**1. Introduction**

178 From Preconception to Postpartum

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#### **1.1 Vessel growth and angiogenesis**

The process that involves vessel formation and growth has been described by the terms vasculogenesis and angiogenesis, as two distinct types [Demir et al., 2007]. Both of these processes are essential for normal uteroplacental development:


The vascularisation of placental villi starts at day 21 post conception (dpc), being the result of local de novo formation of capillaries rather than protrusion of embryonic vessels into the placenta [Demir et al., 1989]. Mesenchymal cells inside the villi transform into hemangiogenic precursor cells that migrate toward the periphery. In the latter, prior to the formation of the first vessels that are observed by about 28 dpc (erythrocytes are detected by 32 dpc), mesenchymal derived macrophages (Hofbauer cells) appear. Those macrophages will express angiogenic growth factors and, as they appear early, they suggest a paracrine role in the initiation of vasculogenesis. Angiogenic growth factors are also expressed by the

Placental Angiogenesis and Fetal Growth Restriction 181

Two types of FGR have been described: early onset, that the growth restriction is symmetrical and late onset that restriction of growth is asymmetrical. The first one is more severe and usually has its underline cause in a specific defect that acts from the beginning of conception such as chromosomal anomalies, infections or substance abuse. Second and more common is less severe with smaller impact on the fetus and the causes may vary. Many elements can be of greater or lesser importance in the progress of this entity. Incomplete placentation and factors controlled by hypoxia are the most common pathophysiologic

A successful pregnancy outcome depends on the proper development of the fetoplacental vasculature in the villous core, which begins with the infiltration of cytotrophoblast in the endometrium and is completed in conjunction with the spiral arteries. It is widely accepted that shallow trophoblast invasion can lead to fetal hypoxia and impaired growth [Mayhew et al., 2004]. The proper and timely proliferation and differentiation of the villous cytotrophoblast stem cells, which are controlled by hypoxia, are crucial for adequate placentation [James et al., 2006]. Thus, the entire repertoire of hypoxia-associated growth

Angiogenesis is a placental factor playing an important role in the development of FGR. FGR occurs as a result of adequate vascular transformation and of terminal villous

Based on the type of FGR (early or late onset) there are two models that may occur in the placental tissue. First is due to uteroplacental insufficiency and results to increased branching angiogenesis. Second and more common is due to placental failure and is accompanied by straight and unbranched capillaries, along with reduced cytotrophoblast proliferation, increased syncytial nuclei and erythrocyte congestion. All these suggest an increased rate of trophoblast proliferation. This situation has been interpreted as placental hyperoxia. Thus, fetoplacental blood flow is severely impaired and transplacental gas exchange is poor, placing the fetus at risk of hypoxia and acidosis [Macara et al., 1996].

Factors that are regulated by oxygen concentration are mainly important for the placental tissue to respond to hypoxic events. The most important known factors of this subgroup are

Hypoxia Inducible Factors (HIF) are transcriptional factors that were discovered in mammalian cells under conditions of low oxygen, and appear to have a fundamental role in the cellular and systematic response to hypoxia, via the regulation of metabolism, cellular cycle, angiogenesis and apoptosis. HIFs are heterodimers constituted from the HIFa (1, 2 or

fundamental

HIFa subunit is found in the cytoplasm and is transported in the nuclei in order to form HIF with the other subunit ARNT, which is expressed regularly in the nuclei, and achieve its transcriptional role. It has been found that transcriptional action of HIFa is achieved via bHLH (basic helix-loop-helix)-PAS domain in N-terminal. For this action the recruitment of

factors is remarkably active during placental development.

**1.3 Fetal growth restriction and angiogenesis** 

formation [Ahmed et al., 2000].

hypoxia inducible factors (HIF).

**2. Hypoxia inducible factors** 

3) and HIFb or ARNT.

mechanism.

maternal decidua and macrophages mediating the trohpoblast invasion [Ahmed et al., 1995]. In the meanwhile since the uterus and its contents demand an increased supply of blood during pregnancy, its vasculature undergoes three main adaptative changes: vasodilation, increased permeability, and growth and development of new vessels. So the villi mature and angiogenesis begins at around 32 dpc [Zygmunt et al., 2003].

Branching or sprouting angiogenesis (lateral ramification of pre-existing tubes) is first observed leading to the formation of a capillary network. During branching angiogenesis multistep processes are taking place including increased vascular permeability, degradation of basement membrane, increase in endothelial cell proliferation and migration, formation of endothelial cell tubes, and recruitment of pericytes to the outside of the capillary to form a stable vessel [Kaufmann et al., 2004]. From the 6th week a basal lamina begins to form around the capillaries that results to a web like arrangement of capillaries within the stroma of mesenchymal villi, and a superficially location of most of the capillaries in the immature intermediate villi (beneath the trophoblast, covering the villous surface). In the latter, from the 15th week onwards, fibrosal stromal core is being formed by the fusion of the adventicia of large central vessels, thus becoming a stem villus. In these larger villi a few central endothelial tubes (or early villous arteries and veins) have larger diameters, up to 100mm, and become surrounded by cells expressing alpha and gamma smooth muscles actins, vimentin and desmin. These contractile cells concentrate around the lumina, acquiring the full spectrum of cytoskeleton antigens. Following branching angiogenesis, non branching angiogenesis is observed from 24 weeks of gestation, due to the formation of mature intermediate villi, specialized in gas exchange. Those villi contain 1-2 long, poorly branched capillary loops, which coil and bulge through the trophoblastic surface, forming the terminal villi. These structures are the main site of diffusional gas exchange between the maternal and fetal circulations. This process includes decreased trophoblast proliferation and increased endothelial proliferation [Benirschke and Kaufmann, 1995].

As gestation increases, the terminal capillaries focally dilate and form large sinusoids, which counterbalance the effects of the long poorly branched capillaries on total fetoplacental vascular impedance. Increasing fetal blood pressure aids this dilation and fetoplacental blood flow rises throughout gestation to 40% of fetal cardiac output at term [Ahmed et al., 2000].

Pseudovasculogenesis is the process which remodelling of maternal uterine vessels occurs. Until 6 weeks post conception uterine arteries (spiral arteries) have high resistance and low capacity. After cytotrophoblast invasion they breakdown smooth muscle cells and replace maternal endothelial cells resulting to low resistance and high capacity vessels. Same process, but less extent occurs also in maternal veins. Pseudovasculogenesis is completed around 20 weeks of gestation.
