The search for gonadal proteins that regulate pituitary FSH led to the isolation of inhibins and activins. As members of the transforming growth factor beta (TGFbeta) superfamily of growth and differentiation factors, these proteins have been shown subsequently to affect a range of tissues and systems beyond their role in reproduction. Studies on the expression and synthesis of activins in the male reproductive tract have localized these proteins in the testis, epididymis and prostate. In general, activins regulate cell proliferation and, consequently, the expression and localization of activin subunit mRNAs and proteins within these organs must be discrete. Activin ligand bioactivity is dependent on the presence of the appropriate receptors and signalling systems, but activin ligand formation or access to receptors is regulated by the formation of inhibins or by activin-binding proteins such as follistatin. This review examines the evidence that the capacity to synthesize activins and to regulate activin bioactivity resides in the cells of the male reproductive tract. It is concluded that activins exert their effects through local (autocrine or paracrine) mechanisms, rather than through endocrine systems. The interplay between the inhibins or follistatins provides a degree of regulation of activin bioactivity before ligand signalling events. The challenge for the future is to determine whether there is any difference between the action of individual activin ligands or whether these proteins are functionally redundant, indicating that compensatory mechanisms are essential for male reproductive tract function.
Placental tissues from humans, rodents and farm animals contain leptin and its receptor. Leptin produced by the human placenta has the same size, charge and immunoreactivity as leptin produced by adipose tissue. However, the expression of human placental leptin appears to be regulated by a placenta-specific upstream enhancer. In this review the occurrence of leptin and its receptor in a range of species and placental types is described, and its significance during pregnancy discussed. Placental leptin contributes to the increase in maternal circulating concentrations of leptin during late pregnancy when it is likely to have an endocrine role in regulating maternal energy balance. Placental leptin may have angiogenic and immunomodulatory activities, which affect the placenta in an autocrine or paracrine manner. It also appears to affect fetal growth and development by binding to leptin receptors present in fetal organs.
The receptor system and the molecular mechanisms by which inhibin acts on its target cells are poorly understood, in contrast to the situation for the structurally related molecule, activin. On the basis of evidence that the biological action of inhibin in a number of systems resembles that of an activin antagonist, it has been contended that inhibin operates by competition for the activin receptor rather than through a specific inhibin receptor. However, mounting evidence indicates that inhibin also interacts with high affinity and specificity with membrane-binding proteins that are likely to be the putative inhibin receptor.
The uterine immune axis holds the key to solving major problems in female reproductive health, including infertility, many pathologies of pregnancy, and sexually transmitted disease. The molecular determinants of tolerance and immunity in the reproductive tract are now being identified, and the governing principles are similar to those in other mucosal tissues. Cytokines are implicated as pivotal regulators at important 'decision-making' points in each phase of the induction and elicitation of a response. Indeed, the flexibility to deal appropriately with antigens as disparate as infectious micro-organisms, spermatozoa and the conceptus is likely to be attributable to the sophistication of the cytokine network in driving immune deviation. A better understanding of the factors controlling the development of immune activity in the uterus, particularly the significance of the inductive cytokine environment in determining the destiny of T-lymphocyte responses, will assist the rational design of new therapeutic strategies to treat immune-based reproductive disorders.
Fats in the diet can influence reproduction positively by altering both ovarian follicle and corpus luteum function via improved energy status and by increasing precursors for the synthesis of reproductive hormones such as steroids and prostaglandins. Dietary fatty acids of the n-3 family reduce ovarian and endometrial synthesis of prostaglandin F2alpha, decrease ovulation rate in rats and delay parturition in sheep and humans. Polyunsaturated fatty acids such as linoleic, linolenic, eicosapentaenoic and docosahexaenoic acids may inhibit prostaglandin F2alpha synthesis through mechanisms such as decreased availability of its precursor arachidonic acid, an increased competition by these fatty acids with arachidonic acid for binding to prostaglandin H synthase, and inhibition of prostaglandin H synthase synthesis and activity. It is not known whether polyunsaturated fatty acids regulate expression of candidate genes such as phospholipase A2 and prostaglandin H synthase via activation of nuclear transcription factors such as peroxisome proliferator-activated receptors. Manipulation of the fatty acid profile of the diet can be used potentially to amplify suppression of uterine synthesis of prostaglandin F2alpha during early pregnancy in cattle, which may contribute to a reduction in embryonic mortality. Feeding fats and targeting of fatty acids to reproductive tissues may be a potential strategy to integrate nutrition and reproductive management to improve animal productivity.
Progesterone is synthesized in the peripheral nervous system in glial cells. The functions of progesterone are indicated by the findings that it stimulates neurite outgrowth from dorsal root ganglia sensory neurones in explant cultures, accelerates the maturation of the regenerating axons in cryolesioned sciatic nerve, and enhances the remyelination of regenerated nerve fibres. The formation of myelin sheaths around axons is a sexually dimorphic process, as the sheaths are thicker in female than in male regenerating nerves. The progesterone-induced myelination is probably mediated by progesterone receptors, as it is impaired by mifepristone (RU486), a progesterone antagonist. The stimulation of neurite growth in the peripheral nervous system may be mediated by a progesterone metabolite, 5alpha-tetrahydroprogesterone, through GABA(A) receptors.
Fifty years after the first successful cryopreservation of spermatozoa, the technique is an integral part of the cattle breeding industry but has failed to establish itself commercially in the production of other breeds of domestic livestock. New assessment techniques have shown that the ejaculate consists of a heterogeneous population of cells, which achieve their full fertility potential at different rates within the female tract and thus maximize the chances of a fertile spermatozoon successfully combining with an egg. It is becoming apparent that the freeze-thaw process results in a more homogeneous cell population, which may be functionally compromised. One aspect of sperm function that has been demonstrated to be affected by cryopreservation is the process of capacitation. Chlortetracycline staining has shown that frozen-thawed spermatozoa undergo an accelerated 'capacitation-like' process which has implications for their interaction with the female tract, ability to establish sperm reservoirs in vivo and hence for their life expectancy after insemination. In addition to heterogeneity within the ejaculate, there is increasing evidence for variation between individuals in the success of sperm freezing. Post-thaw sperm survival may be consistently poor for certain individual animals even though pre-freeze parameters appear normal. The mechanisms that may underlie such differences in cryosensitivity remain unclear. A greater role for the use of frozen semen in livestock production can come only from an improvement in the preservation of the functional competence of the cryopreserved spermatozoon after insemination into the female tract.
Intact reproductive development depends on steroids and other endocrine signals. Although recent discoveries have elucidated important steps of sexual differentiation, the intricate mechanisms that regulate the development of steroid-producing tissues remain elusive. In adults, complex feedback mechanisms determine the hypothalamic and pituitary regulation of steroid hormone biosynthesis. Steroidogenic factor 1 (SF-1), an orphan member of the nuclear receptor superfamily of proteins, plays a critical role in development and differentiation of the endocrine and reproductive systems. This review provides an overview of the function of SF-1, its mechanism of action, and a perspective on the interaction of SF-1 with other determinants of sexual differentiation. Whereas SF-1 is essential for the expression of numerous steroidogenic enzymes, its presence may not correlate with steroidogenic function. Moreover, diverse co-regulators modulate the influence of SF-1 on gene transcription. The significance of these interactions is discussed in the context of reproductive development and function. Other orphan or ligand-dependent nuclear proteins may share similar mechanisms.
The defining characteristic of mammals is that females nurse and care for their young; without this, the neonate has no chance to survive. Studies on wild and domestic species show that the neonatal period is the most critical step in the lifetime of a mammal. This review compares three well-studied species (the rabbit, pig and sheep) that differ in their parental strategies and in the problems that neonates have to overcome. As a general trend, mother-young interactions vary according to the maturity of the newborn, and the size of the litter. Neonatal survival relies to a great extent on an environment that is ecologically appropriate for the developmental stage of the neonate, and on optimum interactions with the mother. Adaptive maternal care supposes that the mother provides the basic needs of the neonate: warmth (in pigs and rabbits) or shelter, food, water and immunological protection (via colostrum) and, in some instances, protection from predators and other conspecifics. A major risk facing all neonates, other than the birth process itself, is inadequate colostrum intake owing to delayed suckling or competition with siblings, which leads to starvation hypothermia or even crushing, as has been observed in pigs.
Spermatogonial stem cell transplantation was first reported by Ralph Brinster's laboratory in 1994. It has proven to be a technological breakthrough in the study of both stem cells and Sertoli cell-germ cell interactions. This technique can be used to transfer testicular stem cells successfully from one animal to another of the same species (referred to as syngeneic transplants) and sometimes to an animal of a different species (xenogeneic transplants). This transfer technique, combined with developments in cryopreservation, long-term culture, and the enrichment of stem cell populations makes more significant breakthroughs likely in the near future. Ultimately, the application of spermatogonial stem cell transfer will allow transplantation of cultured stem cells manipulated genetically in vitro to give rise to functional male gametes with an altered genotype. This achievement will have applications in basic science, human medicine, and domestic and wild animal reproduction. Although progress toward this goal has been swift, potentially significant barriers, such as the stable incorporation of genetic material into stem cells and immunological responses to the introduced germ cells, remain to be overcome. This article is a review of the scientific advances made since the initial report of successful transplantation in 1994.
An extracellular matrix that mediates critical steps in fertilization and early development surrounds all vertebrate eggs. In mice and humans, this matrix is known as the zona pellucida and comprises three glycoproteins: ZP1, ZP2 and ZP3. Homologues of these proteins isolated from other vertebrates have conserved protein motifs that may be important for establishing a common fibrillar structure. However, specific but contradictory biological roles have been assigned to individual egg coat proteins based on assays in vitro in a wide range of species. Mouse lines lacking either ZP1 or ZP3 have been established with abnormal or absent zona matrices and varying degrees of infertility to examine zona structure and function in vivo. By crossing mouse lines lacking individual zona proteins with those expressing human homologues, the structural integrity of the zona matrix can be restored. Because mouse and human spermatozoa exhibit order-specific binding to the zona pellucida, mice with 'humanized' chimaeric zonae may provide an experimental system to elucidate the molecular basis of sperm-zona interaction.
Growth hormone (GH) is not classically considered as a reproductive hormone, although a vast literature indicates that it has roles in reproductive function. It is required for sexual differentiation and pubertal maturation and it participates in gonadal steroidogenesis, gametogenesis and ovulation. GH is also required for fetal nutrition and growth during pregnancy and for mammary development and lactation. Although some of these roles reflect the action of GH on the secretion and action of LH and FSH (Chandrashekar and Bartke, 1998), they also reflect direct actions of GH and indirect actions mediated through the local production of insulin-like growth factor I. Moreover, as GH is produced in gonadal and mammary tissues, these actions may reflect local autocrine or paracrine actions of extrapituitary GH, as well as the endocrine actions of pituitary GH. The roles of GH in reproductive function are considered in this review.
Progesterone is synthesized in the peripheral nervous system in glial cells. The functions of progesterone are indicated by the findings that it stimulates neurite outgrowth from dorsal root ganglia sensory neurones in explant cultures, accelerates the maturation of the regenerating axons in cryolesioned sciatic nerve, and enhances the remyelination of regenerated nerve fibres. The formation of myelin sheaths around axone is a sexually dimorphic process, as the sheaths are thicker in female than in male regenerating nerves. The progesterone-induced myelination is probably mediated by progesterone receptors, as it is impaired by mifepristone (RU486), a progesterone antagonist. The stimulation of neurite growth in the peripheral nervous system may be mediated by a progesterone metabolite, 5 alpha-tetrahydroprogesterone, through GABA(A) receptors.
Evidence from mouse mutants indicates that the Kit gene encoding KIT, a receptor present on the oocyte and theca cells, and the Mgf gene encoding KIT LIGAND, the ligand of KIT, are important regulators of oogenesis and folliculogenesis. Recently, in vitro cultures of fetal gonads, of follicles and of oocytes have identified specific targets for the KIT-KIT LIGAND interaction. In fetal gonads, an anti-apoptotic effect of KIT-KIT LIGAND interactions on primordial germ cells, oogonia and oocytes has been demonstrated. In postnatal ovaries, the initiation of follicular growth from the primordial pool and progression beyond the primary follicle stage appear to involve KIT-KIT LIGAND interactions. During early folliculogenesis, KIT together with KIT LIGAND controls oocyte growth and theca cell differentiation, and protects preantral follicles from apoptosis. Formation of an antral cavity requires a functional KIT-KIT LIGAND system. In large antral follicles, the KIT-KIT LIGAND interaction modulates the ability of the oocyte to undergo cytoplasmic maturation and helps to maximize thecal androgen output. Hence, many steps of oogenesis and folliculogenesis appear to be, at least in part, controlled by paracrine interactions between these two proteins.
Monocyte chemoattractant protein 1 (MCP-1) is a member of the chemokine family of cytokines which are involved in leukocyte physiology and trafficking. Interest in the role of inflammatory cells and their cytokine products in luteolysis has been increasing and there is mounting evidence demonstrating that MCP-1 is involved in luteolysis. Cell sources of MCP-1, such as endothelial cells, are abundant in late stage luteal tissue. Increased amounts of mRNA encoding MCP-1 are found after luteolysis in sheep, pigs, cows, rats and women and its up-regulation is associated with an increase in macrophages within the corpus luteum, indicating that MCP-1 may act as an inflammatory mediator during luteal regression. Luteolytic substances (prolactin in rats and prostaglandin F2alpha in ruminants) appear to be involved in increased expression of MCP-1 within the corpus luteum, although it is unclear whether this is a direct or indirect effect. Cytokines produced within the corpus luteum around luteolysis may also be involved in regulating MCP-1 expression. The field of chemokine biology is expanding rapidly and MCP-1, as well as other chemokines yet to be investigated, may prove to be an important link between the hormonal and cellular events within the corpus luteum around the time of luteolysis.
Eutherian mammals have inherited a typical vertebrate immune system, which protects the body against infectious organisms by detecting and destroying foreign biological material. However, with the evolution of longer gestation periods, this protective mechanism became a potential threat to the 'semi-foreign' fetus and so eutherians have developed systems to prevent immune rejection of their developing fetuses. In many species, this is achieved by reducing placental expression of major histocompatibility complex (MHC) genes, the products of which are responsible for most transplantation rejection reactions. Unexpectedly, however, major histocompatibility complex expression is often re-established in the most invasive trophoblast cells. It is not known why transplantation antigen expression in the fetal cells most exposed to the maternal immune system is advantageous. It is possible that such expression aids the process of invasion or exerts an immunoprotective effect on the fetus. It may prove possible to identify the essential steps that all eutherian fetuses take to ensure their survival in the face of potential maternal immune attack by studying the common features of the placental immunology of different species.
This review focuses on the importance of oestrogen and oestrogen receptors in the male reproductive system, with a special interest in the newly discovered role of oestrogen in the regulation of fluid reabsorption in the efferent ductules of the testis. Early work on oestrogen synthesis indicated that Leydig and Sertoli cells were the only important cells in the production of this steroid in the adult testis. However, more recent work has shown that germ cells and spermatozoa also contain aromatase and produce oestrogen. The observation that germ cells synthesize oestrogen contributed to a new hypothesis that oestrogen in the lumen of the male reproductive tract targets the epithelial lining of efferent ductules and the epididymis. The location of nuclear oestrogen receptors in the male reproductive tract has also been investigated and it has been found that oestrogen receptor alpha is more abundant in the efferent ductules of the testis than in any other tissue of the male or female. In all species examined to date, oestrogen receptor alpha has been found to be abundant in the efferent ductules. The structure and function of the efferent ductules are taken into account as these tubules are responsible for the reabsorption of almost 90% of the luminal rete testis fluid. Thus, it was logical to hypothesize that oestrogen receptors play a role in the regulation of fluid reabsorption in efferent ductules. The oestrogen receptor alpha knockout mouse was used to help define this role of the receptor in males. In this animal model, the efferent ductules are altered markedly from a reabsorptive epithelium to a squamous epithelium devoid of lysosomes and endocytotic organelles. Although the separate roles for oestrogens and androgens in the regulation of fluid reabsorption are controversial and remain to be resolved, it is now established that loss of oestrogen receptor function in males interferes with the resorptive function of efferent ductules, a function that is essential for fertility. Future studies will focus on the biochemical and physiological mechanisms involved in the regulation of water and ion movement by oestrogen in the male reproductive tract.
Fertilin is a sperm surface protein with an essential role in fertilization. It is required for the migration of spermatozoa through the oviduct, for binding to the zona pellucida, and for efficient binding to the egg plasma membrane. Fertilin consists of two subunits, fertilin alpha and beta, both of which belong to the metalloprotease-disintegrin protein family (ADAMs). Fertilin alpha and beta are made as larger precursors that are processed proteolytically at different stages of sperm maturation in the testis and epididymis. Fertilin alpha is processed first, most likely by a pro-protein convertase in the secretory pathway of testicular cells. Fertilin beta is processed later, while spermatozoa are in transit through the epididymis. The processing of fertilin beta in the epididymis correlates with the acquisition of fertilization competence in spermatozoa, exposes an epitope that has a role in sperm-egg interactions, and triggers the relocalization of fertilin from the whole sperm head to the posterior head. These findings indicate that the proteolytic processing of fertilin and perhaps also other sperm proteins plays an important role in sperm maturation and activation in the epididymis. Further evaluation of the functional significance of proteolysis for sperm maturation should lead to new and exciting insights into the mechanism of sperm maturation, and may also uncover the cause of certain types of male infertility. The identification of the responsible proteases could provide novel targets for contraceptive drugs.
The search for gonadal proteins that regulate pituitary FSH led to the isolation of inhibins and activins. As members of the transforming growth factor beta (TGF beta) superfamily of growth and differentiation factors, these proteins have been shown subsequently to affect a range of tissues and systems beyond their role in reproduction. Studies on the expression and synthesis of activins in the male reproductive tract have localized these proteins in the testis, epididymis and prostate. In general, activins regulate cell proliferation and, consequently, the expression and localization of activin subunit mRNAs and proteins within these organs must be discrete. Activin ligand bioactivity is dependent on the presence of the appropriate receptors and signalling systems, but activin Ligand formation or access to receptors is regulated by the formation of inhibins or by activin-binding proteins such as follistatin. This review examines the evidence that the capacity to synthesize activins and to regulate activin bioactivity resides in the cells of the male reproductive tract. It is concluded that activins exert their effects through local (autocrine or paracrine) mechanisms, rather than through endocrine systems. The interplay between the inhibins or follistatins provides a degree of regulation of activin bioactivity before ligand signalling events. The challenge for the future is to determine whether there is any difference between the action of individual activin ligands or whether these proteins are functionally redundant, indicating that compensatory mechanisms are essential for male reproductive tract function.
In male mammals, spermatogenesis proceeds for the reproductive lifetime of the animal. The continuation of this process depends upon a pool of spermatogenic stem cells within the testes that undergo asymmetric division to both maintain the stem cell population and give rise to progenitors that will proceed through spermatogenesis to generate mature spermatozoa. Thus, the development of functional spermatozoa may be divided into two distinct stages. The second, the process of spermatogenesis, is dependent upon the first, the successful formation of spermatogenic stem cells. Although spermatogenesis is characterized by marked cellular differentiation, the initial stages of germ line differentiation involve an avoidance of the differentiation signals acting during embryo development. The germ line is set aside early in embryo development and, while the primordial germ cells remain refractory to the differentiation signals affecting the soma, they undergo a number of phenotypic shifts before and after colonizing the genital ridge. Upon colonization of the genital ridge, the somatic tissue of the male genital ridge directs the final differentiation events that result in the formation of spermatogenic stem cells. It is this cell population that provides the basis for the maintenance of spermatogenesis in the adult.
