Extrapolation of experimental results using mouse and avian embryonic models demonstrates that environmental factors present in utero during the second to third week of human gestation (human, 16 to 19 days after fertilization; mouse, embryonic days ED 6.75 to 7.5; avian, HH stage 4 to stage 5-) can alter early developmental processes resulting in severe cardiac anomalies [1
]. Some perturbations may alter development in a manner that may not be clinically evident at birth, but result in increased susceptibility to cardiac problems after birth, or increasingly as we age. In a recent study published in September 2012 [6
], NIH Common Fund researchers reported on genome-wide association studies (GWASs) of genetic variants, specifically pertaining to noncoding regions of DNA that actively regulate gene expression. They found that 88 percent of GWAS variants are in regulatory DNA regions of genes that are active in fetal development
, including variants associated with adult-onset disease. The finding that a large number of disease-associated variants are located in regulatory DNA regions that are active during embryonic development suggests that environmental exposures affecting the intrauterine milieu during early embryonic stages can influence risk for not only birth defects, but also a large number of adult diseases. This concept of developmental origins of health and adult disease arose with data that the intrauterine milieu influences cardiovascular and mental diseases long into adulthood [7
Early in human gestation following conception, the fertilized embryo implants in the uterine wall. The successful implantation requires adequate maternal uterine perfusion and endogenous hormone preparation to allow initial embryonic survival and later organ development and growth for a term gestation. Shortly after implantation, cardiomyocyte specification and commitment take place between 16–19 days postconception (pc). The circulation and a beating tubular heart are established by 21 days pc in human pregnancy. In mouse development, cardiac specification corresponds to embryonic day (E) 6.75 to E7.5 whereby the cardiac crescent forms. Next, a beating, linear, and tubular heart forms that then loops and septates (E10.5) to form a four-chambered heart (4–6 weeks of human gestation). Human cardiac morphological development is complete by 8 weeks of age.
Abnormal cardiac development leading to congenital heart disease can be associated with abnormal placental development with abnormal trophoblast invasion and remodeling and the resultant abnormal transfer of nutrients and oxygen from the mother to the fetus [10
]. Studies relating to germ-line ablation of specific genes, as p38α-mitogen activated protein (MAP) kinase or of peroxisome proliferator-activated receptors (PPARs)
, demonstrated they were critical for proper placental function and, when ablated only in the placenta
, resulted in cardiac defects. When the placental function was rescued via aggregation with tetraploid embryos (tetraploid morulas failed to contribute to embryonic tissues, only placental), this corrected the cardiac defect [10
]. These studies highlight the existence of a critical heart-placental axis that is just beginning to be recognized.
The heart-placental axis is associated with parallel development of the placenta and heart that utilizes many common molecules and genes [11
] and reflects intimate and synergistic growth of both organs. Two important pathways critical to placental development, implantation, and cardiac development during the same period of early development are canonical Wnt/β
-catenin signaling and pathways associated with folate metabolism [17
]. If these pathways are altered, developmental anomalies ensue. Failure to maintain a pregnancy or have normal cardiac development may be due to abnormal placental function or cardiac function. Fetal loss can occur prior to usual detection of pregnancy at 5 to 6 weeks of gestation (estimated to be a significant percent of all fertilized embryos) or occurs later after initiation of the embryonic heartbeat. Placental volume blood flow is a major determinant of early cardiac output, fetal growth, and well-being [22
]. The associated placental abnormality during gestation of the fetus with congenital heart disease has deleterious effects on the outcome of pregnancy. There may be such severe starvation of the fetal circulation and growth failure that growth arrest, severe hypoxemia and marginal cardiac output occur with spontaneous loss of the pregnancy. Premature delivery may result in birth of a neonate with poor prognosis related to the prematurity and low birth weight. Management of the neonate with prematurity and congenital heart disease is associated with a very high perinatal mortality.
The underlying mechanisms by which environmental exposures can result in congenital birth defects, as well as disease much later in life, have had no clear experimental definitions, but there is evidence that cellular processes associated with cell fate decisions and cell lineage modulation are involved [4
]. In the case of cardiomyocyte specification, it is among the earliest cell-fate decisions to occur, because the heart-cardiovascular system is the first organ-system to develop. The statistic that 49% of all pregnancies are unplanned [25
], would indicate the first month is a high risk period for the early embryo and specifically for cardiovascular development, neurogenesis, and for placentation in that in all three organ systems important steps in cell specification and early differentiation processes are taking place that are regulated by the same signaling pathways, specifically as we have reported, Wnt/β
The focus of my research has been on early stages of heart development, specifically regulation of cardiac cell specification and differentiation using the mouse and avian embryonic models. In the human embryo, a beating, tubular heart with extra- and intraembryonic circulations bringing blood to the heart is present at 21 days. Because cardiac cell fate decisions are taking place several days earlier, approximately at 16 to 19 days of human pregnancy, these early periods of human gestation are not possible to analyze on a cell and molecular level. Thus, the mouse model is an excellent mammalian model to analyze developmental processes leading to a functional heart. With present day technologies, both cardiac structural and functional relationships can be monitored in the mouse embryo using Doppler ultrasound ().
Figure 1 Color Doppler (top left) and pulsed directed Doppler were used to obtain blood waveform patterns. Normal ventricular inflow and outflow (IF-OF) pattern is shown in top right. Locations of waveforms obtained for ductus venosus (DV), descending aorta (DA), (more ...)
Noninvasive evaluation of fetal heart function during early human pregnancy using clinical echocardiography is challenging. The earliest variables of early fetal cardio-circulatory dynamics have been established for 11–14 weeks of gestation [26
]. Those critical developmental processes are already well underway in cardiogenesis by the time pregnancy is usually confirmed at weeks 5/6 after-fertilization which may relate to why congenital heart defects arise in 1% of births world-wide and of all birth defects are the most common. As the heart is the first organ-system to develop, followed closely by the nervous system, our recent research on early development using environmental exposure during gastrulation may help to explain why cardiac birth defects are so prevalent and are the leading cause of infant mortality. In the first month, women may be continuing to use prescribed medications or leisure drugs, to smoke, and to drink alcohol, may be type-2 diabetic or obese, and not be following neither healthy lifestyles or diets, nor protecting their embryos with prescribed perinatal folate supplementation.
Multiple early processes are associated with similar signaling pathways in heart, placental, and neural development, and thus development of different tissues in the same embryos can be affected with the same exposures [27
]. In a recent study of infants in the Atlanta metropolitan area, 28.7% of infants with CHDs also had associated major noncardiac malformations [28
]. If one reads the literature provided to women upon learning of their pregnancy usually at 5 to 6 weeks after conception, pamphlets predominantly focus on neural development and the use of folate during pregnancy with targeting the second to third month of gestation. Possibly due to a lack of knowledge on effects of environmental exposures during early embryonic development, generally there is little discussion of adverse effects on cardiac outcomes, importance of planned pregnancy, and the use of folate before becoming pregnant and continuing until pregnancy is confirmed, so as to protect the earliest stages of embryonic development, specifically of the cardiovascular system [5
], placentation [29
], and neurogenesis [30