When does the prenatal period of development begin?

Prenatal

The prenatal period is defined as the gestational period, from conception to birth. Research has shown that adversity exposure can induce changes in the epigenome as early as in utero and evidence is mounting that there are sensitive periods to stress exposure within this very early life period.20 Pioneering this line of research, Heijmans and colleagues examined DNAm in the maternally imprinted IGF2 gene, a key factor in human growth and development, among adults who were exposed prenatally to the famine during the Dutch Hunger Winter in 1944–45.21 Compared to their unexposed same-sex siblings, less IGF2 methylation was observed in blood among individuals exposed to the famine during the first trimester of gestation, but not those exposed during the third trimester. Remarkably, when the study was conducted, over six decades had passed since the exposure. The findings illustrate that early exposure during gestation may be a sensitive period for a lasting epigenetic mark subsequent to environmental adversity and that such an effect can persist throughout the individual’s lifetime. These findings have since been examined at the genome-wide level in which DNAm in genes involved in development and metabolic processing, such as energy metabolism (PIM3), cell function (TXNIP), glycolysis (PFKFB3), and adipogenesis (METTL8), was found to mediate the relationship between preconceptional exposure to the famine and body mass index and serum triglycerides in adulthood.22

Commonly examined prenatal stressors include maternal prenatal stress and psychopathology (e.g., depression and anxiety), which have been found to confer lifelong risk for physical and mental health in childhood and beyond.20 This fetal programming that occurs is believed to be mediated by such prenatal stress resulting in alterations in the developing hypothalamic-pituitary-adrenal (HPA) axis stress system. As such, initial research on time-dependent effects of prenatal stress exposure has focused on alterations associated with the epigenetic regulation of the HPA axis, such as the glucocorticoid receptor gene NR3C1. In fact, increased methylation of NR3C1 has been found in human cord-blood DNA of newborns prenatally exposed to maternal depressed mood, even at nonclinical levels.23 This effect was observed subsequent to exposure to maternal depressed mood in both the second or third trimester; however, the methylation status of the NR3C1 promoter was more sensitive to the exposure occurring within the third trimester. Furthermore, methylation status in newborns was associated with infant stress cortisol reactivity at 3 months old. As such, epigenetic alternations in NR3C1 occurring in the third trimester may mediate the relationship between prenatal exposure to maternal depressed mood and altered HPA function in infants, with consequences for future health trajectories.

Maternal depressed and anxious mood have been associated with biological changes that could increase fetal exposure to glucocorticoids, and thereby constitute one mechanism mediating the negative effects of prenatal stress on future neurodevelopmental outcomes. To explore this hypothesis, in vitro models have been used to mimic the effect of in utero stress exposure in humans to better understand how and when such epigenetic alterations occur during the prenatal period. In one of these studies, human multipotent hippocampal progenitor cells (HPCs) were exposed to dexamethasone (DEX), a synthetic glucocorticoid, and its immediate and longer-term effects on genome-wide DNAm were examined during proliferation, differentiation, and postdifferentiation.24 Provencal and colleagues found that stress exposure during proliferation and differentiation, but not after cell differentiation, leads to lasting changes in DNAm of the HPCs. Interestingly, the glucocorticoid-induced DNAm changes were later associated with enhanced responsivity of target gene transcripts when exposed to a repeated stressor; however, these changes were not correlated with strong gene transcription changes at baseline. The findings may indicate that in utero stress exposure may have lasting impact on the nervous system development by altering proliferation and neuronal differentiation rates and prime relevant transcripts to alter transcriptional response after further stress exposure. In other words, stress exposure during this developmental window (i.e., proliferation and differentiation) resulted in altered sensitivity to postnatal stress. These findings are in line with the metaplasticity model, which stipulates that prenatal adversity may increase the fetus’s sensitivity to the impact of the environment postnatally.25, 26

2.2.1 Prenatal Exposures

The prenatal period is critical for fetal development and is particularly critical to brain development in the context of psychiatric disorders. Neurogenesis, neuron migration, maturation, and networking, as well as synaptic pruning, all require precise procedures that could be affected by environmental factors.

The fetus is, in general, well protected by the mother's body from biological and physical insults, though viral infection, malnutrition, and maternal substance and medication use pose potential biological threats. Many of these factors have been shown to increase risk of the fetus developing psychiatric disorders, and DNA methylation may play an important role in the process.66 Even though a causal relationship has not been established,67 much evidence connecting prenatal factors to DNA methylation changes are present in animal models.68

At least two studies have connected maternal malnutrition to the risk of schizophrenia.69,70 Perinatal malnutrition was found to be linked to changes in DNA methylation, which may further lead to growth and metabolic disease.71,72 DNA methylation is at its most dynamic period during early embryonic development; maternal intake of methyl-group donors such as folate, betaine, and folic acid was found to be associated with infant buccal cell DNA methylation, though only in the periconception period.73

Prenatal alcohol exposure was found to be associated with DNA methylation changes in genes related to protocadherins, glutamatergic synapses, and hippo signaling.74

Psychological stress experienced by the mothers may have an impact on the well-being of the fetus as well, possibly by affecting DNA methylation. A small study on pregnancy during the 1998 Quebec ice storm identified altered DNA methylation in blood and saliva of children 8 and 11 years old; the DNA methylation changes were correlated with objective maternal stress.75 A new study of 121 subjects showed that even grand-maternal exposure to psychosocial stress during pregnancy had an effect on DNA methylation of the grandchildren.76

It is unlikely that DNA methylation is the only change maternal malnutrition and other stressors impose on fetal development and consequent risk of psychiatric and other disorders. Fetal neuroendocrine alteration77 and inflammation reaction78 could also lead to pathology risk.

Early Development

The prenatal period and early postnatal life are precarious times. Cell duplication and the formation of a diverse number of different cells progresses rapidly, dictated by DNA and mRNA functioning. This process is dynamic in the sense that various endogenous substrates are added as part of the instruction for the developing fetus, and then removed at appropriate times. These modifications necessarily require that particular genes be turned on and off in a well-orchestrated sequence.

Gene transcription that occurs early in embryonic development can instill a stable epigenetic state that has implications for later physiological functioning (Greenberg et al., 2017). Yet, with so many changes occurring during the course of prenatal development, especially those stemming from multiple rounds of cell multiplication, the risk for chance mutations and those that are instigated by toxicants is considerable, and should such an outcome occur, it will be amplified with cell multiplication. At the same time, dynamic epigenetic processes are thought to be important in the regulation of developmental processes, and here again, there is ample room for problems to occur owing to environmental challenges and stressors. During this time, immune system functioning is blunted so that the fetus isn’t swarmed by the maternal immune system, and to some extent, components of the immune system are functional by the second trimester (McGovern et al., 2017). Indeed, during pregnancy a mother’s immune functioning may be down-regulated, which may account for the alleviation of autoimmune symptoms women may experience during this period, but this may also leave her at risk for infection, which could impact the developing fetus. In addition to variations of peripheral immune functioning, cytokine changes are diminished in the maternal brain (Sherer, Posillico, & Schwarz, 2017), which can have profound behavioral consequences that are apparent during the postpartum period.

In the context of developmental problems, there was a time when the focus of teratogenic effects involved drugs and environmental chemicals, and to some extent immunologically related illnesses (e.g. Rubella). But, it has become clear that a wide range of factors, including moderate stressors, may affect the developing brain. It also appears that prenatal stressors may interact with genetic factors in determining the occurrence of developmental disorders as well as pathologies that emerge in adulthood. As we’ll see in Chapter 8, Depressive Disorders, individuals carrying the short variants of the gene for the 5-HT transporter (5-HTT) may be at increased risk for the development of mood disorders, provided that they also experienced strong early life or adult stressors (although there have been indications that these actions might be less straight forward than initially thought). It likewise seems that in mice, prenatal stressors were more likely to promote depressive-like features among female offspring carrying the short allele of the 5-HTT promoter gene (Van den Hove et al., 2011). Moreover, behavioral effects of prenatal immune challenge could develop through epigenetic actions on the gene coding for the 5-HTT transporter (Reisinger et al., 2016).

Placental Cellular Populations and Extraction Process

The antenatal and perinatal periods can provide a high yield of progenitor cells. These can be isolated from a variety of regions, including the amniotic fluid, placenta, yolk sac, umbilical cord, and the umbilical cord blood. In this chapter, we have focused on the cardiogenic potential of stem cells isolated from the placenta and the amniotic fluid. Other perinatal cell sources have been extensively reviewed elsewhere [10,11].

The amniotic fluid is mainly composed of water, and it provides a nutritive and a cushion support for the fetus throughout the embryonic development. Amniotic fluid stem cells (AFSCs) can be easily collected by amniocentesis during a routine prenatal diagnosis or an amnioreduction for the treatment of twin–twin transfusion syndrome. The amniotic fluid is then centrifuged and the cells are seeded and incubated at 37°C for several days to adhere and proliferate in their appropriate culture medium. On confluency, the cells are passaged and the subcultures are cryopreserved for future experimentation. Other isolation variants based on immune selection or a two-stage culture protocol are also reported [12].

The term placenta contains different cellular populations, which can be isolated from four distinct layers. The amnion is the innermost part of the placenta in direct contact with the amniotic fluid. Externally situated, the chorion surrounds the amnion and is separated from the maternal decidua basalis by a rich network of fetomaternal vasculature, constituting the chorion villi. In the 2007 First International Workshop on the placenta-derived stem cells, it was agreed that four different cell populations of multipotent potential can be isolated from the placenta. Two of them can be extracted from the amnion: the amniotic epithelial stem cells (AESCs) and the amniotic mesenchymal stromal cells (AMSCs). The two other cell populations are derived from the chorionic membrane: the chorionic mesenchymal stromal cells (CMSCs) and the chorionic trophoblastic stem cells (CTSCs) [13]. However, as later discussed in this chapter, additional reports in cellular cardiomyoplasty have indicated the isolation of progenitor cells from other placental layers such as the chorionic villi, the decidua basalis, and the complete placental tissue (Fig. 7.2).

To harvest stem cells from the human term placenta, umbilical cord blood is first allowed to drain from the placenta, which is then dissected carefully. Isolation protocols of the various cellular populations in the placenta resemble to a certain extent. The extraction procedure begins with the dissection of the anatomical region of interest followed by its enzymatic digestion. Subsequently, samples are filtered and centrifuged, and the pellets are resuspended in the corresponding medium. Stem cells are then seeded in culture plates and incubated at 37°C for several days. Adherent cells are identified as the placenta-derived stem cells. The culture medium is refreshed, and the cells are allowed to expand for subsequent passages and cryopreservation. It is noteworthy to indicate that isolation protocols differ among investigators. However, the sequential steps are similar to a large extent. The different isolation procedures are detailed in Parolini et al. [14].

Preparation of the Breasts

The prenatal period is a time for a couple to prepare for their new role as parents and to learn as much as possible about breastfeeding. Most mothers do no special preparation and are successful. Carefully controlled studies do not support the contention that fair-skinned women, especially redheads, are more prone to developing cracked, sore nipples than are others. Mothers who have had trouble with tender, cracked nipples when nursing a previous infant will need extra assistance in putting the infant to breast properly in the first few days, but elaborate rituals prenatally may actually cause problems. Nipple preparation has a negative effect on some women who are not ready to handle their breasts for these preparations during pregnancy.

Bathing should be as usual, with minimal or no soap directly on the nipples and thorough rinsing. Some recommend patting the nipple dry with a soft towel, but this should not be done except after a shower or bath. Persistent removal of natural oils of the nipple and areola actually predisposes the skin to irritation. Montgomery glands in the areola secrete a sebaceous material for the cleansing and lubrication of the areola and nipple. This should not be removed by soaps or chemicals. Tincture of benzoin, alcohol, and other drying agents are contraindicated because they predispose the nipples to cracking during early lactation.114 Wearing protective brassieres, modern women do not experience the friction to the nipples that looser clothing causes, which may be why cracked nipples are a common problem in modern society but almost unheard of in developing countries and among other mammals. In Scandinavia, it is suggested that pregnant women get as much air and sunshine as possible directly on the breasts before delivery. Wearing a nursing brassiere with the flaps down to expose the nipples under loose clothing will serve the same purpose. However, aggressive and abrasive treatment of the nipples does not prevent nipple pain postpartum and may aggravate it. Gentle love making involving the breasts is usually safe and is the most effective preparation.68

The use of lanolin, which is miscible with water and thus allows normal evaporation from the skin, does no apparent harm but in controlled studies also made no difference prenatally. Women allergic to wool will also be allergic to lanolin. Use of vitamin A and D ointment prophylactically also makes no difference, having an effect only in the treatment of fissures later.146 In climates with average to high humidity, ointments are not routinely recommended for breasts and may interfere with Montgomery gland secretion. In extremely dry climates, using ointments sparingly is often necessary.

Some believe gentle traction to the point of discomfort, but not pain, reduces perception of pain in the first week of lactation. A study carefully controlled to eliminate subjective discrepancies of interpretation revealed no significant difference in nipple sensitivity or trauma in those who practiced prenatal nipple rolling, application of breast cream, or expression of colostrum compared with those who had untreated breasts.68 No increased pain or trauma was reported among fair-skinned participants in the study, treated or untreated. Because many women are not inclined to manipulate their breasts before delivery and might be discouraged from breastfeeding if it is implied that this must be done, physicians should prescribe treatment only when an indication exists.186

Early life immune regulation

The prenatal period is instrumental in shaping a child’s immune system (“programming”) influenced by a wide variety of factors elucidated below, including microbiome, nutrition, smoke exposure, and infection, among many others (Figure 1). This window of opportunity is thus critical for a wide range of risk and protective influences discussed in more detail below. Multifaceted effects on early immune programming can occur prenatally that are critical for effects on local tissues and relevant for risk for or protection from immune-mediated diseases, which may occur only several years later. Thus, an efficient interplay between innate and adaptive immune regulation can shape the maturing immune system, keeping it balanced over numerous years during childhood. Any default regulation affecting solely parts of the system or even cells can result in different immune-mediated diseases such as infections, more chronic diseases such as, for example, allergies, autoimmune diseases, or lack of tolerance.

Whereas bidirectional interactions between the fetus and mother seem critical for postnatal immune regulation, human studies on causal effects are complex because of multifaceted influences that are difficult to study at this time of maturation.

In addition to genetic factors such as “immutable footprints”, epigenetics, the environment, and their interactions influence early immune programming, subsequently affecting anatomical structures such as the mucosa and epithelium, and influencing barrier function.

The exact nature underpinning intrauterine modulatory mechanisms may be composed of the following: Although the amniotic fluid has been shown not to be sterile, direct and indirect modulation via fetoplacental transfer may occur. Decidual tissue maternal immune cells including macrophages, CD8+ and γδ-T cells, and large granulated lymphocyte cells are able to induce rejection of paternal histocompatibility antigens. Maternal–fetal tolerance to paternal alloantigens is actively mediated, involving pTregs (peripheral Tregs) distinctly responding to paternal antigens for tolerance induction.11 Generally, maturation of the infant adaptive immune system occurs from the 15th to 20th week of gestation and can be Ag-specific.

Postnatal immune maturation influences are comparable to before birth, with the major difference in the absence of direct maternal environment. Whereas effects on immune programming most likely happen continuously with different thresholds on several types of immune cells, numerous factors induce changes directly in the organs subsequently affected by later disease. For allergic diseases, for example, airway antigen-presenting cells are most likely involved in local damage during airway inflammation and are critical to programming of adaptive T cell responses after migration to the lymph nodes.

What age is the prenatal period?

Where does the prenatal period begin?

What are the 3 stages periods of prenatal development?

What is the first stage of prenatal development?