Reviews of reproduction最新文献

The goldfish, a member of the carp family, is a widely used model for reproductive neuroendocrine studies of economically important fish. The two gonadotrophin (GTH) molecules released from the fish anterior pituitary, GTH-I and GTH-II, are structurally similar to tetrapod FSH and LH, respectively. Gonadotrophin II is the best studied, and in goldfish stimulates gonadal growth and steroidogenesis, ovulation and sperm release. Growth hormone also has gonadotrophic actions in fish which enhance gonadal steroidogenesis. The principal stimulatory and inhibitory systems regulating GTH-II release are the gonadotrophin-releasing hormone (GnRH) and dopamine neurones in the preoptic-hypothalamic region. In goldfish there are two native GnRH forms, salmon GnRH and chicken GnRH-I; both stimulate GTH-II release but use different signal transduction pathways. In contrast to mammals, teleost fish do not have a median eminence and the GTH-II cells are thus directly innervated by neurones producing GnRH, dopamine and other stimulatory neurohormones. For most of these factors, the ability to stimulate GTH-II release varies seasonally. The amino acid neurotransmitter, gamma-aminobutyric acid, has the most prominent stimulatory actions which enhance GnRH release and inhibit dopamine turnover in the hypothalamo-pituitary complex. Neuropeptide Y stimulates GTH-II release by a combined direct action on the gonadotroph and also by enhancing GnRH release. Positive and negative sex steroid feedback mechanisms act concurrently to regulate GTH-II release in adults of both sexes. The principal site of positive feedback is the GTH-II cell where testosterone and oestradiol potentiate GnRH-stimulated GTH-II release. Negative feedback by sex steroids involves activation of inhibitory dopamine neurones, thus maintaining tight control over circulating GTH-II concentrations.

As early as 1887, it was postulated that the mature oocyte possesses all of the elements necessary for embryonic development with the exception of an active division centre, and that the spermatozoon contains such a centre, but lacks the substrate in which to operate. This division centre is called the centrosome. The precise definition of this structure is still a subject for debate. It consists of two centrioles in a perpendicular arrangement and pericentriolar material, and is considered to be responsible for nucleation of microtubules and the formation of the mitotic spindle. There is a paternal pattern of inheritance of the centrosome in humans; thus, human oocytes lack centrioles but the spermatozoa carry two. At gamete fusion the sperm tail is incorporated into the ooplasm, and the centriolar region forms the sperm aster while the sperm head is decondensing; this aster acts to guide the female pronucleus towards the male pronucleus. The centriole duplicates during the pronuclear stage, and at syngamy centrioles are found at opposite poles of the first cleavage. The centrosome has several implications for human infertility. It is possible that immotile or nonprogressively motile spermatozoa may possess centriolar abnormalities or an absence of centrioles. Similarly, antisperm antibodies against centrioles may be responsible for mitotic arrest. One way of solving this problem would be the use of donor centrosomes. To this end, we have assessed the ability of embryos injected with physically separated sperm segments (head only, head and tail separated or isolated tail) to develop normally. Fluorescent in situ hybridization revealed an almost universal mosaicism in these embryos, suggesting that physical disruption of the spermatozoa compromises the ability of the centrosome to function in the zygote. Thus far, centrosome donation with centriole-carrier flagellae obtained by this dissection method does not appear to be feasible.

Although high concentrations of reactive oxygen species (ROS) cause sperm pathology (ATP depletion leading to insufficient axonemal phosphorylation, lipid peroxidation and loss of motility and viability), recent evidence demonstrates that low and controlled concentrations of these ROS play an important role in sperm physiology. Reactive oxygen species, such as the superoxide anion, hydrogen peroxide and nitric oxide, induce sperm hyperactivation, capacitation or the acrosome reaction in vitro. The ROS involved in these processes may vary depending on experimental conditions, but all the evidence converges to describe these events as 'oxidative' or 'redox regulated'. Human sperm capacitation and acrosome reaction are associated with extracellular production of a superoxide anion that is thought to originate from a membrane 'oxidase'. The enzymes responsible for tyrosine phosphorylation-dephosphorylation of sperm proteins are possible targets for ROS since mild oxidative conditions cause increases in protein tyrosine phosphorylation and acrosome reaction. The lipid peroxidation resulting from low concentrations of ROS promotes binding to the zona pellucida and may trigger the release of unesterified fatty acids from the sperm plasma membrane. The fine balance between ROS production and scavenging, as well as the right timing and site for ROS production are of paramount importance for acquisition of fertilizing ability.

Recent studies of the nonclassical HLA-G class I gene provide insight into its function(s) during pregnancy. The HLA-G gene can be transcribed in different isoforms resulting from alternative splicings and encoding membrane-bound and soluble proteins. These different mRNA species have been found in the various trophoblast cell subpopulations that constitute the maternofetal interface in the human placenta. The raising of antibodies to HLA-G has introduced new tools to determine in which types of trophoblast cells and in which other tissues these transcriptional isoforms are translated in functional proteins. The HLA-G gene exhibits a certain amount of polymorphism, the exon three that encodes the alpha 2 external domain showing the most extensive nucleotide variability. It remains to be determined whether the homozygosity of some HLA-G alleles constitutes a real disadvantage in terms of pregnancy or resistance to specific pathogens. Regarding the potential antigen-presenting function(s) of HLA-G, two isoforms are capable of binding an identical set of nonamer peptides derived from a variety of intracellular proteins. The ligand motif contains three anchor residues and is similar to that of classical HLA class I molecules. Experiments are being performed to identify the recognizing cells and to determine whether HLA-G induces a cytolytic (including anti-viral) T-cell response or in some other way represses natural killer-cell functions.

Prolactin mediates its effect on target cells through an interaction with membrane-anchored receptors. In the last decade, several subtypes of the receptor have been isolated from different species. This has generated a great deal of interest in the roles of the receptor subtypes and the possible divergent signalling pathways in mediating the pleiotropic effects of prolactin on target tissues. Our current knowledge of the signalling pathway of prolactin is derived mainly from the interaction of the hormone with one of its receptor subtypes (the long form) isolated from rats. In vitro expression studies have led to the identification of the regions within the long form prolactin receptor that are essential for the association of the tyrosine kinase Jak-2, and the phosphorylation events leading to activation of the prolactin responsive beta-casein promoter. To date, a specific target gene that may be activated after interaction of prolactin with the short form of the receptor has not been identified. However, the different receptor subtypes are present in the same cell type in vivo and their expression is hormone regulated, possibly through multiple promoters that control transcription of the prolactin receptor gene. Comparative studies suggest that the signalling pathways and the relevance of different receptor subtypes on prolactin function may vary between species.

Oxytocin is a neurohypophyseal hormone that has long been associated with uterine contraction during parturition and milk ejection during nursing. Recent studies have suggested that oxytocin is also a neurotransmitter that has central effects important for reproduction, including the initiation of parental and sexual behaviours. This review describes oxytocin pathways in the brain and examines their regulation by gonadal steroids. Brain oxytocin receptors are remarkable for their plasticity and for striking species differences in their distribution. The molecular characterization of this receptor has provided several clues to the regulation of its expression. Comparative and transgenic studies suggest that central oxytocin release may influence reproductive behaviours, but the importance of these central effects depends on the pattern of expression of the receptor--a pattern that is species-specific.

Leukocytes, specifically macrophages, lymphocytes and mast cells, are found within the testes of most, if not all, mammals. In some species (for example, rats, mice and humans), the number of 'resident' testicular macrophages, in particular, is quite considerable. However, reproductive biologists are only beginning to explore the characteristics and possible biological significance of these cells. As in other tissues, the testicular leukocytes are involved in immunological surveillance, immunoregulation and tissue remodelling. They are implicated in the mechanisms that make the testis a particularly successful site for tissue transplantation in some experimental animals. Moreover, recent studies have demonstrated that the testicular macrophages have specific trophic effects on Leydig cell development and steroidogenesis. In turn, the development and functions of the testicular leukocyte population are clearly influenced by the testicular environment, and especially by the Leydig cells and Sertoli cells. These data indicate an important role for leukocytes in testicular homeostasis. Balanced against this beneficial role is the fact that these cells possess the potential to damage testicular function in conditions of immune activation, as their inflammatory and cytotoxic activities may disrupt the normal environment of the testis. The importance of the testicular leukocytes to normal and abnormal testicular function is evident. The challenge for future research is to define the details of this relationship.

The GnRH cells represent the final output neurones of an integrated neuronal network used by the brain to generate pulsatile LH secretion from the pituitary gland. Changes in LH secretion profile throughout the ovarian cycle, including the preovulatory LH surge, result principally from alterations in the output of this GnRH network and it has been a key goal of many neurobiologists to elucidate the components and nature of this network. This review documents recent progress in understanding the role of noradrenaline within the GnRH network and highlights and explains its 'enabling' or permissive characteristics. Network behaviour analysis suggests that noradrenaline should be considered as a permissive agent promoting high output states of the GnRH network. On the basis of recent molecular and neuroanatomical data, it is proposed that oestrogen influences brainstem noradrenergic neurones specifically within the nucleus tractus solitarius to facilitate synaptic transmission within the GnRH network. In this manner, noradrenaline is likely to play a role in bringing about the increased GnRH messenger RNA expression and secretion necessary for ovulation.

Excitatory amino acids, such as glutamate, exert a profound stimulatory effect on the reproductive axis of several mammals. Although glutamate receptor agonists stimulate GnRH secretion, both in vivo and in vitro, it is unclear whether GnRH neurones respond directly to glutamatergic excitation. Immortalized GnRH neurones (GT1 cells) express glutamate receptors when grown in culture and also show enhanced GnRH secretion in response to glutamate receptor agonists. In addition, immunocytochemical evidence at the electron microscope level supports the possibility of a direct interaction between glutamatergic and GnRH neurones. In general, however, double-label histochemical studies (using immunocytochemistry, in situ hybridization, or a combination of these techniques) have not shown significant glutamate receptor gene expression in GnRH neurones of adult animals. It remains to be determined whether a higher degree of glutamate receptor gene expression occurs during development. This general lack, or very low amount, of glutamate receptor gene expression in the GnRH neurones of adults supports the view that excitatory amino acids exert their stimulatory action on the reproductive axis primarily through interneuronal pathways that impinge on the GnRH neurones, rather than by stimulating GnRH release directly.

Transgenic mice have become important model systems for studying molecular, cellular, organ, and whole animal physiology. In particular, transgenic technology allows determination of cell lineage differentiation and the role(s) of specific proteins in mammalian development and oncogenesis. Transgenic mice can be created that express wild-type genes, mutant genes, marker genes or cell lethal genes in a tissue-specific manner. In addition, homologous recombination strategies in embryonic stem cells permit more sophisticated manipulation of the mammalian genome, including the functional deletion of specific genes in whole mice or in a specific tissue. Since all transgenic mice created must be bred to study the consequences of transgene or mutant allele expression, a number of effects of these genome manipulations on the reproductive development and function of these mice have been uncovered. In this review, we summarize the transgenic mouse models in which defects and abnormalities in the reproductive axis have been demonstrated.