Regular ArticleGrowth and function of the normal human placenta
Introduction
The placenta is the highly specialised organ of pregnancy that, along with the fetal membranes and amniotic fluid, support the normal growth and development of the fetus. Changes in placental development and function have dramatic effects on the fetus and its ability to cope with the intra-uterine environment. Implantation and the formation of the placenta is a highly coordinated process involving interaction between maternal and embryonic cells. Trophoblast cell invasion of uterine tissues and remodelling of uterine spiral arterial walls ensures that the developing feto-placental unit receives the necessary supply of blood and that efficient transfer of nutrients and gases and the removal of wastes can take place. Different types of placentation are categorised according to the number and types of layers between the maternal and fetal circulations [1]. The human placenta is a haemochorial villous organ, whereby maternal blood comes into direct contact with placental trophoblast cells and allows an intimate relationship between the developing embryo and its supply of nutrients.
This review describes the basic structural arrangements of the mature human placenta and fetal membranes and gives an overview of the processes of implantation, decidualisation and placentation that result in their formation. Finally, the main functions of the placenta are discussed under the headings of transport and metabolism, protection and endocrine function.
Section snippets
The structure of the mature placenta
The utero-placental unit is composed of both fetal tissue derived from the chorionic sac and maternal tissue derived from the endometrium. In the mature placenta, the fetal aspect is called the chorionic plate. This region carries the fetal chorionic blood vessels, which are branching radials from the umbilical vessels. The maternal aspect of the placenta is called the basal plate. In between these two regions is the intervillous space, which contains the main functional units of the placenta,
The placental membrane
The term placental membrane (sometimes called the placental barrier) refers to the layers of cells that separate the maternal blood in the intervillous space and the fetal blood in the vasculature in the core of the villi [4]. Initially, the placental membrane is made up of four layers, the maternal facing syncytiotrophoblast, a layer of cytotrophoblast cells, connective tissue of the villus and the endothelium lining the fetal capillaries (Fig. 1B). By approximately 20 weeks, however, the
The fetal membranes
The fetal membranes contain the fetus throughout the pregnancy and eventually undergo programmed rupture during the first stage of labour. They consist of the fetal-facing amnion and maternal-facing chorion [5]. The amnion comprises five distinct layers. The innermost layer is the amniotic epithelium, which is in direct contact with the amniotic fluid on one side and a basement membrane on the other. The other layers consist of the compact layer, the fibroblast layer and the spongy or
Implantation
The period for uterine receptivity for implantation is relatively short. Physiological preparation of the endometrium is modulated by cyclic secretion of 17β-estradiol and progesterone. These hormones regulate growth factors, cytokines and adhesion molecules that alter the endometrial surface and open the implantation window [3]. Other substances, such as fibronectin, close the window several days later. Prior to attachment to the endometrial epithelium, the zona pellucida surrounding the
The decidual reaction
Decidualisation of the endometrial stroma occurs as part of the normal menstrual cycle; however, in the event of pregnancy, decidual changes become more extensive. Glycogen and lipids accumulate in the cytoplasm of the cells causing them to enlarge and take on the appearance of the pale-staining decidual cells. The cellular and vascular changes of the endometrium as the blastocyst implants is referred to as the decidual reaction. The function of the decidua, however, is not certain. Rather than
Trophoblast development
After successful implantation and initiation of placentation, trophoblast cells undergo extensive proliferation and differentiation. There are two main pathways by which trophoblast differentiation may occur, that is, villous and extravillous (Fig. 2). By days 13 to 14 of pregnancy, cytotrophoblast cells penetrate the layer of syncytiotrophoblast surrounding the early conceptus to form columns of extravillous cytotrophoblast cells. These contiguous cells form the cytotrophoblastic shell that is
Blood vessel formation
The microvascular fetal capillary networks within the terminal villi are the culmination of the vascular tree of the placenta that originates with the macrovascular arteries and vein of the umbilical cord. The umbilical cord envelops a pair of arteries that carry deoxygenated blood and waste products to the placental villi, and a vein that carries oxygenated blood to the fetus. Without proper vascularisation, optimal growth and function of the placenta is not possible.
Vascularisation of the
Placental function
The main functions of the placenta can be categorised under the headings of transport and metabolism, protection and endocrine. The placenta acts to provide oxygen, water, carbohydrates, amino acids, lipids, vitamins, minerals and other nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. It can help to protect the fetus against certain xenobiotic
Transport and metabolism
Much of what is known about human placental transport is derived from studies of term placenta. However, there is increasing evidence that placental transport in early pregnancy may differ from that at term in many respects [25]. An important factor that may alter the expression of particular transporter proteins is oxygen tension and blood flow to the intervillous space. As described above, maternal blood flow to the intervillous space is only established from 10 to 12 weeks of gestation.
Endocrine functions of the placenta
The placenta is devoid of nerves, and therefore any communication between it and the mother and/or fetus would normally occur via blood-borne substances. Substances are also produced by the placenta that can play a localised role, e.g. in the uterus or within the placenta itself.
Endocrine, paracrine and/or autocrine factors that are produced by the placenta include oestrogens (produced in conjunction with the fetal adrenal gland and possibly fetal liver), progesterone, chorionic gonadotrophin,
Protective functions of the placenta
The placenta can act to protect the fetus from certain xenobiotics that could be circulating in maternal blood. Many small xenobiotic molecules can cross the placenta by simple diffusion via transcellular or paracellular routes. Alternatively, some xenobiotics can be transported across the placenta by one or more of the large number of placental transport systems, many of which are not completely specific for the endogenous transported molecule(s). However, there are a number of protective
Acknowledgements
We thank Joanne Bruhn for her assistance with manuscript preparation.
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