**1. Introduction**

#### **1.1. Structure and function of human placenta**

Human placenta is an indispensable organ during pregnancy for supporting the development of the fetus. The placenta is a unique organ since it is a multicellular barrier, in which

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

both maternal and fetal cells coexist. Placenta performs functions of metabolic exchange and endocrine regulation between two genetically distinct individuals, the mother and the fetus, while maintaining immunological tolerance between them [1, 2].

and the internal cytotrophoblast. The cytotrofoblast is constituted by highly proliferative mononucleated cells. Syncytiotrophoblast is formed by fusion of cytotrophoblastic cells and has high invasive capacity. This syncytium is responsible for the implantation or anchorage

Human Placenta-Derived Mesenchymal Stromal Cells: A Review from Basic Research to Clinical Applications

The lytic activity of the syncytiotrophoblast, which is responsible for the degradation of the matrix of the endometrium, reaches the uterine capillaries, eroding them. As a result of vascular damage, maternal blood comes out to the syncytiotrophoblast where it forms lacunae; this lacunar stage is the first one toward a fetomaternal circulation. At the same time, the epithelial-like cells of the cytotrofoblast, which have continued proliferating, form accumulations that project toward the syncytiotrophoblast forming the chorionic villi that penetrate the decidua basalis [4]. These finger-like structures (cytotrophoblast covered with syncytiotrophoblast) are invaded by an extraembryonic mesoderm that, in the fourth week after fertilization, gives rise to blood vessels within each villi which makes possible the establishment of the interaction between the fetal circulation, in these embryo vessels, and the maternal blood contained in the trophoblastic lacunae (**Figure 1**). The different layers of the trophoblast (the cytotrophoblast and the syncytiotrophoblast), the basal membranes of the fetal vessels, and the vascular endothelium of these vessels constitute the placenta barrier that regulates the metabolite exchange between both circu-

lations (fetal and maternal). It has been estimated that this exchange surface is about 5 m<sup>2</sup>

contains two arteries and one vein to transport metabolites between mother and fetus.

thinning throughout pregnancy going from 10 microns at the beginning to 1 or 2 microns at the end of the gestation [6]. The umbilical cord connects placenta to the fetus. It is a narrow tube that

Regenerative medicine is an interdisciplinary field within translational medicine whose purpose is to heal or replace damaged tissues or organs as a result of age, illness or trauma. It may involve the transplantation of stem cells that will repair the damaged tissue, stimulate the body's own repair processes or serve as delivery-vehicles for therapeutic agents such as

Stem cells are unspecialized cells that have the capacity to renew themselves or differentiate toward more specialized cells. The proliferation of stem cells is indispensable for the maintenance of the stemness niche. The differentiation is the process by which, under certain physiological or experimental conditions, unspecialized cells are induced to become tissue- or organ-specific cells. The differentiation potential of stem cells is essential during the development of the embryo. In the adult, the main function of stem cells is the maintenance of the

Both embryonic and adult tissues are sources of stem cells with therapeutic potential. However, embryonic stem cells have some limitations in clinical practice, such as ethical concerns, difficulty in obtaining, and tumorigenicity. Adult stem cells have been identified in many organs and tissues, including brain, bone marrow, peripheral blood, adipose tissue, skeletal muscle, skin, teeth, heart, gut, liver, and placenta. Though the number of stem cells

at term [5]. Moreover, this barrier undergoes a progressive

http://dx.doi.org/10.5772/intechopen.76718

at week

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of the blastocyst within the uterine walls.

28 of gestation and reaches 10–11 m<sup>2</sup>

**1.3. Regenerative medicine and placenta**

genes, cytokines, or therapeutic drugs.

tissue homeostasis acting as an internal repair system.

The term placenta derives from the latin and means "flat cake" because of its discoid shape. At the end of pregnancy, it is about 15–20 cm in diameter, 2–3 cm thick, and 500 g in weight, that is, 1/6 of the fetal weight.

The placenta is constituted by structures of fetal origin, such as, the placental disk, the fetal membranes, divided in amniotic and chorionic membranes, and the umbilical cord. The placenta is also composed by a membrane of maternal origin termed the decidua that originates from the endometrium. The functional unit of the placenta is the chorionic villosity that forms the border between maternal and fetal blood during pregnancy (**Figure 1**).
