Diapause

Pedigreed primate embryonic stem cells express homogeneous familial gene profiles

Human embryonic stem cells (hESCs) hold great biomedical promise, but experiments comparing them produce heterogeneous results, raising concerns regarding their reliability and utility, although these variations may result from their disparate and anonymous origins. To determine whether primate ESCs have intrinsic biological limitations compared with mouse ESCs, we examined expression profiles and pluripotency of newly established nonhuman primate ESC (nhpESCs). Ten pedigreed nhpESC lines, seven full siblings (fraternal quadruplets and fraternal triplets), and nine half siblings were derived from 41 rhesus embryos; derivation success correlated with embryo quality. Each line has been growing continuously for approximately 1 year with stable diploid karyotype (except for one stable trisomy) and expresses in vitro pluripotency markers, and eight have already formed teratomas. Unlike the heterogeneous gene expression profiles found among hESCs, these nhpESCs display remarkably homogeneous profiles (>97%), with full-sibling lines nearly identical (>98.2%). Female nhpESCs express genes distinct from their brother lines; these sensitive analyses are enabled because of the very low background differences. Experimental comparisons among these primate ESCs may prove more reliable than currently available hESCs, since they are akin to inbred mouse strains in which genetic variables are also nearly eliminated. Finally, contrasting the biological similarities among these lines with the heterogeneous hESCs might suggest that additional, more uniform hESC lines are justified. Taken together, pedigreed primate ESCs display homogeneous and reliable expression profiles. These similarities to mouse ESCs suggest that heterogeneities found among hESCs likely result from their disparate origins rather than intrinsic biological limitations with primate embryonic stem cells.

High inter-individual variation in the gestation length of the hedgehog tenrec, Echinops telfairi (Afrotheria)

The gestation length (GL) of Tenrecs (Tenrecinae, Afrotheria) is still uncertain. This lack of knowledge also applies to the lesser hedgehog tenrec, Echinops telfairi, the species most commonly bred and maintained in captivity. The animals used in this study were held under controlled conditions (light, temperature and humidity). In order to determine the GL, groups of female tenrecs were subjected to various mating procedures followed by isolation periods of different lengths. A total of n=249 pregnancies were analysed and the number of offspring per litter was 3.29+/-0.09. The length of gestation could be determined in n=199 pregnancies and a mean GL of 67.53+/-0.36 days was calculated. Initial attempts with isolation periods of less than 16 days did not allow to accurately define the GL. Experiments with longer isolation periods and females subjected to only one mating procedure (n=10) revealed a variation in the GLs of 57-79 days. However, in one female a GL of only 50 days was also observed indicating an even greater range in GL variation. There was a statistically significant tendency for shorter GLs in the animals that conceived later in the mating season, but no statistical evidence was found that age, parity or litter size played an essential role in determining the GL. In conclusion, an unexpected high variability in gestation length in E. telfairi was demonstrated although the study animals were kept under controlled environmental conditions. The factors and mechanisms regulating this high intra-species variability in gestation length need further investigations.

Reproduction and embryonic diapause in a marsupial: insights from captive female Honey possums, Tarsipes rostratus (Tarsipedidae)

The reproductive physiology of the polyoestrous Honey possum (Tarsipes rostratus) is virtually unknown except that it shares with the kangaroos and wallabies the phenomenon of embryonic diapause. Its tiny size necessitates an alternate approach to study their reproductive cycle. We have accordingly utilised faecal steroid analysis. Baseline faecal cortisol levels in the Honey possum, at 4.1+/-0.3 mug g-1, are approximately 100-fold those of other mammals and are associated with adrenal glands that, on a mass-specific basis, are almost 10 times larger than the adrenals of other mammalian, including marsupial, species. Histological examination of the adrenal glands revealed no abnormalities, however, but their hypertrophy and the peaks recorded in faecal levels following disturbance suggest that the Honey possum is vulnerable to chronic stressors in the captive situation. Mean faecal progestagens (124.4+/-107.3 ng g-1) and oestradiol-17beta (4.1+/-1.1 ng g-1) in 4 non-pregnant females maintained long term were not different from those of 5 pregnant females (101.4+/-61.0 ng g-1 and 4.3+/-1.5 ng g-1, respectively) and, on analysis, revealed a cyclicity of 24+/-1.2 days. We would predict from this evidence that the gestation period, in the absence of lactation, is approximately 23 days. Four of the pregnant females, monitored from July to November under conditions of 10:14 L:D photoperiod, showed a fall in levels of progestagens from 175.9+/-10.8 ng g-1 in July and August to 30.9+/-9.4 ng g-1 in October, while mean faecal levels of oestradiol-17beta increased from 3.8+/-0.4 ng g-1 in July to 5.7+/-0.3 ng g-1 in October. September and October are months of peak reproductive activity in the wild and we suggest that these hormonal modulations may represent an entrained reproductive rhythm. Blastocysts appear to develop at varying rates, both within the one uterus, and between the two uteri of a single female. In addition, the time taken to reach the blastocyst stage may be longer than in any other marsupial studied to date. An association of the age of the pouch young with the stage reached by the developing blastocyst does not support the conclusion that blastocysts, once formed, grow slowly during lactation or diapause. Contrary to previous reports, we have documented what appears to be a lactational inhibition on blastocysts in diapause and have estimated the length of the 'delayed' reproductive cycle in two females as less than 2 weeks. Reactivation of blastocysts in Tarsipes has been shown to be stimulated by shortening day lengths after the summer solstice, a response similar to the annual breeding period of macropodid marsupials. Results from studying Honey possums in captive conditions suggest that the control of diapause in Tarsipes appears to be three-fold; lactational, photoperiodic and an entrained rhythm.

Embryo stability and vulnerability in an always changing world

Contrary to the view that embryos and larvae are the most fragile stages of life, development is stable under real-world conditions. Early cleavage embryos are prepared for environmental vagaries by having high levels of cellular defenses already present in the egg before fertilization. Later in development, adaptive responses to the environment either buffer stress or produce alternative developmental phenotypes. These buffers, defenses, and alternative pathways set physiological limits for development under expected conditions; teratology occurs when embryos encounter unexpected environmental changes and when stress exceeds these limits. Of concern is that rapid anthropogenic changes to the environment are beyond the range of these protective mechanisms.

Society for Reproductive Biology Founders' Lecture 2006 - life in the pouch: womb with a view

Marsupials give birth to an undeveloped altricial young after a relatively short gestation period, but have a long and sophisticated lactation with the young usually developing in a pouch. Their viviparous mode of reproduction trades placentation for lactation, exchanging the umbilical cord for the teat. The special adaptations that marsupials have developed provide us with unique insights into the evolution of all mammalian reproduction. Marsupials hold many mammalian reproductive 'records', for example they have the shortest known gestation but the longest embryonic diapause, the smallest neonate but the longest sperm. They have contributed to our knowledge of many mammalian reproductive events including embryonic diapause and development, birth behaviour, sex determination, sexual differentiation, lactation and seasonal breeding. Because marsupials have been genetically isolated from eutherian mammals for over 125 million years, sequencing of the genome of two marsupial species has made comparative genomic biology an exciting and important new area of investigation. This review will show how the study of marsupials has widened our understanding of mammalian reproduction and development, highlighting some mechanisms that are so fundamental that they are shared by all today's marsupial and eutherian mammals.

Evidence of increased endometrial vascular permeability at the time of implantation in the short-nosed fruit bat, Cyanopterus sphinx

Early embryonic development and implantation were studied in tropical short-nosed fruit bat Cyanopterus sphinx. We report preimplantation development and embryo implantation. Different stages of cleavage were observed in embryo by direct microscopic examination of fresh embryos after retrieving them either from the oviduct or the uterus at different days, up to the day of implantation. Generally, the embryos enter the uterus at the 8-cell stage. Embryonic development continued without any delay and blastocyst were formed showing attachment to the uterine epithelium at the mesometrial side of the uterus. A distinct blue band was formed in the uterus. The site of blastocyst attachment was visualized as a blue band following intravenous injection of pontamine blue. Implantation occurred 9+/-0.7 days after mating. This study reports that bat embryonic development can be studied like other laboratory animals and that this bat shows blue dye reaction, indicating the site and exact time of implantation. This blue dye reaction can be used to accurately find post-implantational delay. We prove conclusively that this species of tropical bat does not have any type of embryonic diapause.

Pluripotent stem cells and their niches

The ability of stem cells to self-renew and to replace mature cells is fundamental to ontogeny and tissue regeneration. Stem cells of the adult organism can be categorized as mono-, bi-, or multipotent, based on the number of mature cell types to which they can give rise. In contrast, pluripotent stem cells of the early embryo have the ability to form every cell type of the adult body. Permanent lines of pluripotent stem cells have been derived from preimplantation embryos (embryonic stem cells), fetal primordial germ cells (embryonic germ cells), and malignant teratocarcinomas (embryonal carcinoma cells). Cultured pluripotent stem cells can easily be manipulated genetically, and they can be matured into adult-type stem cells and terminally differentiated cell types in vitro, thereby, providing powerful model systems for the study of mammalian embryogenesis and disease processes. In addition, human embryonic stem cell lines hold great promise for the development of novel regenerative therapies. To fully utilize the potential of these cells, we must first understand the mechanisms that control pluripotent stem cell fate and function. In recent decades, the microenvironment or niche has emerged as particularly critical for stem cell regulation. In this article, we review how pluripotent stem cell signal transduction mechanisms and transcription factor circuitries integrate information provided by the microenvironment. In addition, we consider the potential existence and location of adult pluripotent stem cell niches, based on the notion that a revealing feature indicating the presence of stem cells in a given tissue is the occurrence of tumors whose characteristics reflect the normal developmental potential of the cognate stem cells.

Roadmap to embryo implantation: clues from mouse models

Implantation involves an intricate discourse between the embryo and uterus and is a gateway to further embryonic development. Synchronizing embryonic development until the blastocyst stage with the uterine differentiation that takes place to produce the receptive state is crucial to successful implantation, and therefore to pregnancy outcome. Although implantation involves the interplay of numerous signalling molecules, the hierarchical instructions that coordinate the embryo-uterine dialogue are not well understood. This review highlights our knowledge about the molecular development of preimplantation and implantation and the future challenges of the field. A better understanding of periimplantation biology could alleviate female infertility and help to develop novel contraceptives.

G protein-coupled receptor for diapause hormone, an inducer of Bombyx embryonic diapause

Bombyx diapause hormone was the first chemical substance identified as a maternal control factor that arrests offspring development. However, the molecular mechanisms by which the hormone transduces the signal to the oocyte that induces embryonic diapause immediately after mesoderm segmentation are not fully understood. Here, we describe a cDNA for a G protein-coupled diapause hormone receptor with seven transmembrane domains. Its amino-acid sequence shows a high level of similarity to the receptors of mammalian neuromedin U and insect regulatory peptide, an FXPRL-amide C-terminus. When expressed in a Xenopus oocyte system, the receptor exhibited the highest affinity (EC(50), approximately 70nM) for diapause hormone, when compared with other Bombyx FXPR/KL-amide peptides. Diapause hormone without amidation at the C-terminus, which never induces embryonic diapause in vivo, had no effect in this heterologous expression system. The mRNA is expressed in the ovaries during Bombyx pupal-adult development. These results strongly indicate that the cDNA encodes the diapause hormone receptor.

Female Receptiveity, Embryonic Diapause, and Superfetation in the European Badger (Meles Meles: Implications for the Reproductive Tactics of Males and Females

The European badger Meles meles is thought to mate throughout the year with two mating peaks occurring in late winter/spring and summer/autumn. After mating, fertilized ova enter embryonic diapause (delayed implantation) at the blastocyst stage, which lasts up to eleven months. Even if mating is successful, however, the estrous cycle may continue during embryonic diapause, which suggests that female badgers are capable of superfetation (conception during pregnancy). This may increase female fitness by facilitating polyandry, and reduce the risk of infanticide by resident males through paternity confusion. Detailed understanding of female receptivity, specifically the association of superfetation with embryonic diapause, may explain field observations of seemingly inconsistent reproductive tactics of male badgers with regard to, for instance, whether or not they guard mates or defend territories. The combination of embryonic diapause and superfetation may occur in other mustelids; if so, the sociobiology of mustelids will need reevaluating, and the Mustelidae may prove to be a good model taxon for studies of sexual conflict in the reproduction of eutherian mammals.

Faecal steroid analysis and urinary cytology of the squirrel glider (Petaurus norfolcensis)

Non-invasive techniques were used to investigate the reproductive biology of captive squirrel gliders (Petaurus norfolcensis) for 3 months during the breeding season. The squirrel glider is a medium-sized marsupial glider of eastern Australia and is currently listed as a threatened species as a result of habitat destruction and fragmentation. Urinary cytology was used to determine the timing of oestrus, and the presence of sperm confirmed mating. Progesterone and oestradiol-17β were identified in faecal samples via thin-layer chromatography, and were used to characterise the reproductive cycle. Reproductive activity was observed in three of four females, with births occurring during June and July. A preoestrus increase in faecal oestradiol-17β was detected in a single female, whilst significant increases occurred post partum (±2 days) in two of four females, suggesting that the squirrel glider may undergo a postpartum oestrus. Faecal progesterone profiles showed low concentrations before oestrus and significantly elevated concentrations after oestrus, which were maintained throughout pregnancy. Parturition coincided with a decrease in progesterone concentrations (±1 day). This study successfully used non-invasive monitoring of urinary cytology and faecal steroids to define luteal and gestational length as 16–17 days, a previously unpublished detail.

Sexual maturity, factors affecting the breeding season and breeding in consecutive seasons in populations of overabundant Victorian koalas (Phascolarctos cinereus)

It is important to have knowledge of basic population parameters to understand how these vary geographically and temporally and how they contribute to population dynamics. This paper investigates three of these parameters in Victorian koala populations: sexual maturity, aspects of the breeding season, and the continuity of individuals’ breeding. The investigation was carried out in koalas of known-age in two free-living (Redbill Creek on French Island and Brisbane Ranges) and one semi-captive (the Koala Conservation Centre on Phillip Island) population as well as koalas of unknown age in four Victorian populations of overabundant koalas: Mt Eccles and Framlingham in south-west Victoria, French Island in Western Port and Snake Island in south Gippsland. At sexual maturity, female koalas had a mean age (±95% confidence interval) of 24.4 months (23.5–25.3 months), a mean head length of 125 mm (124–127 mm) and a mean body mass of 6.6 kg (6.3–6.8 kg). Only 7.4% of independent females (of unknown age) were carrying young when they weighed less than 6 kg. The breeding season was more restricted in the south-west populations. At Framlingham and Mt Eccles 85% and 91% of births, respectively, occurred between December and March. At Snake and French Islands only 46% and 53% of births, respectively, were recorded in the same period. In the Chlamydia-free population (Red Bill Creek) none of the koalas that were monitored stopped breeding and then resumed breeding in a subsequent season whereas many females from Chlamydia-infected populations (Brisbane Ranges and the Koala Conservation Centre) did so. This variation in reproductive patterns is likely to make an important contribution to the variation in the demography observed in different koala populations.

System B0,+ amino acid transport regulates the penetration stage of blastocyst implantation with possible long-term developmental consequences through adulthood

Amino acid transport system B(0,+) was first characterized in detail in mouse blastocysts over two decades ago. Since then, this system has been shown to be involved in a wide array of developmental processes from blastocyst implantation in the uterus to adult obesity. Leucine uptake through system B(0,+) in blastocysts triggers mammalian target of rapamycin (mTOR) signalling. This signalling pathway selectively regulates development of trophoblast motility and the onset of the penetration stage of blastocyst implantation about 20 h later. Meanwhile, system B(0,+) becomes inactive in blastocysts a few hours before implantation in vivo. System B(0,+) can, however, be activated in preimplantation blastocysts by physical stimuli. The onset of trophoblast motility should provide the physiological physical stimulus activating system B(0,+) in blastocysts in vivo. Activation of system B(0,+) when trophoblast cells begin to penetrate the uterine epithelium would cause it to accumulate its preferred substrates, which include tryptophan, from uterine secretions. A low tryptophan concentration in external secretions next to trophoblast cells inhibits T-cell proliferation and rejection of the conceptus. Suboptimal system B(0,+) regulation of these developmental processes likely influences placentation and subsequent embryo nutrition, birth weight and risk of developing metabolic syndrome and obesity.

Embryonic diapause and its regulation

Embryonic diapause, a condition of temporary suspension of development of the mammalian embryo, occurs due to suppression of cell proliferation at the blastocyst stage. It is an evolutionary strategy to ensure the survival of neonates. Obligate diapause occurs in every gestation of some species, while facultative diapause ensues in others, associated with metabolic stress, usually lactation. The onset, maintenance and escape from diapause are regulated by cascades of environmental, hypophyseal, ovarian and uterine mechanisms that vary among species and between the obligate and facultative condition. In the best-known models, the rodents, the uterine environment maintains the embryo in diapause, while estrogens, in combination with growth factors, reinitiate development. Mitotic arrest in the mammalian embryo occurs at the G0 or G1 phase of the cell cycle, and may be due to expression of a specific cell cycle inhibitor. Regulation of proliferation in non- mammalian models of diapause provide clues to orthologous genes whose expression may regulate the reprise of proliferation in the mammalian context.

Profiling gene expression in growth-arrested mouse embryos in diapause

A recent report has identified changes in embryonic gene expression that are associated with, and may halt, embryonic cell proliferation during diapause.

In many mammalian species, embryonic cell proliferation can be reversibly arrested in an embryonic diapause at the time of embryo implantation. A recent report has identified changes in embryonic gene expression that are associated with, and may halt, embryonic cell proliferation.

Reproductive activity in captive female Honey possums, Tarsipes rostratus, assessed by faecal steroid analysis

Hormonal changes associated with reproductive activity in the unique pollen and nectar-feeding marsupial Honey possum, Tarsipes rostratus, have been monitored by the measurement of sex steroids excreted via the faeces. From a radio-metabolism study, 63% of administered [(14)C]oestradiol was excreted in the faeces and 37% via the urine. Peak levels in the faeces were reached 6h after injection and by a mean 12h, 95% of steroid was eliminated. The principal metabolic products of progesterone that were identified by chromatographic analysis were the isomers 5alpha- and 5beta-pregnan-3beta-ol-20-one with only trace amounts of progesterone and the isomers 5beta-pregnan-3beta,20beta- and 20alpha-diols. Extended excretory profiles for faecal progestagens (PM) and oestradiol-17beta (E(2)) are reported for the first time in a marsupial. The profiles from 4 females held in indoor cages with an artificial photoperiod suggest that long days inhibit reproductive activity in this species, as is the case in a number of other marsupials. One female appeared to resume cycling after a 5-month period and the time between peak levels of both E(2) and PM suggest that the length of the oestrous cycle in the Honey possum is approximately 25 days. The PM profile suggests that the corpora lutea secrete low levels of progesterone for approximately the first 19 days after ovulation, followed by increased rates of excretion during the final 6 days.

Global gene expression analysis identifies molecular pathways distinguishing blastocyst dormancy and activation

Delayed implantation (embryonic diapause) occurs when the embryo at the blastocyst stage achieves a state of suspended animation. During this period, blastocyst growth is very slow, with minimal or no cell division. Nearly 100 mammals in seven different orders undergo delayed implantation, but the underlying molecular mechanisms that direct this process remain largely unknown. In mice, ovariectomy before preimplantation ovarian estrogen secretion on day 4 of pregnancy initiates blastocyst dormancy, which normally lasts for 1-2 weeks by continued progesterone treatment, although blastocyst survival decreases with time. An estrogen injection rapidly activates blastocysts and initiates their implantation in the progesterone-primed uterus. Using this model, here we show that among approximately 20,000 genes examined, only 229 are differentially expressed between dormant and activated blastocysts. The major functional categories of altered genes include the cell cycle, cell signaling, and energy metabolic pathways, particularly highlighting the importance of heparin-binding epidermal growth factor-like signaling in blastocyst-uterine crosstalk in implantation. The results provide evidence that the two different physiological states of the blastocyst, dormancy and activation, are molecularly distinguishable in a global perspective and underscore the importance of specific molecular pathways in these processes. This study has identified candidate genes that provide a scope for in-depth analysis of their functions and an opportunity for examining their relevance to blastocyst dormancy and activation in numerous other species for which microarray analysis is not available or possible due to very limited availability of blastocysts.

The Escape of the Mink Embryo from Obligate Diapause1

The obligate embryonic diapause that characterizes gestation in mink engenders a developmental arrest at the blastocyst stage. The characteristics of escape from obligate diapause were investigated in embryos reactivated by treatment of the dams with exogenous prolactin. Protein and DNA synthesis showed marked increases within 72 h after the reinitiation of development, and embryo diameter increased thereafter. Trophoblast cells from embryos at Day 5 after activation proliferated more readily in vitro than trophoblasts from diapause or from Day 9 after activation, while in Day 9 embryos, cells from the inner cell mass (ICM) replicated comparatively more readily in vitro. There was evidence of expression of fibroblast growth factor-4 (FGF4) in both diapause and activated embryos and in ICM, but not the trophoblast. FGF receptor-2 was present in embryos from Day 5 after reactivation in both trophoblast and ICM cell lines. Trophoblast cell lines established from mink embryos proliferated in culture in the presence of FGF4 with a doubling time of 1.4 days, while in its absence, the doubling time was 4.0 days. We conclude that, during reinitiation of embryogenesis in the mink after diapause, embryo growth is characterized by gradual increases in protein synthesis, accompanied by mitosis of the trophoblast and ICM. There appears to be a pattern of differential proliferation between cells derived from these embryonic compartments, with the trophoblast phase of replication occurring mainly in the early reactivation phase, while the ICM proliferates more rapidly nearer to the time of implantation.

Suspended animation in C. elegans requires the spindle checkpoint

In response to environmental signals such as anoxia, many organisms enter a state of suspended animation, an extreme form of quiescence in which microscopically visible movement ceases. We have identified a gene, san-1, that is required for suspended animation in Caenorhabditis elegans embryos. We show that san-1 functions as a spindle checkpoint component in C. elegans. During anoxia-induced suspended animation, embryos lacking functional SAN-1 or a second spindle checkpoint component, MDF-2, failed to arrest the cell cycle, exhibited chromosome missegregation, and showed reduced viability. These data provide a model for how a dynamic biological process is arrested in suspended animation.

Amino acid transport regulates blastocyst implantation

Mouse blastocyst outgrowth in vitro and probably implantation in vivo require amino acid signaling via the target of rapamycin (TOR) pathway. This signaling does not simply support protein synthesis and trophoblast differentiation. Rather, it regulates development of trophoblast protrusive activity and may act as a developmental checkpoint for implantation. Moreover, intracellular amino acids per se are insufficient to elicit TOR signaling. Instead, de novo transport of amino acids, and particularly of leucine, stimulate mTOR activity at the blastocyst stage. The activity of the broad-scope and yet leucine-selective amino acid transport system B0,+ could produce such increases in intracellular amino acid concentrations. For example, system B0,+ uses a Na+ gradient to drive amino acid uptake, and the Na+ concentration in uterine secretions increases by nearly two-fold about 18 h before implantation. The resultant mTOR signaling could trigger polyamine, insulin-like growth factor II, and nitric oxide production in blastocysts and the increased cell motility sometimes associated with synthesis of these bioactive molecules.

Matching productivity to resource availability in a small predator, the stoat (Mustela erminea)

Stoats (Mustela erminea), introduced to New Zealand in the late nineteenth century, are common in New Zealand beech (Nothofagus sp.) forests, where populations of feral house mice (Mus musculus) fluctuate between years much as voles do in the northern hemisphere. We present new field evidence and two models demonstrating (i) a strong correlation between density indices for young stoats in summer and for mice in the previous spring, and (ii) a significant linear relationship between productivity per female and spring density of mice up to 25 mice captures per 100 trap-nights. These models confirm that short-lived small mustelid predators dependent on fluctuating populations of prey have evolved means of matching their productivity to the prospects of success across a wide range, from total failure in rodent crash years to >12 independent young per female in rodent peak years. We suggest that the enhanced reproductive success of female stoats when rodents are abundant is due to a combination of critical improvements in both the reproductive physiology and the foraging behaviour of female stoats in rodent peak years. Conversely, a drastic shortage of rodents increases the mortality of embryos and nestlings, while the adult females are able to survive, and even remain relatively fat, on other foods.

Quiet please, do not disturb: a hypothesis of embryo metabolism and viability

This review uses nutritional markers of normal and impaired development to address the question; what makes a viable mammalian preimplantation embryo? Resolution of this question is important to ensure the long-term safety of embryo-based biotechnologies in man and domestic animals, the optimisation of embryo production and culture conditions and the development of methods to select viable embryos for replacement. After considering the nutrition of embryos and somatic cells, and the phenomenon of caloric restriction, it is concluded that preimplantation embryo survival is best served by a relatively low level of metabolism; a situation achieved by reducing the concentrations of nutrients in culture media and encouraging the use endogenous resources.

The First Comprehensive Genetic Linkage Map of a Marsupial: The Tammar Wallaby (Macropus eugenii)

The production of a marsupial genetic linkage map is perhaps one of the most important objectives in marsupial research. This study used a total of 353 informative meioses and 64 genetic markers to construct a framework genetic linkage map for the tammar wallaby (Macropus eugenii). Nearly all markers (93.8%) formed a significant linkage (LOD > 3.0) with at least one other marker, indicating that the majority of the genome had been mapped. In fact, when compared with chiasmata data, >70% (828 cM) of the genome has been covered. Nine linkage groups were identified, with all but one (LG7; X-linked) allocated to the autosomes. These groups ranged in size from 15.7 to 176.5 cM and have an average distance of 16.2 cM between adjacent markers. Of the autosomal linkage groups (LGs), LG2 and LG3 were assigned to chromosome 1 and LG4 localized to chromosome 3 on the basis of physical localization of genes. Significant sex-specific distortions toward reduced female recombination rates were revealed in 22% of comparisons. When comparing the X chromosome data to closely related species it is apparent that they are conserved in both synteny and gene order.

Deciphering the cross-talk of implantation: advances and challenges

Implantation involves a series of steps leading to an effective reciprocal signaling between the blastocyst and the uterus. Except for a restricted period when ovarian hormones induce a uterine receptive phase, the uterus is an unfavorable environment for blastocyst implantation. Because species-specific variations in implantation strategies exist, these differences preclude the formulation of a unifying theme for the molecular basis of this event. However, an increased understanding of mammalian implantation has been gained through the use of the mouse model. This review summarizes recognized signaling cascades and new research in mammalian implantation, based primarily on available genetic and molecular evidence from implantation studies in the mouse. Although the identification of new molecules associated with implantation in various species provides valuable insight, important questions remain regarding the common molecular mechanisms that govern this process. Understanding the mechanisms of implantation promises to help alleviate infertility, enhance fetal health, and improve contraceptive design. The success of any species depends on its reproductive efficiency. For sexual reproduction, an egg and sperm must overcome many obstacles to fuse and co-mingle their genetic material at fertilization. The zygote develops into a blastocyst with two cell lineages (the inner cell mass and the trophectoderm), migrates within the reproductive tract, and ultimately implants into a transiently permissive host tissue, the uterus. However, the molecular basis of the road map connecting the blastocyst with the endometrium across species is diverse (1) and not fully understood. Recent advances have identified numerous molecules involved in implantation (1-4), yet new discoveries have not yielded a unifying scheme for the mechanisms of implantation.

Exogenous amino acids regulate trophectoderm differentiation in the mouse blastocyst through an mTOR-dependent pathway

At the late blastocyst stage, the epithelial trophectoderm cells of the mammalian embryo undergo a phenotypic change that allows them to invade into the uterine stroma and make contact with the maternal circulation. This step can be regulated in vitro by the availability of amino acids. Embryos cultured in defined medium lacking amino acids cannot form trophoblast cell outgrowths on fibronectin, an in vitro model of implantation, but remain viable for up to 3 days in culture and will form outgrowths when transferred into complete medium. The amino acid requirement is a developmentally regulated permissive event that occurs during a 4- to 8-h period at the early blastocyst stage. Amino acids affect spreading competence specifically by regulating the onset of protrusive activity and not the onset of integrin activation. Rapamycin, a specific inhibitor of the kinase mTOR/FRAP/RAFT1, blocks amino acid stimulation of embryo outgrowth, demonstrating that mTOR is required for the initiation of trophectoderm protrusive activity. Inhibition of global protein translation with cycloheximide also inhibits amino acid-dependent signals, suggesting that mTOR regulates the translation of proteins required for trophoblast differentiation. Our data suggest that mTOR activity has a developmental regulatory function in trophectoderm differentiation that may serve to coordinate embryo and uterus at the time of implantation.

Dual control of LIF expression and LIF receptor function regulate Stat3 activation at the onset of uterine receptivity and embryo implantation

Leukemia inhibitory factor (LIF) expression in the uterus is essential for embryo implantation in mice. Here we describe the spatial and temporal regulation of LIF signaling in vivo by using tissues isolated from uteri on different days over the implantation period. During this time, LIF receptors are expressed predominantly in the luminal epithelium (LE) of the uterus. Isolated epithelium responds to LIF by phosphorylation and nuclear translocation of signal transducer and activator of transcription (Stat) 3, but not by an increase in mitogen-activated protein kinase levels. The related cytokines Il-6, ciliary neurotrophic factor, as well as epidermal growth factor, do not activate Stat3, although epidermal growth factor stimulates mitogen-activated protein kinase. In vivo Stat3 activation is induced by LIF alone, resulting in the localization of Stat3 specifically to the nuclei of the LE coinciding with the onset of uterine receptivity. The responsiveness of the LE to LIF is regulated temporally, with Stat activation being restricted to day 4 of pregnancy despite the presence of constant levels of LIF receptor throughout the preimplantation period. Uterine receptivity is therefore under dual control and is regulated by both the onset of LIF expression in the endometrial glands and the release from inhibition of receptor function in the LE.

Oxygen deprivation causes suspended animation in the zebrafish embryo

Continuous exposure to oxygen is essential for nearly all vertebrates. We found that embryos of the zebrafish Danio rerio can survive for 24 h in the absence of oxygen (anoxia, 0% O2). In anoxia, zebrafish entered a state of suspended animation where all microscopically observable movement ceased, including cell division, developmental progression, and motility. Animals that had developed a heartbeat before anoxic exposure showed no evidence of a heartbeat until return to terrestrial atmosphere (normoxia, 20.8% O2). In analyzing cell-cycle changes of rapidly dividing blastomeres exposed to anoxia, we found that no cells arrested in mitosis. This is in sharp contrast to similarly staged normoxic embryos that consistently contain more than 15% of cells in mitosis. Flow cytometry analysis revealed that blastomeres arrested during the S and G2 phases of the cell cycle. This work indicates that survival of oxygen deprivation in vertebrates involves the reduction of diverse processes, such as cardiac function and cell-cycle progression, thus allowing energy supply to be matched by energy demands.

Physiological rationale for responsiveness of mouse embryonic stem cells to gp130 cytokines

Embryonic stem cells are established directly from the pluripotent epiblast of the preimplantation mouse embryo. Their derivation and propagation are dependent upon cytokine-stimulated activation of gp130 signal transduction. Embryonic stem cells maintain a close resemblance to epiblast in developmental potency and gene expression profile. The presumption of equivalence between embryonic stem cells and epiblast is challenged, however, by the finding that early embryogenesis can proceed in the absence of gp130. To explore this issue further, we have examined the capacity of gp130 mutant embryos to accommodate perturbation of normal developmental progression. Mouse embryos arrest at the late blastocyst stage when implantation is prevented. This process of diapause occurs naturally in lactating females or can be induced experimentally by removal of the ovaries. We report that gp130−/− embryos survive unimplanted in the uterus after ovariectomy but, in contrast to wild-type or heterozygous embryos, are subsequently unable to resume development. Inner cell masses explanted from gp130−/− delayed blastocysts produce only parietal endoderm, a derivative of the hypoblast. Intact mutant embryos show an absence of epiblast cells, and Hoechst staining and TUNEL analysis reveal a preceding increased incidence of cell death. These findings establish that gp130 signalling is essential for the prolonged maintenance of epiblast in vivo, which is commonly required of mouse embryos in the wild. We propose that the responsiveness of embryonic stem cells to gp130 signalling has its origin in this adaptive physiological function.