**2. Development**

The placenta is developed from two sources. The principal component is fetal which develops from the chorion frondosum, and the maternal component consists of decidua basalis (**Figure 1**) [2].

© 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.

The fertilized ovum converts into a morula and further differentiates into a blastocyst. The outer layer of the blastocyst proliferates to form the primary trophoblastic cell mass which infiltrates the endometrial lining. By the 7th post-ovulatory day, the trophoblast differentiates into two layers: an inner layer of clear mononuclear cells with well-defined limiting membranes called cytotrophoblast and the outer layer of multinucleated cells with no intercellular membrane called syncytiotrophoblasts [1]. By 10th to 13th post-ovulatory days, a series of intercommunicating spaces or lacunae develop in the rapidly enlarging and dividing trophoblastic cell mass.

The lacunae become confluent, and as the trophoblastic cell erodes the maternal vessels, they become filled with blood to form intervillous spaces. Between the lacunae spaces, there are columns having a central core of cytotrophoblasts surrounded by syncytiotrophoblasts. These form the framework for the development of villi later. From these pillars, branching sprouts appear. Those columns extent as far as the decidua and a mesenchymal core develops in them to form extraembryonic mesenchyme, which forms the villus vessels. In due course, these vessels establish continuity with those developing from the body stalk and inner chorionic mesenchyme. The distal part of the columns is not invaded by the mesenchyme but only serves to anchor it to the basal plate [1, 3]. These cells proliferate and spread laterally separating the syncytiotrophoblasts into two layers, the definitive syncytium on the fetal aspect and the peripheral syncytium on the decidual side which eventually degenerates and is replaced by a fibrinoid material and is known as Nitabuch's layer.

With deeper blastocyst invasion into the decidua, the extravillous cytotrophoblasts give rise to solid primary villi composed of a cytotrophoblast core covered by syncytium. The most deeply implanted portion of these villi forms placenta (**Figure 2**). Beginning on the 12th day

after fertilization, chorionic villi can first be distinguished and form secondary villi. After

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By approximately the 17th day, fetal blood vessels are functional, and a placental circulation is established. The placenta is a vascularized villus structure by the 21st day. The fetal-placental circulation is completed when embryonic blood vessels are connected with chorionic vessels. Groups of cytotrophoblasts also grow into the lumen of the spiral arteries extending as far as the decidual myometrial junction. These cells destroy the muscular and the elastic layer of the vessel wall and get replaced by a fibrinoid material which is derived from the maternal blood and proteins secreted by the trophoblastic cells. This primary invasion dilates the spiral arteriolar wall and thus augments blood flow to the placenta [1, 3]. There is a secondary invasion of trophoblast between 12 and 16 weeks extending up to radial arteries within the myometrium. Thus, spiral arteries are converted to large bore uteroplacental arteries. The net effect is funneling of the arteries that reduce the pressure of the blood to 70–80 mm Hg before

The placental septa appear by 12 weeks protruding into the intervillous spaces from the basal plate and divide the placenta into 15–20 lobes. Until the end of the 16th week, the placenta grows both in thickness and circumference due to growth of the chorionic villi with accompanying expansion

In the first trimester, the villi are large and have a mantle of trophoblasts consisting of an inner layer of cytotrophoblasts and an outer layer of syncytiotrophoblasts with the stroma of small fetal vessels. During the second trimester, the villi are smaller, the mantle is less regular and the cytotrophoblasts less numerous, and the stroma with more collagen. The fetal vessels

of the intervillous space and with continuous arborization and formation of fresh villi [2].

angiogenesis begins in the mesenchymal cores, it results into tertiary villi.

**Figure 2.** Development of placenta.

it reaches the intervillous space. It thus increases the blood flow.

**Figure 1.** Decidual structure differentiating into decidua basalis, capsularis, and parietalis.

**Figure 2.** Development of placenta.

The fertilized ovum converts into a morula and further differentiates into a blastocyst. The outer layer of the blastocyst proliferates to form the primary trophoblastic cell mass which infiltrates the endometrial lining. By the 7th post-ovulatory day, the trophoblast differentiates into two layers: an inner layer of clear mononuclear cells with well-defined limiting membranes called cytotrophoblast and the outer layer of multinucleated cells with no intercellular membrane called syncytiotrophoblasts [1]. By 10th to 13th post-ovulatory days, a series of intercommunicating spaces or

The lacunae become confluent, and as the trophoblastic cell erodes the maternal vessels, they become filled with blood to form intervillous spaces. Between the lacunae spaces, there are columns having a central core of cytotrophoblasts surrounded by syncytiotrophoblasts. These form the framework for the development of villi later. From these pillars, branching sprouts appear. Those columns extent as far as the decidua and a mesenchymal core develops in them to form extraembryonic mesenchyme, which forms the villus vessels. In due course, these vessels establish continuity with those developing from the body stalk and inner chorionic mesenchyme. The distal part of the columns is not invaded by the mesenchyme but only serves to anchor it to the basal plate [1, 3]. These cells proliferate and spread laterally separating the syncytiotrophoblasts into two layers, the definitive syncytium on the fetal aspect and the peripheral syncytium on the decidual side which eventually degenerates and is replaced

With deeper blastocyst invasion into the decidua, the extravillous cytotrophoblasts give rise to solid primary villi composed of a cytotrophoblast core covered by syncytium. The most deeply implanted portion of these villi forms placenta (**Figure 2**). Beginning on the 12th day

lacunae develop in the rapidly enlarging and dividing trophoblastic cell mass.

by a fibrinoid material and is known as Nitabuch's layer.

2 Placenta

**Figure 1.** Decidual structure differentiating into decidua basalis, capsularis, and parietalis.

after fertilization, chorionic villi can first be distinguished and form secondary villi. After angiogenesis begins in the mesenchymal cores, it results into tertiary villi.

By approximately the 17th day, fetal blood vessels are functional, and a placental circulation is established. The placenta is a vascularized villus structure by the 21st day. The fetal-placental circulation is completed when embryonic blood vessels are connected with chorionic vessels. Groups of cytotrophoblasts also grow into the lumen of the spiral arteries extending as far as the decidual myometrial junction. These cells destroy the muscular and the elastic layer of the vessel wall and get replaced by a fibrinoid material which is derived from the maternal blood and proteins secreted by the trophoblastic cells. This primary invasion dilates the spiral arteriolar wall and thus augments blood flow to the placenta [1, 3]. There is a secondary invasion of trophoblast between 12 and 16 weeks extending up to radial arteries within the myometrium. Thus, spiral arteries are converted to large bore uteroplacental arteries. The net effect is funneling of the arteries that reduce the pressure of the blood to 70–80 mm Hg before it reaches the intervillous space. It thus increases the blood flow.

The placental septa appear by 12 weeks protruding into the intervillous spaces from the basal plate and divide the placenta into 15–20 lobes. Until the end of the 16th week, the placenta grows both in thickness and circumference due to growth of the chorionic villi with accompanying expansion of the intervillous space and with continuous arborization and formation of fresh villi [2].

In the first trimester, the villi are large and have a mantle of trophoblasts consisting of an inner layer of cytotrophoblasts and an outer layer of syncytiotrophoblasts with the stroma of small fetal vessels. During the second trimester, the villi are smaller, the mantle is less regular and the cytotrophoblasts less numerous, and the stroma with more collagen. The fetal vessels

• Respiratory • Excretory • Nutritive

sive immunity (**Figure 4**).

**4. Abnormalities of placenta**

**4.1. Placenta succenturiata**

**Figure 4.** Blood supply of placenta.

placenta may lead to:

**b.** Subinvolution

**c.** Uterine sepsis

**d.** Polyp formation

**2.** Endocrine function: placenta is an endocrine gland. It produces both steroid and peptide hormones (like progesterone, estriol, human chorionic gonadotropin, and human placen-

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**4.** Immunological function: maternal antibodies are taken into the syncytiotrophoblasts by pinocytosis and subsequently transferred to fetal capillaries and thus fetus acquires pas-

The accessory lobe is developed from the activated villi on the chorionic leave, may be placed at varying distances from the main placental margin. A leash of vessels connecting the main to the small lobe traverses through the membranes (**Figure 5**). In cases of absence of communicating blood vessels, it is called placenta spuria. The incidence of placenta succenturiata is about 3%. If the succenturiate lobe is retained, the following birth of the

**a.** Postpartum hemorrhage which may be primary or secondary

tal lactogen) to maintain pregnancy and support fetal growth.

**3.** Barrier function: placenta acts as a protective mechanism.

**Figure 3.** Placenta at term.

become larger and more toward the periphery of the villus. In the third trimester, the villi are much smaller in diameter, and the cytotrophoblasts are irregular and thinned out. The fetal vessels are dilated and lie just below the thinned out trophoblasts.

The placenta, at term, is almost a circular disc with a diameter of 15–20 cm and a thickness of about 3 cm at its center [2]. It feels spongy and weighs about 500 g, the proportion to the weight of the baby being roughly 1:  6 at term and occupies about 30% of the uterine wall. It presents two surfaces, fetal and maternal, and a peripheral margin (**Figure 3**) [2].


The placenta consists of two plates. The chorionic plate lies internally. It is lined by the amniotic membrane. The umbilical cord is attached to this plate. The basal plate lies to the maternal aspect. Between the two plates lies the intervillous space containing the stem villi with their branches, the space being filled with maternal blood. A mature placenta has a volume of about 500 mL of blood, 350 mL being occupied in the villi system and 150 mL lying in the intervillous space [2].
