007). Efficacy and Safety of Hormonal Treatment of Cryptorchidism: Current State of the Art. Acta Paediatr. 96, 628–630. doi:10.1111/j.1651-2227.2007.00238.x
Tien, M. Y., Abeydeera, S. A., Cho, H.-J., Sarila, G., Catubig, A., Burton, E., et al. (2020). Does the Apoptosis Pathway Play a Critical Role in Gonocyte Transformation? J. Pediatr. Surg. 55, 1947–1951. doi:10.1016/j.jpedsurg.2019.09.038
Western, P. S., Miles, D. C., van den Bergen, J. A., Burton, M., and Sinclair, A. H. (2008). Dynamic Regulation of Mitotic Arrest in Fetal Male Germ Cells. Stem Cells 26, 339–347. doi:10.1634/stemcells.2007-0622
Winter, J. S. D., Hughes, I. A., Reyes, F. I., and Faiman, C. (1976). Pituitary-gonadal Relations in Infancy: 2. Patterns of Serum Gonadal Steroid Concentrations in Man from Birth to Two Years of Age. J. Clin. Endocrinol. Metab. 42, 679–686. doi:10.1210/jcem-42-4-679
Keywords: gonocyte, spermatogenic stem cell, mini-puberty, luteinizing hormone, follicle stimulating hormone, testosterone
Citation: Shima Y (2022) Functional Importance of Mini-Puberty in Spermatogenic Stem Cell Formation. Front. Cell Dev. Biol. 10:907989. doi: 10.3389/fcell.2022.907989
Received: 30 March 2022; Accepted: 13 April 2022;
Published: 28 April 2022.
Edited by:
Kei-ichiro Ishiguro, Kumamoto University, JapanReviewed by:
Kenshiro Hara, Tohoku University, JapanCopyright © 2022 Shima. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Yuichi Shima, yshima@med.kurume-u.ac.jp