Biochemistry of Andrology E-Book
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- Herausgeber: Bentham Science Publishers
- Kategorie: Fachliteratur
- Serie: Andrology: Current and Future Developments
- Sprache: Englisch
This textbook comprehensively covers the physiology and biochemistry of the processes in the male reproductive system. Readers will gain an understanding of a variety of general topics relevant to andrology through fourteen chapters which include information about male reproductive anatomy, sexual development, spermatogenesis, sperm-egg interactions necessary for fertilization, reproductive toxicology, congenital disorders, sexually transmitted diseases, male sterility and changes due to aging.
The book serves as a primary andrology reference for medical students studying reproductive biology.
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Veröffentlichungsjahr: 2017
Ähnliche
Andrology: Current and Future Developments
(Volume 1)
(Biochemistry of Andrology)
Editors
Marco Aurélio Gouveia Alves
&
Pedro Fontes Oliveira
BENTHAM SCIENCE PUBLISHERS LTD.
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PREFACE
Andrology is the discipline that corresponds to gynecology for men, but while the latter is a well-established discipline, the former is still an emerging field. A simple definition of andrology will include the study of all subjects that may affect the male reproductive system. In the last years, a striking large number of topics has emerged in this field and there is an increasing amount of information that may be useful for graduate, post-graduate and even well-established researchers. Herein, we summarize the available information so far in this area, presenting a review and a critical analysis of the available data on the most relevant subjects with interest to andrology, with emphasis on the biochemistry of the processes. This e-book is divided into 14 chapters, all coordinated by Dr. Marco G. Alves and Prof. Pedro F. Oliveira.The first chapter is a brief introduction to the historical evolution of andrology and how biochemistry has emerged as a partner, contributing to the emerging relevance of this discipline. The second chapter describes general considerations on testis physiology, testicular anatomy and functional organization, as well as its embryonic development. The third chapter discusses the basic aspects of testicular cells physiology and function. The fourth chapter presents the basic aspects of spermatogenesis, a key event for species propagation, from a biochemical perspective. It is mainly focused on the mechanisms responsible for postnatal testis development but it also presents an overview on the complex events that control the spermatogenic cycle. Many of those changes are under the action and function of male-associated hormones that trigger signaling pathways thus, the fifth chapter is dedicated to those issues. The endocrine regulation of sexual maturation and sperm formation is still a matter of great debate and with an enormous interest. In the sixth chapter, the transition period between childhood and adulthood is discussed, particularly the biochemical changes that control pivotal events responsible for the sexual maturation of the individuals. There is also an overview on puberty-associated disorders, pinpointing the clinical features that should be taken into consideration and the deleterious signals that may occur until sexual maturation is achieved. In the seventh chapter, the biochemical events occurring in the epididymis that end-up in sperm maturation, are discussed. It also discussed the structural organization of epididymal epithelial cells and secretory proteins and their involvement on the spermatozoa modifications that occur during the process of maturation. The eighth chapter is dedicated to the formation and biochemical properties of seminal plasma and male accessory glands. Those changes are essential for spermatozoa to acquire fertility capacity. In the ninth chapter, the functional and physiological aspects of spermatozoa, as well as its epigenome are presented, which may have an enormous implication to the success of the pregnancy and latter to the offspring health. The male gamete is very dynamic and has to move, capacitate, migrate through the female tract, bind to the egg membrane and fuse to the oocyte, resulting in a viable embryo. The tenth chapter is dedicated to the sequential modifications and the molecular mechanisms that occur during the journey of spermatozoa through the female reproductive tract since they have a pivotal role in couple’s fertility success and offspring health. Those events may be compromised by several factors that compromise male reproductive health. Congenital disorders, such as hypospadias, undescended testis, testicular atrophy and testicular cancer have increased among young males and even erectile dysfunction and sexually transmitted diseases are still problems that compromise male reproductive health. These issues are discussed in chapter eleven. The twelfth chapter discusses how the pandemic incidence of metabolic diseases is contributing for the worldwide decline in both, sperm quality and male reproductive health. The biochemical changes induced by lifestyle factors and nutrition in the testis are on spotlight to unveil the mechanisms by which metabolic diseases affect nativity rates and the offspring. There is an intense debate whether worldwide sperm quality is decreasing and the factors that may be responsible for that. Environmental contaminants have arisen as main contributors to the decline on sperm quality. Thus, the molecular mechanisms by which environmental cues alter male reproductive health remain a matter of great interest and are discussed in the thirteenth chapter. The last chapter is dedicated to the biochemical changes in the reproductive function of the aging male. Later parenting is very frequent in modern societies. Nevertheless, the quality and the altered patterns of epigenetics/gene expression in aging sperm remain to be disclosed. Thus, the biochemical changes that occur in testis and sperm and that go along with aging will be on the spotlight for the next decades.
Nowadays, there is a huge investment in reproductive healthcare that is mainly applied in assisted reproductive technologies. However, the long-term effects of these treatments and the causes for male infertility are not cautioned. Overall, this book discusses all the major topics of interest for andrology and mainly presents a focus on the biochemistry of andrology without avoiding the debate on the clinical relevance of the discussed topics. This is a fast growing discipline and thus, there is a great need to educate and prepare students, scientists and physicians for the novel challenges. As scientists working in the field, we felt that most books focused on Andrology lack a strong biochemical view on the topics. As biochemists working in the field for more than a decade, we gathered our team and prepared a book that discusses a large spectrum of topics with high relevance for andrologists all over the world. This book will be valuable for all those working on andrology that aim to understand the magnificent biochemical control of the male reproductive health. Our team had great pleasure preparing this book and we are sure that it will be very useful.
List of Contributors
Introduction
Marco G. Alves1,*,Pedro F. Oliveira2,3,*Abstract
Andrology has emerged since the 1950’s, when gynecologists started to consistently refer to this word. However, in 1891, there was already an editorial in JAMA suggesting that andrology could evolve to become an important discipline. It was proposed that, as gynecology is a discipline that is focused on the study of genito-urinary female system, andrology could emerge as the discipline focused on the genito-urinary system of males. For many years, this issue was disregarded and there was a long period until the first societies of andrology appeared and establish it in a definitive way. This historical affirmation of andrology as a discipline will be briefly presented, together with a critical view on some aspects that are still a matter of controversy. Reproductive science is a growing discipline that needs economic support from health care systems, institutions responsible for funding research, and training centers. There was never a greater need for trained and well-prepared scientists and physicians to study human reproductive health. Most countries, developed and developing, are witnessing unprecedented rates of people seeking for assisted reproductive technologies. Decreased sperm quality and male reproductive complications are factors that unquestionably contribute to the observed decline in nativity rates. On the other hand, even though females have various contraceptive methods available, men are still limited. This could be improved if more knowledge on sperm formation, maturation and overall testicular physiology arises. In this introductory chapter, we will discuss some challenges for the upcoming years in the field of Andrology.
In 1891, a JAMA editorial referred to andrology as a possible specialty with difficulties in being established, since it was causing some controversy among the genito-urinary surgeons [1]. Nevertheless, it is usually considered that andrology was firstly used with authority by the gynecologist Dr. Harald Siebke from the University of Bonn in 1957 [2]. Although there was great controversy among some physicians, the first step to establish andrology as a valid specialty was taken when the Congress of American Physicians and Surgeons formed the section of Andrology. This decision was welcomed by many researchers and physicians. Andrology established itself as a growing field of research with potential to rapidly develop into a mandatory discipline to evaluate men’s quality of life and health. After this, it developed within the field of dermatovenereology and gained great interest from several important researchers. Validation also came from the German Society for the Study of Fertility and Sterility that acknowledged andrology as part of its activities in 1958. Though, it was only in 1970 that an international committee of andrology was founded in Barcelona. Later, several other associations of andrology were formed, including the Nordic Association of Andrology (1973), the American Association of Andrology (1974), the German Society of Andrology (1975), and the American Society of Andrology (1976). This ended up with the formation of the International Society of Andrology in 1981. The need for a high level of formation highlighted that the training centers should be grouped and thus, in 1992, the European Academy of Andrology was formed to gather and establish the guidelines for the training in andrology at a European level. Then, all the most relevant societies involved in reproductive medicine recognized and gave attention to this specialty, including the European Society of Human Reproduction and Embryology (ESHRE). This resulted in a rapid development of the discipline in the last three decades [3]. Nevertheless, even today, there are few formal board-certified training programs in andrology. There is a parallelism between andrology and gynecology taking in consideration that the latter is dedicated to the study of the genito-urinary system of females, and the former of males. This is of particular relevance because male’s fertility and the study of their reproductive system has always been overlooked when compared with females, in such a way that the term “diseases of women” is used by several physicians to summarize their specialty, while the same is rare in the case of men. Even today, the field of andrology is not widely recognized by non-experts as a clinical discipline or a research field of great interest like others such as gynecology or urology. Thus, public consciousness of the existence and relevance of this discipline is also mandatory. In summary, andrology is a young interdisciplinary specialty that deals with the male, particularly with the physiology and pathophysiology of male reproductive functions and fertility. We may go further and state that the main focus of andrology is to provide a diagnosis and treatment to males with fertility disturbances.
Andrology evolved as a branch of science that deals with male reproduction and its disorders, including erectile dysfunction, infertility and sexual development. In 1969, the first journal “Andrologie” appeared and gave visibility to this emerging field of research. A few years later, the “Andrologia” journal further contributed to the internationalization of the topic. Initially, the works published were mostly focused on the analysis of the ejaculate, particularly sperm morphology. The clinicians, veterinarians and biochemists started to publish important information on the characterization of sperm and the molecular mechanisms responsible for male fertility. Limited analytical methods, at the time, hampered the initial findings, but enormous progress was made in the first years of those journals. With the advent of molecular biology techniques, omics technologies and hormonal knowledge, andrology entered in a new era of findings. We never had so much information and ways to study testicular physiology, hormonal network, sperm physiology, testicular disorders, and the genetics of the individuals, as we have nowadays. It is also important to highlight that andrology emerged as a discipline that is forced to cooperate with others, including urology, dermatology or endocrinology and thus, it relies on a multidisciplinary work. In addition, concerning the fertility of couples and the treatment of childless couples, it is pivotal that andrologists and gynecologists cooperate and work together to solve the problems beyond the use of assisted reproductive technologies. The diagnosis and therapies of couples would greatly benefit from that. Family planning is also another important matter that benefits from the joint work of both specialties. Nowadays, there are several physicians engaged in andrology and thus, universities and research groups focused on this discipline have highly increased in the last decades. In addition, there is a high number of scientists, besides medical doctors, such as biochemists, veterinaries or biologists that focus their research interest in andrology. Training of highly qualified people is still a major need, as well as support from funding agencies to explore new methods of examination and fundamental research in this field. Basic scientists have also greatly contributed to the exponential growth of this discipline, particularly those with strong formation on biochemistry, biology, pharmacology, genetics and molecular biology. This multidisciplinary approach has allowed a rapid advancement in the understanding of the physiology and biochemical events involved in male reproduction, from the hormonal regulation to the genetic mechanisms responsible for those processes [4, 5]. Nevertheless, the translational gap between basic science and clinical practice still hampers some effective developments that could be useful to improve male reproductive health. We are witnessing an unprecedented need for scientists working on reproductive science. In fact, most developed countries present high rates of induced abortion. Notably, the oral contraceptive method was introduced in 1960, and some authors alerted to the fact that the fundamental biochemical research that served as the basis for this major step in reproductive medicine, was available since the 1920s and 1930s [6]. This clearly highlights that there is a need for a major involvement of basic researchers on translational andrology and andrologists with interest on fundamental research. To encourage both, more funds should be allocated to these areas. In addition, pharmaceutical industry should be more engaged to develop novel strategies to improve the health care of males. The general public and policymakers must be more aware of the need for andrology as an essential discipline, and special funds must be allocated to actively seek for the improvement of men’s health, including the treatment of disorders (i.e. infertility, erectile dysfunction, prostate cancer) and the development of a male contraceptive. In addition, there are several societal challenges that should be considered, including the fact that men are becoming fathers later in life. As the age of father increases, there is a higher risk for miscarriage or disease in the offspring. Another important issue to be considered is the epigenetic information that can be passed along generations [7].
How genetic and environmental factors limit sperm quality and overall male reproductive health should be on spotlight for the next decades. It is of paramount importance to understand those mechanisms and to identify targets to either counteract deleterious effects or to develop a male contraceptive. The pandemic incidence of metabolic diseases and wrong dietary habits are also key factors that are now gaining momentum concerning their effect on the male reproductive health. Thus, physicians, biochemists and basic researchers should also join their efforts to study the mechanisms of action by which those factors may limit male’s fertility.
There is a need for a holistic view of andrology, as an interdisciplinary medical specialty that may evolve even further with a biochemical approach. In addition, the initial problems of the discipline, such as erectile dysfunction or ejaculatory dysfunctions, developed to more dramatic issues in the worldwide trends of ageing male, including neoplastic diseases of the prostate and testis. We may postulate that andrology is still a field with a great margin of progress with the implementation of a multidisciplinary approach. This book aims to contribute to further understand this field of research from a biochemical point of view.
CONFLICT OF INTEREST
The authors confirm that they have no conflict of interest to declare for this publication.
ACKNOWLEDGEMENTS
Declared none.
REFERENCES
[1]Editorial. Andrology as a specialty. JAMA 1891; 17: 691.[2]Schirren C. Andrology. Origin and development of a special discipline in medicine. Reflection and view in the future. Andrologia 1985; 17(2): 117-25. [http://dx.doi.org/10.1111/j.1439-0272.1985.tb00970.x] [PMID: 3890616][3]Prins GS, Bremner W. Andrology in the 20th century: a commentary on our progress during the past 25 years. J Androl 2004; 25(4): 435-40. [http://dx.doi.org/10.1002/j.1939-4640.2004.tb02812.x] [PMID: 15223830][4]Amory JK, Bremner WJ. Regulation of testicular function in men: implications for male hormonal contraceptive development. J Steroid Biochem Mol Biol 2003; 85(2-5): 357-61. [http://dx.doi.org/10.1016/S0960-0760(03)00205-X] [PMID: 12943722][5]Matzuk MM, Lamb DJ. Genetic dissection of mammalian fertility pathways. Nat Cell Biol 2002; 4 (Suppl.): s41-9. [http://dx.doi.org/10.1038/ncb-nm-fertilityS41] [PMID: 12479614][6]Aitken RJ. Age, the environment and our reproductive future: bonking baby boomers and the future of sex. Reproduction 2013; 147(2): S1-S11. [http://dx.doi.org/10.1530/REP-13-0399] [PMID: 24194569][7]Schagdarsurengin U, Steger K. Epigenetics in male reproduction: effect of paternal diet on sperm quality and offspring health. Nat Rev Urol 2016; 13(10): 584-95. [http://dx.doi.org/10.1038/nrurol.2016.157] [PMID: 27578043]Testis Physiology
Bruno P. Moreira1,2,João P. Monteiro2,Maria J. Meneses2,3,*Abstract
In multicellular organisms, and particularly in mammals, both gonadal and germ cell development are essential for the transmission of genetic information to the next generations. The testes are paired ovoid organs located inside the scrotum but outside the abdominal cavity. They have two major functions: spermatogenesis and steroidogenesis. The former corresponds to the production of male gametes, spermatozoa; the latter, to the production of hormones that will influence spermatogenesis and consequently male reproductive function and health. The male and female reproductive organs have the same precursor tissues. Initially, the embryo has a bipotential gonad which may have a testicular or ovarian fate. Accordingly, Müllerian ducts form the uterus and fallopian tubes in females and Wolffian ducts form the epididymis, vas deferens, and ejaculatory duct in males. On the other hand, male sex determination is triggered by sex-determining region Y (SRY), which is located on the Y chromosome and works as a master regulator, initiating SOX9 expression. The latter causes urogenital development, a highly complex process, through a complex cascade of transcription factors and signaling events. These will promote testis differentiation and ultimately the production of hormones that will lead to male development and testicular function during adulthood. In this chapter, we will provide a brief overview of the testicular anatomy and functional organization, as well as its embryonic development.
Introduction
Testes are the primary organs of the male reproductive system. They are central in the production of sperm and responsible for the synthesis of male sex hormones, fundamental to the normal development of male internal and external genitalia. The secondary organs of the male reproductive system support the testes in these tasks. Accessory glands produce secretions that constitute the semen. Other accessory structures support and nourish the developing germ cells. In addition, a network of ducts is responsible for storing and the transport of sperm to the female reproductive tract, where fertilization occurs.
Sexual and asexual reproduction are the two basic processes through which organisms reproduce. In the first, a male and a female are needed and each one of them equally contributes to the formation of the new individual. The egg and the spermatozoon will form a zygote, which contains genetic information from both progenitors. However, and unlike other processes of embryonic development, sex determination is a poorly conserved event among the different species, ranging from being controlled by environmental factors to being genetically determined. In mammals, sex determination is genetically determined at the time of conception, with the formation of either an XX embryo or an XY embryo. This process depends on the chromosome acquired from the father, X or Y chromosome, since the one acquired from the mother is always an X chromosome. Male and female reproductive systems are quite different and thus, evolve independently although sharing a common origin. Bipotential gonad can differentiate into testes or ovaries, depending on the stimuli received. In the male, the presence of the testis determining factor shifts the bipotential gonad into a testicular fate, leading to the development of the testes. This complex process is tightly controlled and involves the action of several different signaling molecules and transcription factors. In this chapter, we present the most relevant aspects of the male reproductive system anatomy. We also discuss the development and differentiation of the testes, from the early stages at the time of conception until they are fully developed.
ANATOMY OF THE MALE REPRODUCTIVE TRACT
The reproductive system is not essential for the survival of the individual; it is, however, required for the survival of the species. It is through the reproductive system that new individuals are born; the species are constantly repopulated and the genetic code is transmitted over generations. In humans, the sexual reproduction is the method used, which has several advantages, namely at the level of variability induced by the combination of progenitors genes. This variability ensures the evolution of the species throughout time.
The reproductive system has some unique features. Unlike any other body systems, it is not fully functional at the time of birth, and it requires the action of sex hormones around the time of puberty to be fully active and ready to perform its purpose. In addition, the gender differences between the male and female reproductive system are clearly observed, a fact that does not occur in the other body systems [1].
Structure of the Male Reproductive System
The male reproductive system has different structures that can be divided in primary and secondary sex organs. In males, the primary sex organs, also known as gonads, are the testes. They are responsible for the production of spermatozoa and the secretion of sex hormones. The secretion of sex hormones is then responsible for the development of secondary sex organs. Surrounding the testes is the scrotum, an outpouching of the abdominal wall that protects the testes. The secondary sex organs are structures responsible for the nourishment and storage or transport of the spermatozoa to the exterior or into the female reproductive tract. One of the organs responsible for this transport is the penis. The penis is the male organ used in sexual intercourse and can be divided into three structures: the root linked to the abdominal wall, the body of the penis that corresponds to the major portion of this organ and the glans, also referred as the head of the penis [2]. There are other secondary sex organs, such as the epididymis, vas deferens, ejaculatory ducts and urethra responsible for the storage, maturation and transport of the spermatozoa and others responsible for the secretion of fluids that are part of the ejaculate, such as seminal vesicles, prostate gland and bulbourethral glands (Fig. 2.1). The sex hormones are also responsible for the development of the secondary sex characteristics, that appear during puberty, such as body hair, deep voice and development of the Adam's apple (see Chapter 6) [3].
Testes
The testes are the male gonads, paired ovoid organs that are responsible for the production of spermatozoa and sex hormones. They are suspended in the scrotum by the spermatic cords. Each one is about 4-5 cm long and 2.5 cm in diameter and weighs between 14-18 g in humans [4]. Both testes are covered by two tunics. The outer tunica is the tunica vaginalis and their visceral layer covers the surface of each testis, except where the testis attaches to the epididymis and spermatic cord. This tunica is a thin closed peritoneal sac that has origin on the peritoneum during the descent of the testes. The parietal layer of the tunica vaginalis covers more tissue than the previous one, extending superiorly onto the distal part of the spermatic cord. The separation between the visceral and parietal layers is filled with fluid, allowing the movement of the testis in the scrotum [1, 3, 5].
Fig. (2.1)) Schematic representation of a human testis. The testis is coated by tunica albuginea, and divided in lobules. The seminiferous tubules are located inside these lobules, highly coiled and organized. The rete testis is responsible for transporting the spermatozoa from the seminiferous tubules into the efferent ducts. From this point forward, spermatozoa enter the epididymis where they go through several processes until they are ready to leave the male reproductive tract into the female reproductive tract where they are fully maturated.The testes have also a tough fibrous outer membrane called tunica albuginea. This tunica has characteristic extensions that move to the inside of each testis dividing it into testicular lobules. Each of these lobules contains long and highly coiled seminiferous tubules that are nearly 80 cm long, if uncoiled in humans. It is inside these tubules, considered the functional unit of the testis, that spermatogenesis occurs (Fig. 2.1). Here, spermatozoa are produced at a rate of about thousands per second. Sertoli cells, which function as a physical support to germ cells and nourish their development into sperm, form the seminiferous tubules playing a key role in this event. Between these numerous tubules is the interstitial space, home to the Leydig cells. As testes are an organ with such important and critical functions in male reproduction, it is expected that they are highly irrigated. This occurs, and the major supplier of blood are the testicular arteries, which in turn derivate from the abdominal aorta. The testicular veins are responsible for the drain of the testes. The right testicular vein enters directly into the inferior vena cava, while the left testicular vein enters into the left renal vein [6, 7]. The irrigation occurs in a similar way in the interstitial space.
Scrotum
Scrotum is the major protector of the testes, being located immediately behind the base of the penis and enclosing the testes. It regulates testes position and helps maintaining constant temperature for spermatogenesis. In this region, the skin is covered by hair and is darker when compared with other parts of the body. Depending on the activity of the scrotal muscles, in a balance between their contraction and relaxation, the external appearance of the scrotum may vary at different time periods. The scrotum is constituted by different types of muscle, specifically the dartos and the cremaster. The first is a layer of connective tissue, mostly consisting of smooth muscle. The tone of this smooth muscle is responsible for the characteristic rugose appearance of the scrotum. The cremaster is a thin layer of skeletal muscle that surrounds the testis and the spermatic cord found between the external and internal layers of spermatic fascia. These two muscles work together in response to temperature shifts moving the testes closer or farther from the abdominal cavity and adjusting the heat loss [1, 3]. This control is crucial due to the importance of temperature to spermatogenesis. The normal temperature of the testes is about 35ºC, oscillating 3-4ºC degrees below normal body temperature due to the contraction or relaxation of these two muscles.
A scrotal septum divides the scrotum into two compartments. This division is pivotal to individualize the testes so that in case of an infection the affected testis can be removed without interfering with the normal function of the other testis. Another characteristic associated with the testes is the fact that the left one is suspended lower and has a bigger size when compared with the right counterpart.
The arteries responsible for the blood supply are the anterior and posterior scrotal artery and the testicular artery. The testicular vein is responsible for the venous drainage. The scrotal nerves are principally sensory, including the pudendal nerves and the posterior cutaneous nerves of the thigh [5, 6].
FUNCTIONAL ORGANIZATION OF THE TESTES
The testes are responsible for the production of the male gametes and male sex hormones, which are processes known as spermatogenesis and steroidogenesis respectively. These two distinct processes take place in two morphologically and functionally different compartments: the seminiferous tubular compartment and the interstitial compartment. Despite being anatomically and functionally different, these two compartments function closely to one another and they are both required in order to achieve the correct parameters in sperm quality and production.
Tubular Compartment
The tubular compartment is composed by the seminiferous tubules. It is responsible for 60-80% of the testicular volume and contains the germ cells, Sertoli cells and the peritubular myoid cells (PMCs). Sertoli and germ cells are organized in a highly-polarized system in order to efficiently support the spermatogenesis. Adjacent Sertoli cells form tight junctions with each other, providing an immunoprivileged microenvironment suitable to the development of germ cells. The tight junctions formed between adjacent Sertoli cells establish the blood-testis barrier (BTB), responsible for controlling the movement of nutrients in the seminiferous tubule [8]. However, there are other types of cooperation used between adjacent Sertoli cells to strengthen the BTB, such as ectoplasmic specializations and desmosomes [9, 10]. This barrier has the function of “gate”, preventing solutes and large molecules from reaching the germ cells and the function of “fence”, restricting the movement of proteins and lipids generating cell polarity. BTB also divides the seminiferous tubules in basal and apical compartments where spermatogonial stem cells and spermatogonia are present in the basal compartment whereas spermatocytes reside in the apical compartment, showing distinct polarity depending on their location [11, 12]. In addition, Sertoli cell nuclei and Golgi complexes are also found in the basal compartment where early phagosomes and early processes involved in the development of the spermatids are all confined to the apical compartment. However, the most obvious form of cell polarity present in the testis is shown in the development of spermatids, where the heads of spermatids point towards the basal compartment while the tails point towards the apical compartment [13, 14].
The testis is divided by a septum of connective tissue into about 250-300 lobules, each one containing 1-3 convoluted seminiferous tubules. Generally, there are about 600 seminiferous tubules present in the human testis and each one has an average length of 30-80 centimeters which varies according to whether they are uncoiled. The total length of the seminiferous tubules is, on average, 300 meters per testis and 600 meters per man [15, 16].
Interstitial Compartment
The seminiferous tubules are surrounded by PMCs, which have several functions. In the male, three or four layers of PMCs surround the seminiferous tubules while in mice, a single layer of PMCs is present [17]. In adult testis, PMCs mediate the contraction of the seminiferous tubules and, during development of the testis and adulthood, they work together with Sertoli cells to deposit the basement membrane (composed by laminin, collagen IV and fibronectin) that surrounds the seminiferous tubules. This is a critical interaction to ensure a correct spermatogenesis and architecture of the seminiferous tubule [18-21]. There are other functions attributed to PMCs. In rats, these cells where shown to be an important part of the barrier function, restricting the entry of substances into the seminiferous tubules [22].
The interstitial compartment is located between the seminiferous tubules and is filled with Leydig cells, the principal component of this compartment. In fact, there are two populations of Leydig cells, the fetal Leydig cells (FLCs) and the adult Leydig cells (ALCs) [23]. After birth, FLCs start degenerating and it is not yet clear if they give rise to ALCs. On the other hand, ALCs derive from Leydig stem cells, capable of self-renewal. These cells develop into Leydig progenitor cells, which express a number of factors such as luteinizing hormone receptors (LHR) and 3β-hydroxysteroid dehydrogenase (3βHSD) [24]. Further differentiation occurs into adult cells that no longer proliferate. In the presence of LH, these cells produce testosterone that is fundamental for the establishment and maintenance of the secondary sex characteristics and the continuation of spermatogenesis [25, 26]. There are other cell types present in the interstitium, namely the immune cells (macrophages, T-cells, dendritic cells), where they respond according to the stimuli received [27]. From this group, macrophages are the most abundant in the interstitium, corresponding to approximately 25% of the interstitial cells present in the adult rodent testis [28]. Several studies have shown that there is cross-talk between immune cells and spermatogonia, where the number of spermatogonia declines after ablation of macrophages [29]. These also establish cell junction with Leydig cells, to facilitate an eventual response. Besides this, 25-hydroxycholesterol secreted by macrophages is used by ALCs to produce testosterone. Furthermore, cytokines produced by macrophages after responding to a stimulus can be used to modulate Leydig cell production of male sex hormones [30-32].
PRENATAL DEVELOPMENT OF TESTES
Human reproduction requires the intervention of individuals from two genders, each of them being characterized by a distinct number of anatomic and genetic characteristics. The phenotype of the sex is dictated by the type of gonad that develops in the embryo. This event depends on the expression of the sex-determining region Y (SRY), located in the male-specific region of the Y chromosome that encodes the sex-determining region Y protein (SRY) [33]. The presence of SRY leads to a chain of events that culminates in the development of the testes (instead of ovaries) from the gonadal primordium [34]. The early development of the embryo is similar in both sexes. Only in later stages, the difference is manifested by the formation of testes in men or ovaries in women, according to the stimuli and genetic information received by the undifferentiated gonads [35]. Sex differentiation, characterized by the development of secondary sexual characteristics (such as external genitalia), begins when gonads start producing sex-specific hormones [36]. In early stages, the embryonic gonad has the unique ability to differentiate into one of two fully functional distinct organs, thus being called bipotential.
The pathway that leads to the formation of one or another relies on a tightly regulated network of cell signaling molecules and transcription factors [37]. Thus the phenotype of sexual differentiation is the result of several consecutive events: the appearance of the bipotential gonad, gonad development, production of sex hormones and ultimately sex differentiation [38].
Sex Determination
Sex determination refers to the key point where the bipotential gonad, particularly the somatic cells, start to differentiate either as Sertoli cells or granulosa cells during fetal life. Despite the difficulty of identifying genes involved in sex determination, some studies have made clear progresses in that field. Nowadays, several genes are already known to play a major role in that process [39, 40]. In humans, the presence of the Y chromosome, more specifically of the SRY gene, acts as the major male sex determinant, shifting the bipotential gonad towards a testicular fate. Several studies have shown that mice or humans carrying point mutations or deletions in this gene display an XY female sex-reversed phenotype. Moreover, XX mice where a 14-kb genomic fragment of this gene was introduced, developed sex reversal, showing that SRY is required to initiate male development [41-43]. One of the most important events in testis development is the differentiation of pre-Sertoli cells. This event is induced by the expression of SRY, which is strictly controlled by several genes. For example, embryos homozygous for the boy/girl (byg/byg) mutation show sex reversal due to a decrease in the expression of SRY. This mutation is an A to T transversion, which causes the appearance of a premature stop codon in the gene encoding for the mitogen-activated protein kinase kinase kinase 4 (MAP3K4), making this kinase nonfunctional [44, 45]. The decrease in SRY expression is partially controlled by growth arrest and DNA damage-inducible gene family 45 (GADD45γ), a protein involved in several important tasks, such as regulation of growth and apoptosis, cell cycle control and senescence [46]. Evidence towards the pattern of GADD45γ expression shows that it is quite similar to the expression pattern of SRY suggesting that they work together in sex determination. This pattern can explain why, in mice deficient for GADD45γ, sex reversal is observed due to a decrease in SRY expression. However, in these same mice, overexpression of MAP3K4 can prevent sex reversal, showing that MAP3K4 is also imperative for sex determination [47].
Expression of SRY starts in the central region of the genital ridges and occurs in a very strict time window. In mouse genital ridges, expression of SRY occurs around the 11th day of embryonic development, peaks on the day 11.5 of embryonic development and stops around the 12.5 day of embryonic development [48-51]. SRY binds to steroidogenic factor 1 (SF1), forming a complex that triggers the expression of SRY-box 9 (SOX9) in Sertoli cells precursors. SOX9 expression is upregulated when expression of SRY is already at its highest, around the day 11.5 of embryonic development. After a short period of time, SOX9 starts downregulating SRY expression, leading to its interruption [52-54]. After being expressed, SOX9 recruits and activates several proteins that play a key role in testis development, such as prostaglandin D synthase (PGDS), an enzyme that catalyzes the isomerization of PGH2. This event is particularly important in order to create a feedforward loop with SOX9, keeping the level of SOX9 high enough to ensure the activation of other crucial genes such as fibroblast growth factor 9 (FGF9) and SF1 [55, 56].
SRY and SOX9 are members of the SOX family of developmental transcription factors that contain an amino acid motif commonly known as high mobility group (HMG-box) domain. This HMG-box is a DNA binding domain and is highly conserved among eukaryotes. SOX9 (and other transcription factors that are a part of this family) bind to the minor groove in DNA [57, 58]. The SOX transcription factors were first identified based on the molecular conservation of the 79 amino acid HMG DNA binding domain present in the SRY and their ability to bind to DNA in the minor groove region [59]. This highly conserved domain allows SRY and the SOX family of proteins to bind to the DNA sequence A/TA/TCAAA/TG with high affinity [60]. Several mutations found in human patients showing sex reversal (male to female) have been associated with the inability of SRY to bind to DNA [61, 62]. Additionally, SRY mutations in the nuclear localization signal present in the N-terminal end of the HMG-box can cause a reduction of nuclear importation, which can be the explanation for some cases of sex reversal [63, 64]. As referred before, one of these two genes individually can promote testicular differentiation. Several studies have shown that a SOX9 transgene can promote testicular differentiation instead of SRY [65-67]. XX individuals who possess an extra copy of SOX9 develop as male even though they have no SRY [68]. Those results suggest that the primordial function of SRY in male development is the upregulation of other transcription factors, such as SOX9. Other possibility is linked with the fact that the SOX9 transgene could activate other SRY target genes required to induce male development making SRY meaningless for the testicular development. There are several genes involved in the pathway following the activation of SRY, however it is not yet clear if the activation of SRY is strictly required for the activation of these genes or if SRY is only required for the upregulation of SOX9 [69].
Other genes are known to be involved in early stages of sex determination, including NR5A1 (encoding SF1), GATA4 (GATA-binding protein 4) and NR0B1 (encoding DAX1, dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1) [70-72]. The presence or the absence of one of these genes results in significant changes in the sexual differentiation of an individual. For example, mice where SF1 is not present manifest a phenotype where one or both gonads are not formed, due to an improper gonadogenesis [73]. SF1 is linked with the transcriptional regulation of genes that encode several hormones involved in the hypothalamic-pituitary- gonadal axis [74]. SF1 is also essential to maintain the bipotential gonad and while its expression decays in the XX genital ridge of mouse embryos, in XY mouse embryos SF1 stays expressed. SF1 expression is essential in both Sertoli and Leydig cells. In Sertoli cells, SF1 works together with SOX9 and increases the level of transcription of Anti-Müllerian hormone, essential for the degeneration of the Müllerian ducts. In Leydig cells, SF1 activates genes that will result in the production of testosterone [75, 76].
Development of the Bipotential Gonad
Formation of the Genital Ridge
In human embryos, the gonadal precursors appear on the 32nd day of embryonic development while in mice they appear earlier, on the day 10.5 of embryonic development. Unless the differentiation process starts, it is not possible to observe any morphological differences between male and female genital ridges [77]. Unlike other organs, like the lung primordium that can only differentiate into lung, the gonadal primordium can either differentiate into testes or ovaries. Before differentiation occurs, the mammalian gonad has an undifferentiated or bipotential stage and at this point it has neither male or female characteristics [78]. On the 4th week of embryonic development, the urogenital ridges appear as two raised edges, parallel to the midline of the body, in the posterior abdominal wall. This urogenital ridge arises from the intermediate mesoderm that generates the urogenital system (the kidneys, the gonads and their ducts) [79]. It develops sequentially into three sets of tubular nephric structures: the pronephros, the mesonephros and the metanephros. The pronephric tubules and part of the pronephric ducts degenerate, while the more caudal portions persist and function as an embryonic kidney, precursor of the adult kidney. The remnants of this structure form the Wolffian duct. While this occurs, the middle portion of the nephric duct induces the formation of the mesonephros that, similarly to the pronephros, has a brief existence. However, it has important functions such as being the source of hematopoietic stem cells and in male mammals, some of these tubes remain and differentiate into the adult vas deferens and efferent ducts. The metanephros is the result of several interactions between epithelial and mesenchymal components of the intermediate mesoderm [80-82]. Parallel to the development of the kidney, the gonadal primordium appears in the intermediate mesoderm around the 4th week of embryonic development and remains sexually undifferentiated until the 7th week of embryonic development. The more lateral portions of the gonadal primordium form the mesonephric ridge, while the medial portion of the gonadal primordium is constituted by coelomic epithelium and forms the genital ridge. During the indifferent stage, the coelomic epithelium in the genital ridges goes through proliferation and thickening, expanding into the coelomic cavity. This proliferation causes the invasion of the subjacent mesenchymal tissue [83]. At this point, primitive sex cords are formed and remain connected to the surface epithelium. An important player appears in this moment, laminin. The production of laminin, a glycoprotein that is the major structural component in the basal lamina of most cells, increases in the genital ridge, as the coelomic cells proliferate [84]. This glycoprotein seems to be essential for the ensuing germ cell colonization. At this point, the cell types of the genital ridge are equally capable of originating testes or ovaries. Here, SRY plays a key role triggering the male development leading to the development of the testis and male reproductive tract. Without interference from the SRY gene and the interference of other genes such as RSPO1 and the WNT gene family, the pathway of events will lead to the formation of ovaries [85]. It is important to bear in mind that the development of a testis or an ovary involves different molecular regulation pathways that antagonize each other [86-88].
Testes Differentiation
The expression of a single gene, SRY, located on the Y chromosome, is the primary responsible for male determination [89]. It occurs in the gonadal blastema, with the coelomic-derived somatic cells, which are called supporting cells, being responsible for it [49]. Several studies have shown that a large part of this group of cells is originated from the coelomic epithelium situated above the genital ridges. However, the mechanism by which these cells enter the gonad is not clear [90]. It is known though that SRY is activated in the genital ridges only after these cells fully colonize the gonad, showing that their migration cannot be a consequence of the SRY expression [49]. The latter begins in the middle of the genital ridges with consecutive expansion to the anterior and posterior poles. After this, SRY expression stops in the anterior and central regions, being only expressed in the posterior region until it completely disappears from the genital ridges [91].
One of the major events in testis differentiation is the formation of the testis cords, which transforms the genital ridge into a highly-structured organ. Testis cords are specialized tubular structures and its formation is fundamental to the structure and function of the testis. They are originated from the sex cords and after sexual differentiation, particularly SRY expression, they differentiate into testis cords. Once this formation starts, the fate of the bipotential gonad is settled and the male sex determining pathway has overcome competitive signals trying to push the bipotential gonad into an ovarian fate [92
