New Concepts for Human Disorders of Sexual Development
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In the last few years, impressive research has been done on disorders of sex development (DSD), largely expanding the physiopathology with relevant effects on practice. Thus, management of individuals with DSD requires updated scientific knowledge, integrating basic and clinical information. In addition, doctors involved in the care of individuals with DSD need to develop specific personal skills in communication, ethics, legal issues and the ability to work together in dedicated multidisciplinary teams. National or even international networks are also mandatory to create stringent structures for correctly addressing the care of individuals with DSD and to offer them a better long-term outcome. This special issue of Sexual Development covers several of these hot topics and highlights some aspects of research and management. A special paper from a patient support group has been enclosed to present the opinion of affected people. All the authors have been selected regarding their expertise and documented competencies.



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Date de parution 28 septembre 2010
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EAN13 9783805595698
Langue English
Poids de l'ouvrage 2 Mo

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Sexual Development

New Concepts for Human Disorders of Sexual Development
Silvano Bertelloni , Pisa
Olaf Hiort , Lübeck
26 figures, 20 in color, and 20 tables, 2010
S. Karger
Medical and Scientific Publishers
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Sexual Development
Vol. 4, No. 4–5, 2010

Bertelloni, S. (Pisa); Hiort, O. (Lübeck)
The European Disorder of Sex Development Registry: A Virtual Research Environment
Ahmed, S.F.; Rodie, M.; Jiang, J.; Sinnott, R.O. (Glasgow)
Ontogenesis of Testis Development and Function in Humans
Stukenborg, J.B.; Colón, E.; Söder, O. (Stockholm)
New Technologies for the Identification of Novel Genetic Markers of Disorders of Sex Development (DSD)
Bashamboo, A. (Paris); Ledig, S.; Wieacker, P. (Münster); Achermann, J.C. (London); McElreavey, K. (Paris)
Copy Number Variants in Premature Ovarian Failure and Ovarian Dysgenesis
Ledig, S.; Röpke, A.; Wieacker, P. (Münster)
Impact of Molecular Genetics on Congenital Adrenal Hyperplasia Management
Balsamo, A.; Baldazzi, L.; Menabò, S.; Cicognani, A. (Bologna)
Klinefelter's Syndrome: A Clinical and Therapeutical Update
Forti, G. (Florence); Corona, G. (Bologna); Vignozzi, L.; Krausz, C.; Maggi, M. (Florence)
Tumor Risk in Disorders of Sex Development
Pleskacova, J. (Prague); Hersmus, R.; Oosterhuis, J.W. (Rotterdam); Setyawati, B.A.; Faradz, S.M. (Semarang); Cools, M. (Ghent); Wolffenbuttel, K.P. (Rotterdam); Lebl, J. (Prague); Drop, S.L.; Looijenga, L.H. (Rotterdam)
Bone Health in Disorders of Sex Differentiation
Bertelloni, S.; Baroncelli, G.I. (Pisa); Mora, S. (Milan)
A Study of Gender Outcome of Egyptian Patients with 46, XY Disorder of Sex Development
Ismail, S.I.; Mazen, I.A. (Cairo)
Management of Vaginal Hypoplasia in Disorders of Sexual Development: Surgical and Non-Surgical Options
Deans, R.; Berra, M.; Creighton, S.M. (London)
Ethical Guidelines for the Clinical Management of Intersex
Wiesemann, C. (Goettingen)
Disclosing Disorders of Sex Development and Opening the Doors
D'Alberton, F. (Bologna)
Importance of Support Groups for Intersex (Disorders of Sex Development) Patients, Families and the Medical Profession
Cull, M.L. (Lichfield); Simmonds, M. (London)
Author Index Vol. 4, No. 4-5, 2010
Sexual Development

Sex Dev 2010;4:191 DOI: 10.1159/000315960
The disorders of sex development (DSD) represent an important field in both research and the clinical setting. In the last years, impressive improvements in both clinical management as well as structured research approaches have been made and they had major effects on clinical practice. Thus, the correct management of individuals with DSD requires updated scientific knowledge in this very specialized area, integrating basic and clinical information. In addition, doctors involved in the care of individuals with DSD need to develop specific personal skills in communication, ethics, legal issues, and, last but not least, the capacity to work together in a dedicated team, comprising biologists, genetists, neonatologists, pediatric endocrinologists, pediatric surgeons, psychologists, etc. The development of national as well as international networks is also mandatory to create stringent structures for correctly addressing the care of more difficult cases, creating data bases for improvement of epidemiology, and to offer better long-term outcome for our patients in the future.
In the last years, several initiatives renewed the interest of clinicians for DSD, for example: the ESPE/LWEPS Consensus Conference on Intersex Disorders (Chicago, USA, 2005), the constitution of a DSD Working Group within the European Society for Paediatric Endocrinology (ESPE), the EuroDSD network funded by the European Commission in the 7th Framework Programme. In addition, some dedicated specialized meetings were held in Germany and in Italy. These experiences permitted to exchange ideas and to create a well-integrated group of specialists working on DSD in Europe.
This themed issue of Sexual Development presents a series of articles mainly reflecting these experiences and written by colleagues who have been recognized as true experts in each chosen area. They cover several hot topics, provide short ‘state of the art’ papers on specific disorders and update some research or management aspects. We are also honored to host a special contribution from a patient support group to additionally open the eyes of scientists on the opinions and requests of affected people. All the authors have been selected based on their expertise, their documented distinctive competencies and their abilities to make complex topics available to all health care providers.
We hope that the readers will find this themed issue not only enjoyable but also helpful in the clinical care of individuals with DSD. In addition, we would like to take the opportunity to express our gratitude and thanks to all the authors involved in this selected issue for the high-level papers they submitted. We also thank the reviewers for their time and expertise as well as the Chief Editors of Sexual Development for giving us the honor to publish this elaborate issue. In particular, we need to thank Michael Schmid for his invaluable advice, support and friendship.
Silvano Bertelloni (Pisa) Olaf Hiort (Lübeck) May 2010
Sexual Development

Sex Dev 2010;4:192–198 DOI: 10.1159/000313434
Published online: May 26, 2010

The European Disorder of Sex Development Registry: A Virtual Research Environment
S.F. Ahmed a M. Rodie a J. Jiang b R.O. Sinnott b
a Developmental Endocrinology Research Group, Royal Hospital for Sick Children, and b National e-Science Centre, University of Glasgow, Glasgow, UK
Key Words
Database Intersex Network
Disorders of sex development (DSD) are a rare group of conditions which require further research. Effective research into understanding the aetiology, as well as long-term outcome of these rare conditions, requires multicentre collaboration often across national boundaries. The EU-funded EuroDSD programme ( ) is one such collaboration involving clinical centres and clinical and genetic experts across Europe. At the heart of the EuroDSD collaboration is a European DSD registry and a targeted virtual research environment (VRE) that supports the sharing of DSD data. Security, ethics and information governance are cornerstones of this infrastructure. This paper describes the infrastructure that has been developed, the inherent challenges in security, availability and dependability that must be overcome for the enterprise to succeed and provides a sample of the data that are stored in the registry along with a summary analysis of the current data sets.
Copyright © 2010 S. Karger AG, Basel
Suspected cases of a disorder of sex development (DSD) are usually present in early infancy with a variable abnormality of the development of the external and/or internal reproductive organs. There is a large amount of variation in how these patients are managed across the world. In addition, there are enormous gaps in our knowledge about the aetiology of these conditions and the long-term outcome in affected adults. The management of these patients requires multidisciplinary input, and, increasingly, this service is being delivered through organised clinical networks which rely on research as a means of auditing and improving their service.
The Consensus Workshop on DSD which was jointly hosted by the European Society of Paediatric Endocrinology (ESPE) and the Lawson Wilkins Pediatric Endocrine Society of North America stressed the need for the creation and maintenance of a database in centres of expertise [ Hughes et al., 2006 ]. Such databases do exist in many regional and national centres and have provided valuable insight into many aspects of DSD, including epidemiology [ Ahmed et al., 2004 ], aetiology [ Ahmed et al., 2000a ; Gottlieb et al., 2004 ], variation of disease expression [ Bebermeier et al., 2006 ], initial adjustment of parents to their affected child’s condition [ Duguid et al., 2007 ], and long-term outcome [ Lux et al., 2009 ]. However, these databases and registers lack international uniformity and have not been integrated - a key feature particularly desirable when dealing with a rare group of conditions. With the initial help of ESPE and, more recently, from the European Union, a European web-based register and research environment for DSD has now been in operation for approximately 2 years. This infrastructure is currently helping the EuroDSD programme ( ) and has the potential to address many unanswered questions in the future.

Fig. 1. The central role of the European DSD registry as the platform used for exchange of all information between the clinical and research partners with targeted tools supporting a VRE. Clinical partners enter data into the registry. Research partners subsequently design studies, assign recruitment criteria and search the registry for suitable cases. Research partners contact clinical partners for further details of positive cases, perform studies and enter data into the VRE so that new studies can be designed and knowledge promulgated.
This paper describes the construction of the current EuroDSD registry, the operating procedures, general description of the data that are held within the registry, and the future direction of the registry as it develops into a complete virtual research environment (VRE) for research into DSD.
The Creation of the Registry
The cornerstones of the European DSD registry model are site autonomy and the detailed definition and enforcement of standard operating procedures on access, usage and contribution of data to the registry. Details of the standard operating procedures are available at . Each clinical site is solely responsible for deciding what datasets it can share, with which partner sites and in what context. To support this, the design of the registry has been driven by security, incorporating both the needs of the clinical community and ethical oversight required on information governance. The registry provides various functionality including querying, uploading/edit/deletion of DSD cases and crosssearching of cases. The greatest challenge to date related to data heterogeneity, software and data storage heterogeneity and language heterogeneity has been overcome by developing a consensus around a core data model which is largely based on the revised DSD nomenclature [ Lux et al., 2009 ]. The registry platform allows data entry of sequential clinical examinations and development of specific modules, such as a genetic module which records details of method and results of genetic analyses, the mutations that may have been found and the methods of analysis used.
The development of the registry and associated VRE draws heavily on e-Science tools and expertise in information security at the National e-Science Centre (NeSC) at the University of Glasgow ( ). This VRE provides an extensible and personalisable framework that integrates applications, services and resources targeted to the specific-research needs of the DSD clinical and research collaborators. Successful VREs allow aspects of distribution of resources and heterogeneity of data to be made seamless and transparent, targeted to the needs and roles of the scientists. VREs ensure that the data can only be accessed by those with sufficient privileges. A variety of security-supporting portal-based tools [ Sinnott et al., 2007 ] and advanced authorisation solutions [ Sinnott et al., 2008 ] have been utilized for this purpose. User and institute-oriented access control is achieved through the Internet2 Shibboleth technologies ( ) which supports federated access control and delivery of digitally signed X509-based attribute certificates. These are used for automatic configuration of portal contents, e.g. for restriction of access and usage of associated datasets according to the assigned user role within the portal. Figure 1 shows how the registry is central to the EuroDSD research programme and how it acts as a key component of the VRE where clinical data and research results are deposited securely and shared by centres across Europe with appropriate privileges to develop and design new studies.
Registry Users and Their Roles
The European DSD Registry has a panel that consists of members of the ESPE DSD Group ( ). Prospective users are expected to complete a simple online-application form to apply to this panel for approval. There are 2 broad categories of users of the registry: clinical partners and research partners.
Clinical partners are eligible to enter data into the registry. Only full members of a national or international clinical professional society are allowed to become a clinical partner and need to show proof of membership. To ensure maximum levels of governance of clinical data, applications from more than one clinical partner from the same institution is discouraged. Each clinical partner can identify other members of their team who will require access and act as local data contributor. Thus, the clinical partner will remain responsible and accountable for data entry. The level of data sharing is configurable and can be done at a local level, a national level, a Euro-DSD level, or a wider international level as deemed appropriate by the clinical collaborator. Thus, non-Euro-DSD members are able to use the registry to add their own data and use it as a local store. In this case, these data sets are not accessible to other partners or EuroDSD members. The clinical partner is responsible for provision of information to the patient and obtaining consent.
Currently, the research partners within the EuroDSD consortium are the only research partners. It is envisaged that after the lifetime of the EUFP7-funded EuroDSD programme new research partners shall be able to apply to the ESPE DSD registry panel with brief details of their proposed study and search criteria. Research partners are required to demonstrate that they have obtained ethics approval for their respective studies. The panel shall be able to indicate the number of cases that fulfil the recruitment criteria of the investigator’s studies and, for a fixed fee, provide contact details of the clinical partner responsible for the cases.
Some partners may have joint clinical and research partner status. These partners need to continue renewing their research partner status.
Eligibility of Cases in Registry
Any adult or child with a DSD at a centre with an approved clinician is eligible to be included in the EuroDSD registry. Participating cases and their legal guardians (if patients are less than 16 years old) are approached by the clinical partner or a member of their team for approval to include the details on the registry. It should be emphasised that the registry only contains non-identifiable data and although there may be no need to obtain informed consent in some countries such as the UK to share such data with European Economic Area (EEA) members, which includes the 27 countries of the EU and the 3 countries of Norway, Iceland and Liechtenstein, it is recognised that some countries within the EEA, as well as out-with Europe, may have different national regulations which require opt-in consent models. For uniformity as well as for compliance with the feedback received from patient and user support groups, the opt-in system is the recommended standard of consent. As the registry includes children, an information sheet has been created for those under the age of 14 years. Over the age of 14 years, these young adults can be provided with the adult information sheet. Minors (under 16 years) may only participate if both, the minor and a parent or legal guardian, do not raise any objections. If the minor lacks the capacity to provide assent, parent or legal guardian permission is sufficient. On turning 16 years old, the registry will automatically remind the clinical partner to send the participant an adult information sheet. At any time, a participant may request that his or her data or their child’s data no longer be made available in the registry. Participants can make this request to their local clinician who is the clinical partner and who will inform the panel. The participant can also make this request directly to the panel. A confirmation of withdrawal shall be sent to the clinical partner.
The current European DSD registry has been approved by the local Caldicott Guardian in Glasgow, by the UK Research Ethics Committee and the Ethics Committee of the EuroDSD programme. The Congenital Adrenal Hyperplasia support group and the Androgen Insensitivity Support Group in the UK have also been consulted on the development of the registry. All information stored in the registry, and access to that information, conforms to the UK Data Protection Act (1998). However, all participating clinical partners and research partners are encouraged to follow their own national regulations and provide assurance to the registry panel that national regulations are being followed for data handling as well as research. Generic information sheets and consent forms have been developed and are available at . The information sheets can be adapted to include the name of the local clinical partner, local institution and local institutional contact.
Data Flow and Security
Figure 2 summarises how information flows in the European DSD registry as well as the security checks that are supported. Audit tracking software monitors access patterns, machine locations and user access more generally. With this information, it is possible to accurately track and identify both legitimate and any potential illegal access attempts - this is achieved in part through tools provided by Google Analytics. The VRE and registry are themselves hosted in a portal that is protected through a targeted Shibboleth Identity Provider (IdP) at the NeSC. In addition to supporting Single Sign-On (SSO) authentication, the IdP also provides digitally signed attribute certificates which are subsequently used to restrict access to and use of data resources available through the portal itself. Assignment of these privileges is made as part of the panel evaluation. Current roles supported include for local contributors only, for EuroDSD contributors, for EuroDSD investigators and for EuroDSD research collaborators. A user not in possession of any of these roles will not be able to access any data resources within the portal.

Fig. 2. Data flow within the European DSD registry. EEA = European Economic Area; IBAC = Identity Based Access Control; RBAC = Role Based Access Control.

Fig. 3. Distribution of cases according to country (February 2010).
The registry itself does not include any identifying information on patients directly. Instead, every participant on the registry is assigned a unique identifier generated automatically following entry of a case into the registry. This identifier needs to be kept and associated with local records at the contributing partner site. A record in the registry may also have a local identifier which is kept by the clinical partner, physically and electronically, separately to the registry. The unique identifier contains no identifying information within it. This unique identifier is used to track all information about the participant in the registry. The only way for research partners to find out more about the participants in the registry is to contact the clinical partners whose details shall be linked to the unique identifier. The complete research staff at the NeSC maintains up-to-date training in protection of data on human subjects as detailed at .
Table 1. Distribution of 548 cases in the European DSD registry by disorder and actual diagnosis
Disorder of gonadal development
Partial gonadal dysgenesis
Complete gonadal dysgenesis
Testicular DSD
Ovotesticular DSD
Gonadal regression
Disorder of androgen synthesis
17β-HSD deficiency (HSD17B3)
P450 oxidoreductase deficiency (POR)
5a reductase deficiency (SRD5A2)
Combined 17a-hydroxylase/17,20 lyase deficiency
Isolated 17,20 lyase deficiency
P45O scc deficiency (CYP11A1)
Disorder of androgen excess
21a hydroxylase deficiency (CYP21A)
11β hydroxylase deficiency (CYP11B1)
Disorder of androgen action
Complete androgen insensitivity syndrome
Partial androgen insensitivity syndrome
Nonspecific disorder of undermasculinisation
Complex anomalies
EMS of less than 5
EMS of greater than 8
EMS between 5 and 8
Isolated bilateral cryptorchidism
Isolated hypospadias
Leydig cell defects
Persistent Mullerian Duct Syndrome
Cloacal anomaly
Defects of mullerian development
Mayer-Rokitansky-Kuster-Hauser Syndrome
Mullerian duct aplasia, renal agenesis/ectopia cervical somite dysplasia (MURCS)
Description of Cases in Registry
At last review (February 2010), there were 548 cases on the register with a variable number of cases from a variety of clinical centres in Europe which are participating in the EuroDSD research programme ( fig. 3 ). The median year of birth of these cases was 1993 (range, 1927-2009) and the age of presentation ranged from less than 1 month to 62 years. Sex assigned was female in 371 (68%) cases and male in the remaining 177 (32%). Out of the 371 cas-es, consent from clinical centre was available to access metadata in 307 cases. Out of these 307 female cases, 229 were 46XY, 58 were 46XX, 8 were 45X/46XY, 2 cases were of complex rearrangements of sex chromosomes and one case was a trisomy of an autosome. Out of the 177 male cases, consent from clinical centre was available to access metadata in 143 cases. Out of these 143 cases, 110 were 46XY, 19 were 45X/46XY, 8 were 46XX and there were 4 cases of another sex chromosome abnormality. The median external masculinisation score (EMS) [ Ahmed et al., 2000b ] at the first presentation of the cases of 46XY raised as boys and girls was 5.5 (1, 12) and 4 (0, 11), respectively (p < 0.0001, Mann Whitney U test). The median EMS at first presentation of the cases of 45X/46XY raised as boys and girls was 5 (2.5, 12) and 4 (0, 10) (NS), respectively.

Fig. 4. Associated malformations in cases in registry (February 2010).
Amongst the diagnostic criteria based on the revised DSD nomenclature, ‘disorders of androgen action’ is the commonest disorder type with 186 (34%) cases ( table 1 ). Of these cases, 144 (77%) have been diagnosed as complete androgen insensitivity syndrome, 40 (22%) as partial androgen insensitivity syndrome and 2 (1%) cases have a diagnosis of ‘other’. The next most common category is ‘disorders of gonadal development’ with 137 (25%) cases. Amongst the 548 cases in the registry, a family history of infertility, DSD and parental consanguinity was reported in 45 (8%) cases, 130 (24%) cases and 37 (7%) cases, respectively. In 383 (70%) cases, DNA analysis had already been performed and a DNA abnormality had been detected in 248 (65%) of these 383 cases. Associated malformations are present in 121 (22%) cases, and multiple associated malformations have been reported in a number of cases ( fig. 4 ). Out of the total cohort of 548 cases, a stored sample of DNA was available in 430 cases consented for use within the EuroDSD research programme.
The registry has the ability to perform finer grained search of cases by combining the above fields including search for genetic screens and mutations found, however, this is beyond the scope of this paper.
In summary, the European DSD registry is a live, security-oriented web-based platform that can act as a resource for research into a number of aspects of DSD. The future direction of the registry is to facilitate the collection of standardised data internationally, thereby, allowing collaborative research to be performed across the globe. The registry has already allowed access for local use to partners from numerous countries (including Argentina, the Czech Republic, Estonia, and Turkey). Its cornerstone is adherence to the highest standards of data security and information governance. The work on this project is still very much ongoing both from a software development and a clinical research/usage perspective. The work has demonstrated that development of advanced VREs is now realistic and moves beyond the ‘proof of concept in a software research centre’ to production use in a real world clinical and research environment. We continue to work in this space and refine the software solutions to meet a range of criteria deemed important for ethics, information governance and importantly for software guarantees, e.g. on availability. The lessons that have been learnt can be applied to international collaborative research in other rare conditions.
The research leading to these results has received funding from the European Society of Paediatric Endocrinology Research Unit and the European Community’s 7th Framework Programme (FP7/2007-2013) under grant agreement number 201444 and from the European Science Foundation. The authors would like to acknowledge the ESPE DSD Group and the EuroDSD consortium that have contributed to the development of these solutions.
Ahmed SF, Cheng A, Dovey L, Hawkins JR, Martin H, et al: Phenotypic features, androgen receptor binding, and mutational analysis in 278 clinical cases reported as androgen insensitivity syndrome. J Clin Endocrinol Metab 85:658-665 (2000a).
Ahmed SF, Khwaja O, Hughes IA: The role of a clinical score in the assessment of ambiguous genitalia. BJU Int 85:120-124 (2000b).
Ahmed SF, Dobbie R, Finlayson AR, Gilbert J, Youngson G, et al: Prevalence of hypospadias and other genital anomalies among singleton births, 1988-1997, in Scotland. Arch Dis Child Fetal Neonatal Ed 89:F149-F151 (2004).
Bebermeier JH, Brooks JD, DePrimo SE, Werner R, Deppe U, et al: Cell-line and tissue-specific signatures of androgen receptor-coregulator transcription. J Mol Med 84:919-931 (2006).
Duguid A, Morrison S, Robertson A, Chalmers J, Youngson G, et al: The psychological impact of genital anomalies on the parents of affected children. Acta Paediatr 96:348-352 (2007).
Gottlieb B, Beitel LK, Wu JH, Trifiro M: The androgen receptor gene mutations database (ARDB): 2004 update. Hum Mutat 23:527-533 (2004).
Hughes IA, Houk C, Ahmed SF, Lee PA; LWPES Consensus Group; ESPE Consensus Group: Consensus statement on management of intersex disorders. Arch Dis Child 91:554-563 (2006).
Lux A, Kropf S, Kleinemeier E, Jürgensen M, Thyen U; DSD Network Working Group: Clinical evaluation study of the German network of disorders of sex development (DSD)/intersexuality: study design, description of the study population, and data quality. BMC Public Health 9:110 (2009).
Sinnott RO, Ajayi O, Jiang J, Stell AJ, Watt J: User-oriented security supporting inter-disciplinary life science research across the grid, in Konagaya A, Arzberger P, Tan TW, Sinnott R, Angulo D (eds): Special Edition on Life Science Grids, New Generation Computing Journal, Springer, 2007,pp 339-354.
Sinnott RO, Grid Security, Grid Computing: Technology, Service and Application. Boca Raton, CRC Press, 2008,pp 307-334.
S. Faisal Ahmed, MD Department of Child Health Royal Hospital for Sick Children Glasgow G62 8NT (UK) Tel. +44 141 201 0571, Fax +44 141 201 0837, E- Mail
Sexual Development

Sex Dev 2010;4:199–212 DOI: 10.1159/000317090
Published online: July 27, 2010

Ontogenesis of Testis Development and Function in Humans
J.B. Stukenborg a E. Colón a , b O. Söder a
a Paediatric Endocrinology Unit, Department of Women’s and Children’s Health and b Department of Clinical Pathology and Cytology, Karolinska Institutet and University Hospital, Stockholm, Sweden
Key Words
Germ cells Human spermatogenesis Somatic cells
Functional gonads are mandatory for sexual reproduction and survival of higher animal species. However, at the level of the individual subject, acquired or inherited gonadal dysfunction and infertility are not commonly associated with severe life-threatening phenotypes. Medical progress and increased societal interest have led to more prioritised agendas for reproductive health problems. Increasing attention is focused on disorders of sex development, fertility and sexual function. Despite this engagement, our understanding of the detailed molecular and cellular adverse events behind such problems is still incomplete. Critical early steps, such as determination of the gonads, occur at precise temporal windows of development. The sex chromosomes are obvious critical contributors, but many other human chromosomes also contribute to sex differentiation, engaging multiple genes and proteins. The aim of this review is to give an up-to-date and comprehensive summary of the events required for gonadal ontogenesis in the human male, from the stage of embryonic sex determination to postnatal maturation including puberty. The principal genes involved in these processes are tabulated and discussed. Morphological events relevant for human gonadal development are covered, in particular in connection with early germ cell maturation and spermatogenesis. Consequences of maldevelopment caused by, e.g. cryptorchidism, are discussed.
Copyright © 2010 S. Karger AG, Basel
In humans, the important first steps of sexual differentiation occur during the initial 7 weeks of embryonic development and appear as several successive events starting with establishment of the genetic sex, development of the gonadal ridge and immigration of primordial germ cells followed by a sexually dimorphic differentiation of the gonadal anlagen into either testes or ovaries. Until this point of time, referred to as the indifferent stage of gonadal development, no morphologically distinct sex differences can be noticed in developing human gonads. One critical event in sex differentiation is the determination of the gonads. This developmental phase establishes the hormonal dimorphism which, in turn, has a major impact on several later events of the male as well as female paths. This review will deal with differentiation of the male gonad covering its maturation from its first appearance to the pubertal activation. Recent discoveries added to increase the knowledge of testicular ontogenesis together with the growing list of genes involved in this process will be presented and discussed.
Table 1. Chronology of important early events in human male sex differentiation
Age at start (dpc)
Size CRL (mm)
Genetic sex

PGC migration from yolk sac
Formation of gonadal ridge
PGCs reach gonadal ridge
Male sex determination
Leydig cells appear
Androgen, INSL3 detectable
Testicular descent (1st phase)

dpc = Days post conception; CRL = crown rump length (‘sitting height’).
The Primitive Gonad
By day 32 post conception (pc), the gonadal anlagen can be recognised as paired bipotential structures in the developing human embryo. They are situated at the ventromedial surface of the mesonephros and appear from the mesoderm by contributions from somatic mesenchymal cells from the mesonephros and epithelial cells migrating from the coelomic surface of the gonadal ridge. As mentioned before, no sexual dimorphism can be distinguished morphologically at this stage of development. Primordial germ cells (PGCs), which become gonocytes later on, cannot be observed at this early time of gonadal formation [ Shawlot and Behringer, 1995 ; Torres et al., 1995 ; Miyamoto et al., 1997 ; Birk et al., 2000 ; Failli et al., 2000 ; Park and Jamieson, 2005 ]. The temporal scale of the important early events of human gonadal differentiation is displayed in table 1 .
The mesonephros also constitutes the primordium of the adrenal glands and the urinary system. Disruption by gene targeting of any of several involved transcription factors (online supplementary table 1 ; see ) during genital ridge development results at all times in severely affected phenotypes with multiple malformations of the urogenital tract, adrenals and other structures. Therefore, a number of peptide growth factors have been compromised in gonadal development from the indifferent gonadal anlagen, most notably the insulin-like growth factor (IGF) superfamily [ Nef et al., 2003 ]. The formation of the urogenital ridge is mainly controlled by 2 transcriptional regulators: the tumour suppressor gene Wilms’ tumour-associated gene-1 (WT1) and the orphan nuclear receptor steroidogenic factor-1 (SF1). WT1 is a DNA- and RNA-binding protein with transcriptional and posttranscriptional regulation capacity. It is expressed in gonadal stromal, coelomic epithelial cells and immature Sertoli cells, interacts with the cAMP-responsive element-binding protein CITED 2 and is regulated by ‘paired box gene 2’ (PAX2) (see online suppl. table 1 ). In rodents, disruption of Wt1 leads to lack of formation of kidneys, gonads and adrenals. Distinct but not identical phenotypes can be observed after WT1 loss-of-function (LOF) mutation in humans, resulting in pseudoher-maphroditism and/or urogenital and other malformations in boys with WAGR, Deny-Drash or Frasier syndromes [ Pritchard-Jones et al., 1990 ; Park and Jamieson, 2005 ; online suppl. table 1 ]. SF1 , expressed in gonadal ridges, is a transcriptional regulator of steroid hydrolases, gonadotropins and aromatase and involved in the stabilisation of intermediate mesoderm, follicle development and ovulation (online suppl. table 1 ). Additionally, SF1 regulates the anti-müllerian hormone (AMH), dosage-sensitive sex reversal-congenital adrenal hypoplasia critical region on the X-chromosome protein 1 (DAX1) and steroidogenic acute regulatory protein (StAR). These genes are expressed in the somatic testicular compartment and important for normal testicular cord formation, Leydig and Sertoli cell differentiation as well as the initial step of steroidogenesis (online suppl. table 1 ). Therefore, deletion of Sf1 in mice results in failure of gonadal and adrenal development, whereas the corresponding LOF mutation in humans has a less prominent gonadal phenotype and adrenal insufficiency [ Biason-Lauber and Schoenle, 2000 ; Achermann et al., 2002 ; Park and Jamieson, 2005 ; online suppl. table 1 ]. For a comprehensive account of more genes involved in gonadal formation, please consult online suppl. table 1 .
Table 2. Selection of protein expression patterns specific for different male germ cell types in the human testis

Primordial Germ Cells
By the end of the 5th week pc of human embryonic development, 3 different lineages of somatic cell types with bipotential fate, dependent on their future paths (see below), are forming the gonadal anlagen. At this stage, immigrating PGCs are colonising the gonadal structures. After they have permanently been situated in the gonad, they are specified as gonocytes. The PGCs differentiate from epiblast-derived stem cells in the yolk sac. Due to their expression of molecular and cellular markers for pluripotency or early germ cells such as alkaline phosphatase, OCT3/4 and c-kit, they can be distinguished from other cells within the forming gonad (see also table 2 and online suppl. table 1 ). Guided by extracellular matrix proteins expressed along the dorsal mesentery of the hind gut, the PGCs migrate to the gonadal ridges. During this phase, PGCs exhibit active mitotic proliferation and have expanded in numbers while reaching the gonadal anlagen [ Bendel-Stenzel et al., 1998 ; Wylie, 1999 ]. In the early testis shortly after determination, gonocytes will continue their mitotic proliferation and then become mitotically quiescent. They will not be recruited into meiosis until much later in time. The decision to enter into meiosis or not is thought to be governed by somatic cells in the male gonad since XY PGCs residing in an ovary follow the female path [ McLaren and Southee, 1997 ]. However, meiotic entry might also be activated by mechanisms intrinsic to germ cells [ Morelli and Cohen, 2005 ]. In males, the absence of germ cells still allows differentiation of somatic cells including Leydig cells with steroidogenic activity. Affected males will undergo pubertal development but are infertile due to a Sertoli-cell-only syndrome.
Somatic Cell Lineages in the Male Gonad
At the end of the 6th week pc of human embryonic development, the indifferent gonad consists of 4 different cells lineages, including gonocytes, with predefined maturational paths dependent on the sex. The crucial somatic cell lineages are Sertoli cells, Leydig cells and peritubular cells. Failure of differentiation and function of any of these lineages will result in severe phenotypes with respect to adult gonadal function and fertility.
Sertoli Cells
Sertoli cells are crucial for testicular histogenesis and future function. In the adult testis, the Sertoli cells are nurse cells for spermatogenesis, creating niches for differentiation of spermatogonial stem cells and providing structural support, nutrients and growth factors for the developing germ cells. Due to the fact that the sperm output in the adult testis is related to the number of Sertoli cells, the control of Sertoli cell proliferation in the developing testis is crucial for future production of male germ cells [ Petersen and Söder, 2006 ]. Pituitary follicle stimulating hormone (FSH) and its receptor (FSHR) are important factors for Sertoli cells development. Functional impairments of the FSHR result in reduced fertility, a reduction of Sertoli cell numbers and, therefore, a reduction of testicular size combined with a reduction in circulating testosterone levels [ Huhtaniemi et al., 1987; Simoni et al., 1997 ; Krishnamurthy et al., 2001 ; O'Shaughnessy et al., 2007a ; online suppl. table 1 ]. Nevertheless, during the first phase of Sertoli cell differentiation and proliferation, the fetal hypothalamic-pituitary-gonadal (HPG) axis is not yet operative and FSH is not available [ Söder, 2007 ]. Therefore, other growth factors are considered to control the early phase of Sertoli cell proliferation [ Petersen et al., 2001 , 2002 ; Petersen and Söder, 2006 ]. Sertoli cell differentiation and proliferation are important first steps of male sex determination and are controlled by several genes (for details see online suppl. table 1 ). Therefore, they are exposed targets for disruptive actions of endogenous factors and xenobiotics such as inflammatory mediators and endocrine disrupting compounds (EDCs) [ Petersen et al., 2002 , 2004 ; Petersen and Söder, 2006 ; Söder, 2007 ].
Pre-Sertoli cells are first defined as cells of the supporting lineage expressing the sex determining region on the Y (SRY) chromosome. After SRY expression, the SRY related HMG box 9 (SOX9) , a gene with predominantly testis promoting activity, is expressed by Sertoli cells and leads to an upregulation of AHM , fibroblast growth factor 9 (FGF9) and prostaglandin D2 (PGD2) (for details see online suppl. table 1 ). These genes are affecting the differentiation of the reproductive tract and therefore defining male sex determination. This process is rapidly followed by morphological changes in the primitive gonad thus adopting testicular features such as formation of testicular cords.
Leydig Cells
Leydig cells constitute another crucial testicular cell lineage in the developing male. They originate from steroidogenic precursor cells immigrating from the coelomic epithelium and the mesonephric mesenchyme to colonise the indifferent gonad [ Merchant-Larios and Moreno-Mendoza, 1998 ; Schmahl et al., 2000 ; O'Shaughnessy et al., 2006 ]. At week 7 pc of human development, they start to differentiate. The differentiation and proliferation of functional Leydig cells is influenced by signals from Sertoli cells, as AMH, desert hedge hog (DHH) and FGF9 (for details see online suppl. table 1 ). [ Clark et al., 2000 ; Colvin et al., 2001a ].
Leydig cells may be classified into 3 different subtypes according to their morphological and maturational stage of development. The development and function of human fetal-type and the adult-type Leydig cells were discussed recently in a review by our group [ Svechnikov et al., 2010 ]. The first Leydig cell type to appear after testis determination is the fetal type. It shows a different steroidogenic pattern and regulation as compared with the immature-type and the adult-type Leydig cells. Both other Leydig cell types appear postnatal before puberty and in adults after completed pubertal development, respectively [ Ge et al., 1996 ; Colvin et al., 2001a ]. At the 8th week of human gestation, fetal-type Leydig cells start to produce testosterone and other androgens [ Svechnikov et al., 2010 ]. Initially they are regulated by the placental human chorionic gonadotropin (hCG), which shares on Leydig cells signalling receptors with pituitary LH. However, this latter hormone appears much later in development when the HPG axis becomes established in the beginning of the 2nd trimester of human pregnancy. At mid-gestation they may constitute 40% of the total testicular cell mass. Leydig cells are situated in the interstitial tissue compartment of the testis and increase their number during the first 2-3 months after birth [ Svechnikov et al., 2010 ].
Additionally to testosterone, a crucial hormone for differentiation of male external and internal genitalia, Leydig cells also produce SF1 necessary for steroidogenesis [ Achermann et al., 2002 ] and insulin-like factor-3 (INSL3) (online suppl. table 1 ). INSL3 and its receptor RXFP2, together with androgens and AMH are involved in the process of testicular descent (online suppl. table 1 ). The first transabdominal phase of testicular descent occurs in human fetuses during week 8-16. Even if LOF mutations affecting INSL3 or RXFP2 result in cryptorchidism, it remains a rare cause of this common malformation in boys [ Ivell and Hartung, 2003 ; Ferlin et al., 2006 ; online suppl. table 1 ]. In addition to its important role behind testicular descent, INSL3 seems to have other functions as a paracrine mediator in the male gonad and may also serve as a useful differentiation marker of Leydig cells in clinical practice [ Ferlin et al., 2006 ].
As discussed already in a recent review of our group [ Söder, 2007 ], adrenocortical and gonadal steroidogenic cells seem to share embryonic origin in the coelomic epithelium and may exist as one lineage before divergence into the gonadal and adrenocortical paths [ Mesiano and Jaffe, 1997 ]. In line with this, adrenocortico-tropic hormone (ACTH) has been implicated as a regulatory factor for fetal Leydig cells expressing ACTH receptors in the early phase of gonadal differentiation. A common origin is also supported by the testicular adrenal rest tumours that are often found in male patients with congenital adrenal hyperplasia. These benign tumours are thought to be due to ACTH-driven expansion of adrenocortical or steroidogenic common precursor cells present in the testis [ Stikkelbroeck et al., 2001 ], and, although much rarer, adrenal rest tumours have also been found in the ovary [ Claahsen-van der Grinten et al., 2006 ].
In a similar way to Sertoli cells, Leydig cells represent an obvious target of disruptive actions of xenobiotics and EDCs [ Söder, 2007 ]. In adult animals, these cells demonstrate a large regenerative capacity. Several growth factors have been implicated in Leydig cell regeneration and survival [ Yan et al., 2000 ; Colón et al., 2007 ]. If this regeneration is driven by the resident Leydig precursor cells is not yet clearly defined. A second possible hypothesis suggests that peritubular testis cells (see below) also represent a reserve pool of steroidogenic cells.
Peritubular Cells
Along the basal membrane of the seminiferous tubuli, peritubular cells (PTCs) are required for early histogenesis of the seminiferous cords. Together with the basal membrane and the Sertoli cells they form the blood-testis barrier and provide physical support for Sertoli cells. In the postpubertal testis, they may add contractile forces thought to be necessary for tubular fluid and sperm release. Via chemotactic signals from Sertoli cells, early PTCs and cells contributing to the vasculature of the testis migrate from the adjacent mesonephros [ Cupp et al., 2003 ]. This migration process is an important step in sex determination and is SRY dependent. Normal SRY expression is related to GATA4 , a gene expressed also by PTCs. GATA4 also activates steroidogenic genes such as StAR, CYP11A, CYP17, CYP19 , and HSD3B2 , which are mainly expressed in Leydig cells (online suppl. table 1 ). Considering this and the fact that they are highly proliferative cells, PTCs are demonstrating important features for normal testis development [ Capel et al., 1999 ; Schmahl and Capel, 2003 ], but their precise role in adult testicular function is still not known. Data accumulated lately indicate their possible role as a reserve or stem cell pool [ Haider et al., 1995 ] and that they might be involved in the regeneration of Leydig cells after a disruptive injury.
Sex Determination
The undifferentiated gonadal anlagen in XY individuals show the first signs of testicular development at day 42 pc. This critical event is referred to as sex determination. As described above, it is related to the expression of the testis determining gene SRY , followed by expression of SOX9 (for details see online suppl. table 1 ). Gonocytes in the testis primordium proliferate rapidly by mitoses and subsequently become quiescent without entering meiosis. Morphogenesis of the testis is apparent when Sertoli cells supported by PTC form testicular cords, also hosting the gonocytes. The morphology of the testes remains stable until puberty when the cords are differentiated into seminiferous tubules at the beginning of spermatogenesis. Activation of the spermatogenic process results in an increase of the testicular volume; this observation is used as a clinical definition of the onset of puberty in boys. When looking at the tissue level, this is reflected by quantitative onset of meiotic activity of testicular germ cells and formation of a lumen shifting the seminiferous cords to tubules.
Testicular Descent
Function of the postpubertal testes is dependent on their scrotal position. The process of testicular descent consists of 2 phases: the first transabdominal phase of descent followed by the inguino-scrotal phase aiming to transfer the testes to a scrotal position. The first phase starts soon after testis determination and differentiation of Leydig cells and guides the testis from a position in the upper abdomen to the inner opening of the guinal channel in the pelvic part of the abdomen. After week 18 of gestation, the testes with the epididymis and the proximal part of vas deferens finally move through the inguinal canal. During the final 2 months of pregnancy before birth, the testes usually have a scrotal position.

Fig. 1. Histology of normal human testicular germ cells. Gonocytes, spermatogonia, spermatocytes, round and elongating spermatids are shown on the left side in cross sections of seminiferous tubules of a 1-year-old boy ( a ), a 5-year-old boy ( b ), a 12-year-old boy ( c ) and a 23-year-old man ( d ). Postnatal male germ cells of pre-meiotic, meiotic and post-meiotic phases during spermatogenesis are shown in detail in the bottom row ( e ). Abbreviations: gonocyte (G), spermatogonia (Spg); spermatocytes (Spc), round spermatids (rSpd), elongating and elongated spermatids (eSpd). Staining: haematoxylin/eosin (HE). Scale bar = 50 μm.

Fig. 2. Histology of testicular anomalies in humans. Fibrosis and Leydig cell hypoplasia are shown in cross sections of seminiferous tubules from an undescended testis of a 6-year-old boy ( a, b ), whereas only discrete fibrosis is shown in samples from an undescended testis of a 3-year-old boy ( c, d ). While all seminiferous tubules observed in the testis of the 6-year-old patient are lacking germ cells ( b ; white arrows), the seminiferous tubules of the 3-year-old patient still contain few spermatogonia and gonocytes ( d ; black arrows). A mixture of female and male gonadal cell types is shown in cross sections of an ovotestis of a newborn child ( e, f ). Primordial ovarian follicles ( f ; white arrowheads) are enclosed by an interstitial compartment remaining morphological features normally found in the testis. Abbreviations: blood vessels (bv), gonocyte (G), spermatogonia (Spg). Staining: haematoxylin/eosin (HE). Scale bar: a and c = 2 mm; e = 1 mm; b, d and f = 100 μm.
Approximately 3-4% of newborn boys suffer from uni- or bilateral undescended testes, which results in adverse effects on germ cell maturation ( fig. 2a-d ) [ John Radcliffe Hospital Cryptorchidism Study Group, 1992 ; Berkowitz et al., 1993 ]. However, recent prospective data show large regional differences with figures as high as 9% in Denmark compared with 2.4% in Finland [ Boisen et al., 2004 ]. Undescended testes can be grouped into 4 groups with respect to their location: (a) cryptorchidism: the testes are located proximal to the inguinal channel (intra-abdominally or retroperitoneally) and cannot be seen or palpated; (b) inguinal testes: testes are located within the inguinal channel and cannot be moved; (c) retractile testes: testes can be pushed into the scrotum but return back into the inguinal channel afterwards, and (d) testicular ectopy: the testes are located outside the normal route of descent, e.g. in the groin or femoral. In 75% of these cases, the testes descend within the first 3 months after birth [ Berkowitz et al., 1993 ; Cortes et al., 2008 ]. However, 1% or more of the boys still have cryptorchid testes at the age of 12 months. These patients require treatment during early childhood to minimise the risk of infertility and of testicular germ cell tumour development later in life [ Kollin et al., 2007 ]. During the last century, the influence of cryptorchidism on infertility, as well as on testicular cancer formation in adulthood, has been thoroughly investigated. Despite this fact, cryptor-chidism-related long-term effects are not well defined so far [ Wood et al., 2009 ] because of limited access to testicular material from these patients. Many genes as FGFR1, WT1, AR, ARX, INSL3 or HOXA13 , have been identified to be related to cryptorchidism (for details see online suppl. table 1 ) and high intra testicular testosterone is of importance for the 2nd phase of testicular descent.
Spermatogenesis requires an intact testicular environment with functional interactions between germ cells and somatic cells [for review see Sharpe, 1994 ; Wistuba et al., 2007 ]. In humans, these cellular interactions are established during the first months after birth, a process highly impaired in cryptorchid testes [ Zivkovic et al., 2007 ]. Differentiation of gonocytes into type A spermatogonia within the first 6 months of postnatal life is essential for ongoing spermatogenic maturational processes and sperm production later in life and might be related to an increased testosterone production occurring during this time (‘mini-puberty’) [ Forest et al., 1974 ; Hadziselimovic et al., 2001a , b ; Zivkovic et al., 2007 ].
The formation and fate of spermatogonia from prepubertal boys suffering from cryptorchidism has been investigated in testicular biopsies in several studies [ Ritzén et al., 2007 ; Wood et al., 2009 ]. There are indications that an increased testicular temperature might cause a disturbed formation of A dark spermatogonia which leads to infertility and/or testicular germ cell tumour development later in life [ Hadziselimovic et al., 2001a , b ]. However, it is still unclear if changes in germ cell proliferation and differentiation in cryptorchid testes are related to a disturbed somatic cell compartment or not.
Perinatal Events in Testicular Maturation
During the 3rd trimester of pregnancy, the fetal testes still produce large quantities of androgen but less than the peak activity at mid gestation. Closer to birth, at term age, the hormonal activity of the testes declines albeit being still clearly measurable. However, soon after birth in both sexes of primates, but best recognized in human males, the first few months are a period of high hormonal activity of the testes and the hypothalamic-pituitary axis [ Grumbach, 2005 ]. The period is often referred to as the mini-puberty and characterised by a hormonal surge of gonadotropins and testosterone. This is associated with proliferation of Sertoli cells and some degree of germ cell development, i.e. transformation of gonocytes to Ad spermatogonia, at a time when gonadotropin, testosterone and inhibin B reach high levels. More detailed studies have shown that LH values begin to increase 2 weeks after birth and decline to prepubertal values by 1 year of age in both sexes. FSH values also begin to increase 2 weeks after birth and decline to prepubertal levels by 1 year of age in boys and 2 years of age in girls. In parallel, testosterone levels in boys often reach a peak of 10-15 nmol/l during the 2nd month of postnatal life but then decline to prepubertal low levels at 6 months of age [Forest et al., 1975]. However, the bioavailability of these high androgen levels has been questioned since high levels of SHBG in early infant boys may result in less circulating free and available testosterone levels in this age group. The biological role of the ‘mini-puberty’ for future testicular and male reproductive function is unknown but it has been speculated that it may add to aspects of germ cell maturation and the development of male gender identity.
Prepubertal Maturation of the Testis
After the postnatal activation in early infancy the testes become relatively quiescent during childhood due to a constant inhibition of the prepubertal HPG axis by hypothalamic neuronal circuits.
During this period, lasting to the first signs of pubertal activation, commercial assays of sexual hormones rarely catch elevated hormonal levels. However, employing methods with ultrahigh sensitivity and specificity, enabling limits of quantification in the low picomolar level for estrogens and sub picomolar level for androgens, it has been demonstrated that explicitly these sex hormones are detectable in prepubertal boys and girls and show gender differences in children before the appearance of any clinical signs of puberty [ Courant et al., 2010 ].
Pubertal Activation of the Testis
Start of puberty is defined by an increase of the testicular volume to >3ml, which is due to onset of spermatogenesis and production of large numbers of post-meiotic germ cells. Initially before this is evident, the number of mature Leydig cells increase and become activated, indicating that precursor cells (e.g. peritubular-like Leydig stem cells and perivascular cells) are being recruited for this purpose [ Chemes et al., 1985 , 1992 ; Cigorraga et al, 1994 ]. These peritubular-like Leydig stem cells express platelet-derived growth factor receptor-α but not the LH receptor or steroidogenic enzymes [ Ge et al., 2006 ]. After experimental ablation of Leydig cells with selective toxicants, peritubular spindle-shaped cells act as precursors of Leydig cells [ Jackson et al., 1986 ; Teerds et al., 1988 ]. These peritubular cells are myoid in nature and express α-actin at high levels, which can be used for their identification. Although such testicular cells are considered to be the immediate source of Leydig cells in postnatal humans, perivascular cells of the interstitial tissue, testicular macrophages and neural crest cells have also been proposed as precursor Leydig cells [ Jackson et al., 1986 ; Schulze et al., 1987 ; Teerds et al., 1988 ]. The latter suggestion is based on several findings that human Leydig cells express a number of proteins typical for cells of neuronal origin. INSL3 not only controls the early phase of testicular descent during embryonic development, but is also expressed and secreted by adult-type human Leydig cells [ Foresta et al., 2004 ]. Recently, the INSL3 concentration in the peripheral blood of men has been shown to decline continuously in a linear fashion between the ages of 35 and 80 years; a phenomenon that probably reflects a reduction in Leydig cell functionality with age [ Anand-Ivell et al., 2006 ]. In contrast, onset of puberty in boys is associated with a significant rise in INSL3 levels [ Wikstrom et al., 2006 ].
In quantitative terms, spermatogenesis starts at puberty and is reflected by a rapidly increasing testicular volume, constituting the most common first sign of puberty in boys. Spermatogenesis is divided into 3 distinct but linked processes: mitotic self-renewal proliferation of spermatogonia, meiotic division of primary spermatocytes and non-proliferative maturation of round spermatids to spermatozoa (spermiogenesis). To investigate and visualise the different stages of spermatogenesis immunohistochemical staining with specific markers is an excellent method (see a list of suitable markers for humans in table 2 ). This production of mature fertile spermatozoa as the product of a functional spermatogenesis includes proliferation and differentiation of male germ cells starting with undifferentiated spermatogonia, also known as spermatogonial stem cells (SSCs). In contrast to the situation in rodents, whose germ cell populations can be categorised as A single , A pair , A aligned , A1, A2, A3 and A4 [ de Rooij and Grootegoed, 1998 ; de Rooij and Russell, 2000 ; Dettin et al. 2003 ], less different types of spermatogonia have been identified in primates: the A dark , the A pale , the Atransition and several types of B spermatogonia [ Clermont, 1969 ; Meistrich and van Beek, 1993 ; Zhengwei et al., 1997 ; Ehmcke et al., 2005 ]. Among these cell subtypes, Adark spermatogonia are considered as reserve stem cells. This spermatogonial cell subtype does not divide when spermatogenesis is intact, but starts proliferating upon severe testicular damage [ van Alphen et al., 1988 ; Ehmcke et al., 2005 ]. The A pale spermatogonia divide during every spermatogenic cycle and provide either self-renewing A pale spermatogonia or A transition . After the last mitotic divisions, primary spermatocytes which are derived from B-spermatogonia enter meiosis. The first meiotic division starts with a long prophase and can be subdivided into 5 different stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. Diakinesis ends with the migration of the chromosomes to the metaphase plate during metaphase I. During anaphase I, homologous chromosomes are separated, followed by telophase I where 2 daughter cells (secondary spermatocytes) are formed, each containing 1 of the 2 chromosomes from the homologous pair of chromosomes. After a short interkinesis, the second meiotic division starts with the metaphase II, followed by anaphase II and telophase II, which finally results in the formation of round haploid spermatids.
The formation of elongated spermatids during spermiogenesis is divided into 4 phases: the Golgi phase, the cap phase, the acrosome phase, and the maturation phase. Throughout the last phase, the nucleus condenses, histones are replaced by protamines, the remaining cytoplasm becomes the cytoplasmic droplet, and the mitochondria form a ring around the base of the flagellum. After completion of spermiogenesis, the immotile not fully differentiated testicular spermatozoa are transported by the peristaltic activity of the peritubular cells together with fluid excreted by Sertoli cells through the seminiferous tubules into the rete testis before entering the epididymis for final maturation.
The duration of spermatogenesis and spermiogenesis takes approximately 34 days in mice and, including the proliferation of spermatogonia, around 74 days in humans [ Oakberg, 1957 ; Heller and Clermont, 1963 , 1964 ; Amann, 2008 ].
Original work cited in this study was supported by the Swedish Research Council, Swedish Children’s Cancer Fund, EU (EuroDSD;FP7 Grant No. 201444). J.B.S. is supported by the Deutsche Forschungsgemeinschaft (DFG-Grant No. STU 506/3-1).
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