Normal neurulation in mammals

During mammalian neurulation regional differences are evident between the cranial region, in which neurulation is most complex, the trunk as far as the caudal neuropore and the secondary neurulation region of the caudal trunk plus tail. Differences among these three regions are characterized by specific patterns of morphogenesis and by specific patterns of gene expression. During cranial neurulation distinct regions develop in the brain and the presomitic hindbrain forms seven rhombomeric divisions. The first clear morphological boundary is the preotic sulcus (later transformed into the gyrus between rhombomeres 2 and 3), which may limit cell movement as neuroepithelial cells rostral to it flow towards and into the rapidly expanding forebrain region. The formation of rhombomeres as morphological entities and the development of a normal rhombomere-specific pattern of homeobox and other gene expression domains depend on relatively low levels of retinoic acid. Retinoic acid receptors, which are retinoic acid-activated transcription factors, and retinoid binding proteins, which control the availability of retinoic acid to the receptors, show regional patterns of expression in the cranial, trunk and caudal regions of the neuroepithelium during neurulation. These patterns suggest a possible mechanism for region-specific gene expression during neurulation.

Pax genes and neural tube defects in the mouse

The Pax genes encode a family of transcription factors that are expressed in restricted regions of the developing embryo. Several Pax genes are expressed in the developing nervous system where they are believed to regulate the morphogenesis of neural structures. Loss-of-function mutations in the Pax-3 gene have been identified in a number of alleles of the mouse mutant splotch. In homozygous splotch embryos closure of the neural tube is defective with embryos exhibiting spina bifida and/or exencephaly. Other structures in which Pax-3 is expressed are also affected, most notably those tissues derived from the neural crest and somites.

Formation and patterning of the avian neuraxis: one dozen hypotheses

Formation of the neuraxis is dependent on cell-cell interactions and cell movements beginning during stages of gastrulation. Cell movements bring together new combinations of cells, allowing sequential inductive interactions to occur and leading to the specification of the neural plate and to its ultimate mediolateral (subsequently dorsoventral) and rostrocaudal patterning. Formation of the neural plate involves changes in the shape of its constituent cells and the first appearance of neural-specific cell markers. Shortly after the neural plate forms it undergoes 'shaping', in which the pseudostratified columnar epithelium constituting the neural plate thickens apicobasally, narrows transversely and extends longitudinally. Shaping is driven by three principal intrinsic types of cell behaviour: changes in cell shape, position and number. The next stage of neurulation begins while shaping is underway--bending of the neural plate. Bending involves two main processes, furrowing and folding. Furrowing of the neural plate is associated with the formation of the hinge points; these are localized, longitudinal areas where the neuroepithelium is attached to adjacent tissues and where wedging of neuroepithelial cells occurs. Cell wedging in the median hinge point occurs as a result of inductive interactions with the notochord; such wedging drives furrowing, thereby facilitating subsequent folding. Folding of the neural plate requires extrinsic forces generated largely by the surface ectoderm. Types of cell behaviour that could provide such forces include changes in cell shape, position and number. As a result of shaping and bending of the neural plate, the neural folds are brought into apposition in the dorsal midline. Final closure of the neural groove is mediated by cell surface glycoconjugates coating the apical surfaces of the neural folds. Patterning of the neuraxis begins during shaping of the neural plate and continues throughout stages of neurulation and into early postneurula stages. Patterning probably involves inductive interactions with adjacent tissues and the expression of putative positional identity genes such as homeobox-containing genes.

Neurulation in the normal human embryo

The neural groove and folds are first seen during stage 8 (about 18 postovulatory days). Two days later (stage 9) the three main divisions of the brain, which are not cerebral vesicles, can be distinguished while the neural groove is still completely open. Two days later (stage 10) the neural folds begin to fuse near the junction between brain and spinal cord, when neural crest cells are arising mainly from the neural ectoderm. The rostral (or cephalic) neuropore closes within a few hours during stage 11 (about 24 days). The closure is bidirectional; it takes place from the dorsal and terminal lips and may occur in several areas simultaneously. The two lips, however, behave differently. The caudal neuropore takes a day to close during stage 12 (about 26 days) and the level of final closure is approximately at future somitic pair 31, which corresponds to the level of sacral vertebra 2. At stage 13 (4 weeks) the neural tube is normally completely closed. Secondary neurulation, which begins at stage 12, is the differentiation of the caudal part of the neural tube from the caudal eminence (or end-bud) without the intermediate phase of a neural plate.

Molecular genetics of neurulation

The formation of the neural tube begins during gastrulation when ectoderm, an epithelial sheet on the outside of the embryo, is induced to form the neural plate. During the process of neural induction, the epithelium of the neural plate is regionalized along both the dorsoventral and anteroposterior axes of the embryo; this regionalization is likely to contribute to the cellular processes underlying neurulation. Genes whose expression marks the formation and regionalization of the neural plate and which encode cell adhesion molecules or putative transcription factors have been recently identified. The differential expression of these genes apparently subdivides the epithelium of the neural plate into small regions. Evidence from transgenic embryo experiments supports the idea that the differential expression of these genes in the neural plate plays a role in neural tube formation.

Creation and characterization of E-selectin- and VCAM-1-deficient mice

A variety of adhesion molecules have been identified which mediate the interaction of leukocytes with endothelial cells. In order to define the role of individual molecules in inflammation we have produced lines of mice which are deficient in the synthesis of specific adhesion molecules. Null mutations were introduced into the genes encoding E-selectin or vascular cell adhesion molecule-1 (VCAM-1) in embryonic stem cells and these cells were used to produce lines of mice carrying the mutation. E-selectin-deficient mice were viable and exhibited no developmental defects. The roles of E- and P-selectin in the influx of neutrophils were examined using these mice. The data suggest that the two selectins are functionally redundant in mediating neutrophil emigration in a model of chemically induced peritonitis. VCAM-1-deficient mice are not viable. Analysis of VCAM-1 gene expression in wild-type embryos and phenotypic analysis of VCAM-1 -/- embryos suggests that VCAM-1 is required for development of the extraembryonic circulatory system and the embryonic heart.

Clonal analysis of the origin of primordial germ cells in the mouse

Qualitative and quantitative clonal analysis has been used to answer three basic questions about the establishment of the germ cell lineage in the mouse. Where do primordial germ cells originate? What is the size of the founding population at the time of lineage restriction? When and where does lineage restriction occur? Single epiblast cells of 6.0 dpc and 6.5 dpc mouse embryos were injected with a short-term lineage label (lysinated rhodamine dextran, LRDX) and their descendants traced after 40 h embryo culture at neural plate and early somite stages, respectively. An objective matching technique was used to detect the lineage marker in primordial germ cells identified by their characteristic alkaline phosphatase staining. Precursors of the primordial germ cells were found in the proximal epiblast close to the extraembryonic ectoderm in both pregastrulation and early-streak stage embryos. They form part of the presumptive extraembryonic mesoderm and are not lineage restricted while in the epiblast. Quantitative analysis gives a best fit to a model of a founding population of 45 at the time of lineage restriction. The data indicate that the generation time lengthens at the time of allocation. Calculation of clonal histories gives a best fit of 16 h generation time after allocation compared with < 7 h before allocation, with lineage restriction occurring at the early midstreak stage, presumably in the region posterior to the streak in which primordial germ cells are first identifiable. Therefore primordial germ cells are probably allocated early during gastrulation in a group of > 40 cells already segregated in the extraembryonic mesoderm.

Development of the left-right axis

Left-right is not an axis in the conventional sense but rather two mirror-image proximodistal axes, upon which a quantal piece of positional information (leftness or rightness) is superimposed for laterally asymmetric organ development. We are attempting to establish the stages at which left-right is specified and determined, but this is complicated by the apparent loss of normal handed development in embryos that are cultured from pre-neural plate stages. Experiments suggest that left-right is determined by the first somite stage. The loss of normal left-right development in early cultures is probably not due to removal of some maternal signal, even though embryos do develop in vivo with their axes in a specific orientation relative to the uterus. The fact that there are two random embryonic axis orientations, 180 degrees opposed to one another, and that the axes of the two uterine horns are mirror-images of each other make it unlikely that the uterus could impart a sense of left-right to the embryo. The right ovary produces more eggs than the left one; this is reversed in iv/iv situs inversus mice. Analysis of iv/iv mice shows a correlation of left-right abnormalities with sex and close relationships between the abnormal left-right development of some organs, for example the heart and spleen, that have no obvious developmental connection.

Normal neurulation in amphibians

How does cell behaviour accomplish neurulation in amphibian embryos? During neurulation, the neural plate (while preserving the same volume) doubles its length, triples its thickness, narrows 10-fold, greatly decreases its surface and rolls into a tube. Cells that compose the neural plate produce these changes in three ways. They change shape, change neighbours and attempt to crawl beneath the contiguous epidermis. Plate width, length and area are decreased and the plate thickens when apical surfaces of plate cells contract radially, but plate length increases and width is further decreased when cells reposition themselves and collect along plate boundaries. Contraction of the apical surfaces of plate cells also helps roll the plate into a tube. Poisson buckling resulting from elongation of plate borders may contribute bending forces that help tube formation. The main folding force in tube formation is a rolling moment toward the midline produced by neural plate cells attempting to crawl beneath the contiguous epidermis. Experiments, observations and computer simulations support these assertions, reveal the organization of cell behaviour and implicate contraction of actin filaments as the main source of the necessary forces.

Ontogeny and nutritional manipulation of the hepatic prolactin–growth hormone–insulin-like growth factor axis in the ovine fetus and in neonate and juvenile sheep

The somatotrophic axis is the main endocrine system regulating postnatal growth; however, prenatal growth is independent of growth hormone (GH). Fetal development relies on the coordinated actions of a range of hormones, including insulin-like growth factors (IGF), and prolactin (PRL), in the control of differentiation, growth and maturation. In the sheep the abundance peaks for liver IGF-II and PRL receptors occur during late gestation while that for IGF-I receptor occurs at birth. All receptors, with the exception of GH receptor subsequently decrease by age 6 months. It has been proposed that maternal undernutrition during gestation regulates the maturation of the fetal hypothalmic–pituitary–adrenal axis and endocrine sensitivity. Critically, the timing of the nutritional insult may affect the magnitude of reprogramming. Maternal malnutrition during early to mid-gestation (3·2–3·8 MJ/d (60% total metabolisable energy requirements) v. 8·7–9·9 MJ/d (150% total metabolisable energy requirements) between 28 and 80 d of gestation) had no effect on body or liver weight. Nutrient-restricted (NR) fetuses sampled at 80 d (mid-gestation) showed up-regulation of hepatic PRL receptor, but following refeeding the normal gestational rise in PRL and GH receptors did not occur. Hepatic IGF-II receptor was down regulated in NR fetuses at both mid- and late gestation. Conversely, 6-month-old offspring showed no difference in the abundance of either GH receptor or PRL receptor, while IGF-II mRNA was increased. Offspring of ewes malnourished during late gestation (9·1 MJ/d (60% total metabolisable energy requirements) v. 12·7 MJ/d (100% total metabolisable energy requirements) from 110 d of gestation to term) showed reduced abundance of hepatic GH and PRL receptor mRNA. In conclusion, maternal undernutrition during the various stages of gestation reprogrammed the PRL–GH–IGF axis. Nutritional regulation of cytokine receptors may contribute to altered liver function following the onset of GH-dependent growth, which may be important in regulating endocrine adaptations during subsequent periods of nutritional deprivation.

Timing of nutrient restriction and programming of fetal adipose tissue development

It is apparent from epidemiological studies that the timing of maternal nutrient restriction has a major influence on outcome in terms of predisposing the resulting offspring to adult obesity. The present review will consider the extent to which maternal age, parity and nutritional restriction at defined stages of gestation can have important effects on fat deposition and endocrine sensitivity of adipose tissue in the offspring. For example, in 1-year-old sheep the offspring of juvenile mothers have substantially reduced fat deposition compared with those born to adult mothers. Offspring of primiparous adult mothers, however, show increased adiposity compared with those born to multiparous mothers. These offspring of multiparous ewes show retained abundance of the brown adipose tissue-specific uncoupling protein 1 at 1 month of age. A stimulated rate of metabolism in brown fat of these offspring may act to reduce adipose tissue deposition in later life. In terms of defined windows of development that can programme adipose tissue growth, maternal nutrient restriction targetted over the period of maximal placental growth results in increased adiposity at term in conjunction with enhanced abundance of mRNA for the insulin-like growth factor-I and -II receptors. In contrast, nutrient restriction in late gestation, coincident with the period of maximal fetal growth, has no major effect on adiposity but results in greater abundance of specific mitochondrial proteins, i.e. voltage-dependent anion channel and/or uncoupling protein 2. These adaptations may increase the predisposal of these offspring to adult obesity. Increasing maternal nutrition in late gestation, however, can result in proportionately less fetal adipose tissue deposition in conjunction with enhanced abundance of uncoupling protein 1.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

Growth retardation in Down syndrome in relation to insulin-like growth factors and growth hormone

Growth retardation is a cardinal characteristic of Down syndrome (DS). It is most pronounced from the age of 6 months, when growth starts to become growth hormone (GH) regulated. DS children have normal serum levels of GH. GH regulates the production of insulin-like growth factors (IGFs), which act as growth hormones. Therefore, the serum IGF pattern and the levels of their receptors were studied in fetuses with trisomy 21 and in patients with DS throughout life. Serum levels of IGF were determined by radioimmunoassays for insulin-like growth factors 1 and 2 (RIA-IGF-1 and RIA-IGF-2) showing normal serum RIA-IGF-2 levels throughout life. However, serum RIA-IGF-1 did not rise during childhood and remained at a low level throughout life. Determination of serum IGF by a radioreceptor assay (RRA-IGF), which detects both IGF-1 and IGF-2 as well as enhanced activity in the fetal circulation, showed a deficit in serum RRA-IGF in fetuses with trisomy 21, but at birth and throughout life elevated serum RRA-IGF levels. In spite of this, no differences were observed in fetal brain or liver binding sites for IGF-1, IGF-2, or insulin. Since in the RRA-IGF method IGF-1, IGF-2, and a fetal form of IGF-1 cross-react, it is possible that there is a delayed maturation with incomplete switching from production of the fetal form of IGF to production of the GH-regulated IGF-1 in DS. The deficit in IGF-1-like peptides might account for the growth retardation in DS. In order to study the effect of human growth hormone (hGH) therapy in DS, 5 growth-retarded children with DS were treated with hGH for 6 months. During this period the growth velocity doubled and the serum IGF-1 levels were restored to normal. Thus, DS children respond to hGH treatment.

Expression of Dlx and Lhx family homeobox genes in fetal thymus and thymocytes

Homeobox genes comprise a nearly ubiquitous and highly conserved superfamily of developmental regulatory genes that encode transcription factors involved in the determination of axis and tissue identity. While homeobox gene expression has been well characterized in a variety of embryonic tissues, their expression has not been extensively studied in lymphoid progenitor cells or in sites of lymphogenesis. To examine homeobox gene expression in the developing thymus, we screened an embryonic day 13.5 thymus cDNA library by polymerase chain reaction (PCR) using degenerate oligonucleotides within the highly conserved homeodomain region of eight homeobox gene families. The resulting PCR products were then cloned and sequenced. Transcripts for multiple Dlx family members and Lhx2 were repeatedly detected in this screen. Screening of embryonic day 16.5 and adult murine thymus and Thy1+ thymocytes was performed for selected members of these homeobox gene families. Transcripts encoding Lhx2, Lhx3, and Lhx9, as well as Dlx1 and Dlx2 were detected in both thymus and purified thymocytes. Dlx1 is a member of the distal-less homeobox gene family that has been shown to regulate embryonic craniofacial development. Significantly, Dlx1 is expressed in the third branchial arch, which contributes to the thymus. Although Dlx1 knockout mice did not display any obvious developmental defects in thymus or thymocyte development, the expression of these homeobox genes in neural crest derivatives suggests a possible role in cell migration and development that may overlap with other homeobox genes.

Retinol improves development of bovine oocytes compromised by heat stress during maturation

The objectives of this study were to evaluate: 1) effects of a physiologically relevant elevated temperature on in vitro development of maturing oocytes, 2) effects of retinol on in vitro development of maturing oocytes, and 3) effects of retinol to improve development of oocytes compromised by an elevated temperature. Bovine oocytes were matured for 24 h at 38.5 or 41.0 degrees C (first 12 h) in 0 or 5 microM retinol. After insemination, cleavage and blastocyst development were assessed on d 3 and 8, respectively. Temperature, retinol, and their interaction were included in the statistical model. Culture of oocytes at 41.0 degrees C decreased the proportion of 8- to 16-cell embryos and increased that of 2-cell embryos. In addition, culture at 41.0 degrees C decreased the ability of oocytes to develop to the blastocyst stage. Blastocysts derived from oocytes cultured at 41.0 degrees C had fewer total nuclei. In 3 of the 7 experimental replicates, effects of 41.0 degrees C to reduce blastocyst development were minimal (difference in the development of the control vs. heat stress group was <20%). To provide a more precise test of our hypothesis (retinol administration may improve development of oocytes compromised by heat stress), data were analyzed, including only those replicates (n = 4) in which heat stress reduced development to blastocyst >20%. When this was done, a significant temperature x retinol interaction was noted. The addition of retinol to the maturation medium prevented heat-induced reductions in development of oocytes to blastocyst stage. Results indicate that retinol may protect oocytes from some of the deleterious effects of heat stress.

Molecular cloning and characterization of mouse LITAF cDNA: role in the regulation of tumor necrosis factor-alpha (TNF-alpha) gene expression

The inflammatory response to bacteria and bacterial products, such as lipopolysaccharides (LPSs), is mediated by a variety of secreted factors, but cytotoxic effects of LPS have been ascribed to the tumor necrosis factor alpha (TNF-alpha) activity. TNF-alpha is probably the most pleiotropic cytokine and, given the deleterious effects to the host of this factor, it has been postulated that its expression must be tightly regulated. Our laboratory has recently isolated, cloned and characterized a novel human transcription factor named LITAF or LPS-induced TNF-alpha factor. The present study reports the isolation, cloning and characterization of the mouse LITAF cDNA. Chromosomal localization revealed that mouse LITAF mapped to mouse chromosome 16, in a region highly homologous with the area on which human LITAF was previously located. Northern blot analysis shows that mouse LITAF is already expressed at embryonic day 7 of development, and is highly expressed in adult liver, heart and kidney. Moreover, upon LPS stimulation, we show that: (i) LITAF expression is increased in a mouse monocyte/macrophage cell line; and (ii) TNF-alpha expression is reduced in ES cell-derived macrophages lacking one copy of LITAF gene. Taken together, these results highlight the important role of LITAF in the regulation of TNF-alpha gene expression and suggest a potential role of LITAF in mouse organogenesis.

Convergence of p53 and TGF-beta signaling networks

p53 is a protein with many talents. One of the most fundamental is the ability to act as essential growth checkpoint that protects cells against cellular transformation. p53 does so through the induction of genes leading to growth arrest or apoptosis. Most of the studies focusing on the mechanisms of p53 activity have been performed in cultured cells upon treatment with well-established p53-activating inputs, such as high doses of radiations, DNA-damaging drugs and activated oncogenes. However, how the tumor suppressive functions of p53 become concerted with the extracellular cues arriving at the cell surface during tissue homeostasis, remains largely unknown. Intriguingly, two recent papers have shed new light into this unexplored field, indicating that p53 plays a key role in TGF-beta-induced growth arrest and, unexpectedly, in the developmental effects of TGF-beta in early embryos. Here we review and comment on these findings and on their implications for cancer biology.

Overlapping, complementary and site-specific expression pattern of genes of the EMILIN/Multimerin family

The EDEN gene superfamily comprises genes that contain the EMI domain, a structural motif recently identified in proteins of the extracellular matrix. We report here the detailed expression pattern of genes of the EMILIN/Multimerin family, the most numerous group of EDEN superfamily, during mouse development. In situ hybridization has revealed that the EMILIN/Multimerin genes are particularly expressed in the cardio-vascular system and in mesenchymal cells. In general, the territories of expression of each gene are partially overlapping or complementary with that of other members of the family and, usually, more than one gene of the family is active in different tissues, consistent with the possibility of functional compensation. The analysis is particularly relevant in the interpretation of gene targeting experiments.

Programming of adult blood pressure by maternal protein restriction: role of nephrogenesis

BACKGROUND

Modest maternal protein restriction leads to hypertension and a reduced number of glomeruli in adult male but not female offspring. This study determined whether a more severe protein restriction has equivalent effects on male and female rat offspring, and examined the role of nephrogenesis in this programming.

METHODS

Sprague-Dawley rats were fed a protein-restricted (5% protein) diet throughout (LLP), or during the first (LLP/NP) or second (NP/LLP) half of pregnancy. Controls ate a normal diet (NP, 19% protein). Adult offspring were chronically instrumented at 22 weeks; glomerular number and volume were estimated using stereologic techniques.

RESULTS

Mean arterial pressures in male offspring were significantly higher in LLP (136 +/- 2 mm Hg) or NP/LLP (137 +/- 2 mm Hg) than in LLP/NP (125 +/- 1 mm Hg) or NP (125 +/- 2 mm Hg). Moreover, the hypertension was salt-sensitive (increase of 16 +/- 4 mm Hg in LLP on a high Na(+) diet compared to 2 +/- 2 mm Hg in NP). Glomerular number (per kidney) was reduced (15,400 +/- 2,411 in LLP vs. 27,208 +/- 1,534 in NP) but average individual glomerular volume was not different (1.98 +/- 0.18 106 micro(3) in LLP vs. 2.01 +/- 0.14 106 micro(3) in NP). Female offspring showed qualitatively similar results.

CONCLUSION

Severe maternal dietary protein restriction reduces glomerular number and programs for salt-sensitive adult hypertension in both female and male offspring. The window of sensitivity of adult blood pressure to prenatal protein restriction falls within the period of nephrogenesis in the rat. These data are consistent with the hypothesis that maternal protein restriction causes adult hypertension in the offspring through impairment of renal development.

Influenza-B-virus-induced eye and brain malformations during early chick embryogenesis and localization of the viral RNA in specific areas

Influenza is prevalent worldwide, and the teratogenic effects of influenza infection have been suspected to occur within the developing central nervous system. We herein report the sequelae of influenza B viral infection during early chick embryogenesis. Chick embryos at Hamburger-Hamilton stage 9 were infected by an in ovo injection under the blastoderm of influenza B virus (B/Taiwan/25/99). At 48 h after infection, gross malformations of the eye and brain, ranging from 25 to 58% of 168 infected embryos, were observed, in contrast to 3-6% among 71 mock-infected controls (p < 0.0001 for both eye and brain malformations). Histological analyses showed extensive tissue degeneration and aggregates of cells in the head mesenchyme, suggesting cell death and heterotopia. Influenza B viral RNA was directly localized by in situ hybridization with probes specific for the HA segment. Viral RNA was extensively detected in the head surface ectoderm and in the lung bud. In the developing brain, viral RNA was specifically located in the anterior neural retina, habenular area, mid-thalamus, and rhombencephalon. Our data show that influenza B virus can be a teratogenic agent in neural and nonneural embryonic tissues, raising concern for transplacental infection during early pregnancy.

Evidence for the role of oviduct secretions in sperm function, fertilization and embryo development

The oviduct is a dynamic organ which facilitates gamete function, fertilization and embryo development. Secretions of the oviduct, recovered by tissue culture or cannulation techniques have been used to define the composition of the oviduct milieu, as well as functions associated with stage of the reproductive cycle or region of the oviduct. Several oviduct proteins have been shown to associate with the gametes and embryos. Ongoing studies are directed at identifying oviduct proteins and determining their function. Oviduct-specific glycoproteins (OSG) have been purified from the oviduct and shown in vitro to have positive affects on sperm capacitation, sperm-ovum binding, ovum penetration and embryo development. Osteopontin, another oviduct secretion, also has been shown to stimulate fertilization and embryo development. The picture emerging is that some components of the oviduct milieu have overlapping functions to collectively provide a failsafe system to ensure fertility in vivo so that success is not dependent on a single component.

DNA methylation in the preimplantation embryo: the differing stories of the mouse and sheep

In mammals, active demethylation of cytosine methylation in the sperm genome prior to forming a functional zygotic nucleus is thought to be a function of the oocyte cytoplasm important for subsequent normal development. Furthermore, a stepwise passive loss of DNA methylation in the embryonic nucleus has been observed as DNA replicates between two-cell and morula stages, with somatic cell levels of methylation being re-established by, or after the blastocyst stage when differentiated lineages are formed. The ability of oocyte cytoplasm to also reprogram the genome of a somatic cell by nuclear transfer (SCNT) has raised the possibility of directing reprogramming of a somatic nucleus ex ovo by mimicking the epigenetic events normally induced by maternal factors from the oocyte. Whilst examining DNA methylation changes in normal sheep fertilization, we were surprised to observe no demethylation of the sheep male pronucleus at any point in the first cell cycle. Furthermore, using quantitative image analysis, we observed limited demethylation of the sheep embryonic genome only between the two- and eight-cell stages and no evidence of remethylation by the blastocyst stage. We suggest that the dramatic differences in DNA methylation between the sheep and other mammalian species examined call in to question the requirement and role of DNA methylation in early mammalian embryonic development.

Effects of maternal nutrition on fetal and neonatal reproductive development and function

Maternal undernutrition and, under certain circumstances overnutrition, before or during pregnancy or during early postnatal life can alter reproductive function of the offspring. Effects can be exerted at many stages of development, from prior to conception until after birth and may be expressed at the time of the nutritional insult or later. Since patterns of development differ between species, it is probably more appropriate to consider effects in relation to a stage of development rather than relative to the time of birth. Effects exerted at one stage of development may be expressed later, even if the nutritional influence is no longer present. The signals by which maternal nutrition affects the offspring must be related to maternal nutritional state and must have the capacity to reach the embryo, to be 'read' by it and to modify expression of selected genes. It is suggested that single nutrients and/or metabolites are unlikely to have direct impacts on the pattern of development of the reproductive system and it is postulated that multiple endocrine and metabolic signals are involved. Whilst it has been shown that many components of the hypothalamic-pituitary-gonadal system are modified by early life nutritional influences, understanding of the mechanisms through which these effects are exerted remains limited.

How are genes measured? Examples from studies on iron metabolism in pregnancy

As the 21st century moves forward, it is becoming more and more apparent that the genetic makeup of any individual strongly influences the way they metabolise nutrients. It is very important, therefore, to understand the techniques and technologies used to assess the contribution genes make to the physiology of an individual. Clearly, it is not possible to provide a comprehensive overview, but in the present review an attempt will be made to show, using examples from the authors' research, how these methods have contributed to this understanding. Studies are being undertaken into Fe transport across the placenta, from the mother to the fetus, and the consequences of maternal anaemia on pregnancy outcome. Levels of gene transcript and protein have been measured using Northern and Western blotting respectively. During the course of this work a new protein has been identified using the available human genome database. Following this 'in silico' or 'cyber biology', techniques such as real-time RT-PCR and RNA interference have been used to examine expression of this gene and its protein. The methods used, briefly how they work and some of their limitations will be explained. The objective of the present review is primarily to give a better perception of how molecular biology can be used in research and to help gain a clearer understanding of some of the techniques used.