Continuing growth and development of the third-trimester human placenta

Low birth weight and its associated factors in Ethiopia: a systematic review and meta-analysis

Background

Different primary studies in Ethiopia showed the burden of low birth weight. However, variation among those studies was seen. This study was aimed to estimate the national prevalence and associated factors of low birth weight in Ethiopia.

Methods

PubMed, Web of Science, Cochrane library, and Google Scholar were searched. A funnel plot and Egger’s regression test were used to see publication bias. I-squared statistic was applied to check heterogeneity of studies. A weighted inverse variance random-effects model was applied to estimate the national prevalence and the effect size of associated factors. The subgroup analysis was conducted by region, study design, and year of publication.

Result

A total of 30 studies with 55,085 participants were used for prevalence estimation. The pooled prevalence of LBW was 17.3% (95% CI: 14.1–20.4). Maternal age < 20 years (AOR = 1.7; 95% CI:1.5–2.0), pregnancy interval < 24 months (AOR = 2.8; 95%CI: 1.4–4.2), BMI < 18.5 kg/m² (AOR = 5.6; 95% CI: 1.7–9.4), and gestational age < 37 weeks at birth (AOR = 6.4; 95% CI: 2.5–10.3) were identified factors of LBW.

Conclusions

The prevalence of low birth weight in Ethiopia remains high. This review may help policy-makers and program officers to design low birth weight preventive interventions.

A role for cellular senescence in birth timing

Senescence contributes to the local and systemic aging of tissues and has been associated with age-related diseases. Recently, roles for this process during pregnancy have come to light, the dysregulation of which has been associated with adverse pregnancy outcomes such as preterm birth. Here, we summarize recent advances that support a role for senescence in birth timing and propose new aspects of study in this emerging field.

Differential expression profile of growth hormone/chorionic somatomammotropin genes in placenta of small- and large-for-gestational-age newborns

CONTEXT

The human growth hormone/chorionic somatomammotropin (hGH/CSH) locus at 17q22-24, consisting of one pituitary-expressed postnatal (GH1) and four placenta-expressed genes (GH2, CSH1, CSH2, and CSHL1), is implicated in regulation of postnatal and intrauterine growth. A positive correlation has been reported between the offspring's birth weight and serum placental GH (coded by GH2) and placental lactogen (coded by CSH1, CSH2) levels in pregnant women.

OBJECTIVE

The objective of the study was the investigation of the hypothesis that the mRNA expression profile of placental hGH/CSH genes contributes to the determination of birth weight.

DESIGN AND SUBJECTS

We developed a sensitive, fluorescent-labeled semiquantitative RT-PCR assay coupled with gene-specific restriction analysis, capable of distinguishing alternative splice-products of individual placental hGH/CSH genes and quantification of their relative expression levels. The detailed profile of alternative transcripts of GH2, CSH1, CSH2, and CSHL1 genes in placenta from uncomplicated term pregnancies of the REPROMETA sample collection was addressed in association with the birth weight of newborns, grouped as appropriate for gestational age (AGA; n = 23), small for gestational age (SGA; n = 15), and large for gestational age (LGA; n = 34).

RESULTS

The majority of pregnancies with SGA newborn showed down-regulation of the entire hGH/CSH cluster in placenta, whereas in the case of LGA, the expression of CSH1-1, CSH2-1, and CSHL1-4 mRNA transcripts in placenta was significantly increased compared with AGA newborns (P < 0.0001, P = 0.009, P = 0.002, respectively).

CONCLUSION

The expression profile of placental hGH/CSH genes in placenta is altered in pregnancies accompanied by SGA and LGA compared with AGA newborns, and thus, it may directly affect the circulating fetal and maternal placental GH and placental lactogen levels.

Fetal response to intrauterine stress

Many human infants are born inappropriately small as a result of stress suffered during intrauterine life. Acute reductions in oxygen delivery to fetal tissues have therefore been studied in animals so that insight can be obtained into the adaptive mechanisms that underlie human developmental abnormalities. It is now known that during moderate hypoxic stress fetal arterial blood pressure is variably increased while heart rate and cardiac output are depressed; blood volume is reduced but cardiac output is redistributed to spare the myocardium, brain and adrenal glands at the expense of most other organs. Also a greater fraction of oxygen-rich venous blood from the placenta is returned to the heart for distribution. Spared organs are those that grow disproportionately well in human asymmetrical intrauterine growth retardation (IUGR). These cardiovascular responses are not fully understood although elevated fetal plasma levels of catecholamines and a host of fetal hormones are undoubtedly important. Chemical sympathectomy does not abolish the blood flow redistribution phenomenon, which implies that autoregulatory effects may be responsible for some of the redistribution of blood flow. Fetal hypoxaemia and metabolic abnormalities are sequelae often found with human IUGR, suggesting placental exchange defects. IUGR placentas appear to have defective transport mechanisms for many nutrients. Animal studies suggest that the placenta will give priority to its own needs over those of the fetus, when necessary, to support its own growth and function.

SNAT4 isoform of system A amino acid transporter is expressed in human placenta

The system A amino acid transporter is encoded by three members of the Slc38 gene family, giving rise to three subtypes: Na+-coupled neutral amino acid transporter (SNAT)1, SNAT2, and SNAT4. SNAT2 is expressed ubiquitously in mammalian tissues; SNAT1 is predominantly expressed in heart, brain, and placenta; and SNAT4 is reported to be expressed solely by the liver. In the placenta, system A has an essential role in the supply of neutral amino acids needed for fetal growth. In the present study, we examined expression and localization of SNAT1, SNAT2, and SNAT4 in human placenta during gestation. Real-time quantitative PCR was used to examine steady-state levels of system A subtype mRNA in early (6-10 wk) and late (10-13 wk) first-trimester and full-term (38-40 wk) placentas. We detected mRNA for all three isoforms from early gestation onward. There were no differences in SNAT1 and SNAT2 mRNA expression with gestation. However, SNAT4 mRNA expression was significantly higher early in the first trimester compared with the full-term placenta (P < 0.01). We next investigated SNAT4 protein expression in human placenta. In contrast to the observation for gene expression, Western blot analysis revealed that SNAT4 protein expression was significantly higher at term compared with the first trimester (P < 0.05). Immunohistochemistry and Western blot analysis showed that SNAT4 is localized to the microvillous and basal plasma membranes of the syncytiotrophoblast, suggesting a role for this isoform of system A in amino acid transport across the placenta. This study therefore provides the first evidence of SNAT4 mRNA and protein expression in the human placenta, both at the first trimester and at full term.

Predictive value of hormone measurements in maternal and fetal complications of pregnancy

Intrauterine tissues (placenta, amnion, chorion, decidua) express hormones and cytokines that play a decisive role in maternal-fetal physiological interactions. The excessive or deficient release of some placental hormones in association with gestational diseases may reflect an abnormal differentiation of the placenta, an impaired fetal metabolism, or an adaptive response of the feto-placental unit to adverse conditions. This review is focused on the applicability of hormone measurements in the risk assessment, early diagnosis, and management of pregnancies complicated by Down's syndrome, fetal growth restriction, preeclampsia, preterm delivery, and diabetes mellitus. Combined hormonal tests or the combination of hormones and ultrasound may achieve reasonable sensitivity, but research continues to simplify the screening programs without sacrificing their accuracy. Only in a few instances is there sufficient evidence to firmly recommend the routine use of hormone tests to predict maternal and fetal complications, but the judicious use of selected tests may enhance the sensitivity of the risk assessment based solely on clinical and ultrasound examination.

Placental pathology of fetal growth restriction

It is evident that there is not a single uteroplacental or villous lesion that results in fetal growth restriction. It is more likely that it is the accumulation (or total burden) of placental injury that, when present for a sufficient time interval, leads to FGR. Future studies that focus on patterns of lesions, rather than on individual lesions, may prove to be more rewarding in elucidating the causal pathways by which placental histopathology is translated into FGR and its attendant neonatal and pediatric sequelae.

Human placental growth hormone

Placental growth hormone is the product of the GH-V gene specifically expressed in the syncytiotrophoblast layer of the human placenta. Placental growth hormone differs from pituitary growth hormone by 13 amino acids. It has high somatogenic and low lactogenic activities. Assays by specific monoclonal antibodies reveal that in the maternal circulation from 15 to 20 weeks up to term placental growth hormone gradually replaces pituitary growth hormone, which becomes undetectable. It is secreted by the placenta in a nonpulsatile manner. This continuous secretion appears to have important implications for physiologic adjustment to gestation and especially in the control of maternal insulin-like growth factor-I levels. Placental growth hormone secretion is inhibited by glucose in vitro and in vivo and is significantly decreased in the maternal circulation in pregnancies with intrauterine growth restriction. Placental growth hormone does not appear to have a direct effect on fetal growth because this hormone is not detectable in the fetal circulation. However, the physiologic role might also include a direct influence on placental development through an autocrine or paracrine mechanism, as suggested by the presence of specific growth hormone receptors in this tissue.

Stereology and its impact on our understanding of human placental functional morphology

The new stereology permits the objective, quantitative description of morphology by efficient and design-based methods. Applications to placentas in normal and abnormal pregnancies have proved of great value for challenging earlier misconceptions and interpreting better the processes of growth, morphogenesis, adaptation, and functioning at the whole-organ level. This contribution reviews the essential features of the stereological approach, identifies useful structural quantities, and provides examples of their application in various experiments of nature. We focus particularly on normal gestation and the effects of pregnancies associated with high altitude, maternal diabetes mellitus, preeclampsia, and maternal smoking.

Decreased expression of placental growth hormone in intrauterine growth retardation

During normal pregnancy, the levels of placental GH in the maternal circulation increase significantly until 35 wk of gestation. We have previously shown that these levels are significantly reduced in cases of intrauterine growth retardation (IUGR). To better understand the basis of this observation, we have studied the expression of placental GH in placentas from normal births (n = 6) and births with IUGR (n = 5). In situ hybridization histochemistry was used to determine the mean number of cells per area expressing this message, as well as the mean level of specific mRNA per cell. We have found that the mean mRNA signal level per cell of placental GH did not differ between normal or IUGR placentas. However, the mean number of cells/ area expressing this mRNA was significantly greater in normal placentas compared with IUGR placentas (normal 12.8 +/- 0.9 cells/unit area, IUGR 4.9 +/- 2.4 cells/unit area, analysis of variance: p < 0.004). These data suggest that the decreased levels of placental GH in the maternal circulation in IUGR are not due only to the reduced size of the placenta, but also to changes in the placental tissue which result in a reduced number of cells per area that are capable of producing this peptide.

A stereologic study of postmature placentas fixed by dual perfusion

OBJECTIVE

Placental insufficiency has been considered the cause of increased morbidity in infants delivered postmaturely. Former quantitative studies have indicated a decrease in some placental structures just before term. In this study we describe a method of dual perfusion fixation to provide tissue for stereologic examination. Postmature placentas were examined with this method.

STUDY DESIGN

Eleven postmature placentas and 14 placentas delivered at term were fixed by dual perfusion. The volume and the surface area of villi, the trophoblast volume, and the volume, surface area, and length of villous capillaries were estimated by stereologic examination. The Mann-Whitney test (p < or = 0.05) was used for statistical analysis.

RESULTS

Morphologic features were normal in all placentas. No significant differences were disclosed in the stereologic estimates of placentas delivered at term and postmature placentas.

CONCLUSION

No morphologic or significant quantitative changes were found in postmature placentas.

Epidermal growth factor receptor and human fetoplacental development

Alteration of placental development directly interferes with fetal growth. Epidermal growth factor (EGF) plays a major role in placental implantation, growth and differentiation. EGF acts on its placental target cells, i.e. the trophoblasts, via a specific receptor (EGFR) which belongs to the tyrosine kinase receptor family. Abundant placental EGF receptors are located in the brush border at the fetomaternal interface. EGFR expression is modulated by trophoblast differentiation and by hormones or toxic substances such as smoke. Interestingly, in microvilli purified from placentae of infants with intrauterine growth retardation (IUGR) a decrease or absence of tyrosine kinase activity is observed. This suggests that an alteration of EGFR biological activity might interfere with the fetoplacental unit development.

Alteration of epidermal growth factor receptor in placental membranes of smokers: relationship with intrauterine growth retardation

OBJECTIVE

The study of a possible alteration of epidermal growth factor receptor in placental membranes of smokers was performed.

STUDY DESIGN

We investigated the binding capacity and the autophosphorylation of epidermal growth factor receptor in membranes of 33 term placentas from smokers and nonsmokers with appropriate-for-gestational-age or small-for-gestational-age babies.

RESULTS

The binding capacity of epidermal growth factor receptor was similar in the four subgroups of placental membranes. In contrast, epidermal growth factor receptor autophosphorylation was impaired in the membranes from smokers relative to those from nonsmokers. This decrease was significant (p < 0.001) in the membranes from smokers with small-for-gestational-age babies. Insulin receptor phosphorylation was similar in the four groups.

CONCLUSION

Intrauterine growth retardation in women who smoke is associated with an alteration of placental epidermal growth factor receptor bioactivity and suggests that the regulatory role of epidermal growth factor in placental growth and differentiation is defective in this setting.

Use of proliferation cell nuclear antigen immunoreactivity for distinguishing hydropic abortions from partial hydatidiform moles

AIMS

To determine whether the expression of proliferating cell nuclear antigen (PCNA) in villous cytotrophoblast could distinguish between placental tissue from a hydropic abortion and that from a partial hydatidiform mole.

METHODS

Tissue from 18 partial hydatidiform moles, 15 hydropic abortions, five normal first trimester placentas and five normal full term placentas were immunostained for expression of PCNA, using the monoclonal antibody PC10.

RESULTS

PCNA immunoreactivity was very much higher in the cytotrophoblast of normal first trimester placentas than in normal term placentas. Villous tissue from partial hydatidiform moles showed, on average, less immunoreactivity for PCNA than did villous tissue from hydropic abortions.

CONCLUSIONS

Immunostaining for PCNA is of no value for differentiating between partial hydatidiform moles and hydropic abortions. The findings indicate that trophoblastic proliferation or hyperplasia is not a feature of partial hydatidiform moles.

From 13 weeks to term, the trophoblast of human placenta grows by the continuous recruitment of new proliferative units: a study of nuclear number using the disector

A method is presented for obtaining assumption-free estimates of the number of nuclei in the trophoblast of the human placenta and for defining the size of the trophoblast proliferative unit (TPU). The method relies on the disector, a stereological device for counting arbitrary particles in 3-dimensional space using pairs of parallel sections separated by a known distance. It is applied to investigate factors which contribute to trophoblast growth from 13 weeks of gestation to term. Physical disectors were sampled systematically using adjacent 4-4.6 microns thick paraffin sections. Nuclei in the trophoblast (syncytial and cellular) were counted if they appeared in an unbiased counting frame on one section but were absent from the adjacent section. Nuclear packing densities were converted to absolute numbers of nuclei by using placental volume as the reference space. At 37-39 weeks, the average placenta contained 6.4 x 10(10) trophoblast nuclei of which 90 per cent were located within the syncytium and the remainder in the cytotrophoblast. From a knowledge of total trophoblast volume, it was found that each nucleus is associated with 970 microns3 of trophoblast and each cytotrophoblast cell with 11,000 microns3. The latter may be regarded as the volume of a TPU. From 13 weeks of gestation to term, there was a ninefold increase in nuclear number but the trophoblast volumes associated with nuclei, including the size of the TPU, remained constant. Growth of trophoblast is purely hyperplastic and occurs by recruitment of new TPUs.

Quantitative description of the elaboration and maturation of villi from 10 weeks of gestation to term

Stereological methods have been applied to a cross-sectional sample of human placentae collected at 10-41 weeks of gestation in order to provide a quantitative description of the growth and maturation of villi. Random tissue sections were analysed to derive volumes, surface areas, lengths, diameters and membrane thicknesses for villi and their fetal capillaries. Expansion of the total volume and surface area of villi can be explained by a dramatic linear growth of terminal villi which begins at about the middle of the second trimester. Growth of intermediate villi also occurs but to a more limited extent. Linear growth is accompanied by villous maturation which involves increases in the relative volume of capillaries and in villous capillarization coupled with decreases in villous diameter, capillary diameter and harmonic thickness of the villous membrane. These findings confirm that placental growth and development depend greatly on growth and maturation of terminal villi. They do not confirm sinusoidal dilation of fetal vessels as a generalized phenomenon. They also support the contention that changes in effective diffusion distances across the villous membrane have real adaptive significance.

A contemporary view of the human placenta

Our current knowledge of the human placenta is briefly reviewed. Particular stress is placed upon the considerable functional reserve capacity of the placenta, the unimportance of most visible abnormalities of the placenta, the lack of any evidence that the placenta ages during gestation and the lack of significance of placental weight. The effects on the placenta of infection and of maternal cigarette smoking are considered and the concept of placental insufficiency critically discussed. It is concluded that most cases of 'placental insufficiency' are, in reality, examples of maternal vascular insufficiency resulting from inadequate placentation during the early stages of pregnancy.

Ultrasonographic assessment of placental abnormalities

Current ultrasonographic techniques offer a novel approach for the identification of a wide variety of placental abnormalities usually described postnatally by the pathologist. Placental vascular lesions, placental tumors, and abnormal placentation are potentially associated with perinatal complications and their diagnosis in utero may influence the pregnancy management. An ultrasonographic classification of placental lesions that is based on their location, size, echogenicity, and number is proposed. Repeated ultrasonographic examination, together with biologic investigations, is important for the prenatal differential diagnosis of most these lesions and for full understanding of their pathophysiologic characteristics and significance.

Methodological problems in placental morphometry: apologia for the use of stereology based on sound sampling practice

Several problem areas in morphometry of human and animal placentae are reviewed. Attention is given to methods of tissue processing (handling, mode of fixation, embedding, shrinkage) and sampling (of organs, tissue blocks, sections, micrographs). Principal sources of bias and sampling variability are identified and the crucial importance of randomized sampling is emphasized. Methods for obtaining structural quantities from sections are compared. The case is made for estimating absolute values (volumes, surface areas, lengths, numbers, thicknesses) using stereological principles rather than relying on planar data (profile areas, perimeter lengths, numbers, apparent thicknesses). Absolute values may be obtained simply and efficiently without resort to expensive measuring devices. Finally, morphological descriptors suitable for correlating with functional data or for comparing normal and diseased organs are surveyed.