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Placenta

The placenta is the site of nutrient and gas exchange between the fetus and mother. Shape: The full term placenta is discoid in shape. Diameter = 15-25 cm, 2-3 cm thick, Weight = 0.5 kg. Umbilical cord is attached to its center.

Surfaces: - It has 2 surfaces: 1- fetal surface: which is smooth and shinny because it is covered by an amniotic membrane. The umbilical cord is attached centrally to this surface. 2- maternal surface: which is rough, reddish, and has 15 – 20 elevated areas called cotyledons with deep grooves in between made by the decidual septa.

Development: Maternal part: decidua basalis Development: Maternal part: decidua basalis. Fetal part: chorion frondosum.

Villi: -primary chorionic villi. -secondary chorionic villi. -Tertiary chorionic villi.

A- Villi opposite decidua basalis proliferate  chorion frondosum. Villi of chorion frundosum are divided into: 1- Anchoring villi: reach decidua basalis. They are used for fixation of the fetus to the uterus. 2- Free villi ( nourishing): do not reach decidua basalis. They are used for nutrition of the fetus. B- Villi opposite decidua capsularis degenerate  chorion laeve.

Tertiary chorionic villi contain embryonic blood vessels that develop in loose connective tissue core. These blood vessels connect up with vessels that develop in the chorion and connecting stalk.

Maternal blood is derived to the placenta by 80 – 100 spiral arteries in the uterus. Erosion of these maternal vessels to release blood into the intervillous spaces is done by trophoblast cells which invade the terminal ends of spiral arteries.

During the 4th and 5th month, the decidua forms a number of decidual septa, which project into the intervillous space. As a result of this septum formation, the placenta is divided into a number of compartments (cotyledons).

Sructure of placenta

Placental Circulation: 1. Fetal circulation. 2. Maternal circulation. Maternal circulation - Uterine artery → Spiral artery → intervillous space (where exchange of oxygen and CO2 between mother and fetus occurs through the placental barrier) - The venous blood will collect in the marginal vein → uterine vein.

Fetal circulation - The CO2 and wast products of the embryo reaches to the placenta through 2 umbilical arteries. These 2 arteries end by capillaries inside the tertiary villi where exchange of CO2 and O2 will occur through placental barrier. - The oxygen reaches the embryo through the umbilical vein.

Fetal Circulation

Placental (membranes) barrier: -These are layers separating the fetal blood in the villi from maternal blood in the intervillous space. -These layers prevent mixing of the fetal and maternal blood but they allow passage of certain substances throw it. -It is not a true barrier because only few substances are unable to cross it. Most drugs in the maternal blood can pass through it to the fetal circulation and cause major fetal congenital anomalies. 10 weeks full term (placental membrane)

- Early in pregnancy (till about 20 week gestation), it is formed of four layers (Fig. 56): a. The endothelial lining the fetal vessels. b. The connective tissue (mesoderm) of the villus. c. The cytotrophoblast layer. d. The syncytiotrophoblast. - After 20 weeks the cytotrophoblasts degenerate so increases the permeability of the placenta.

- Towards the end of pregnancy, a fibrinoid material made of fibrin is formed on the surface of the villi to decrease the permeability, so the placental barrier is formed of this fibrinoid material, the syncytiotrophoblasts, and the endothelium of the fetal blood vessels.

Functions Of The Placenta Exchange of gases (lung). Exchange of Nutrients and Electrolytes (GIT). Excretion of waste products (kidney). Transmission of Maternal antibodies resulting in passive immunity of the fetus. Hormone Production as Progesterone (maintains the corpus luteum so prevents other ovulations and prevents menses during pregnancy), estrogenic hormones (estriol), HCG, relaxin hormone to soften the uterine ligaments to help delivery and Somatomammotropin.

Functions of placental barrier: It prevents most organisms from passing to the fetus, so it acts as a protective mechanism against damaging factors, many viruses such as Rubella, Coxackie virus, German measles and poliomylitis virus traverse the placenta. These viruses may result in congenital malformations. Most of the drugs in addition to cocaine, heroin cross the placenta and cause serious damage.

Abnormalities of placenta 1- According to site: Placenta Praevia the placenta is attached to the lower uterine segment (due to low level of implantation of the blastocyst). It causes severe antepartum haemorrhage. There are three types:

2- Abnormal penetration into the uterus: 1- Placenta accreta: due to abnormal adhesion between the chorionic villi and the uterine wall. 2- Placenta percreta: The chorionic villi penetrate the myometrium all the way to the perimetrium. - the placenta fails to separate from the uterus after birth and may cause severe postpartum hemorrhage.

3- Abnormal attachment of umbilical cord: a- Velamentous attachment: The cord does not reach the placenta itself but is attached to amniotic membrane over the fetal surface of placenta. The umbilical vessels pass in the membrane to reach the placenta. It is easly torn. B- Battle- dore placenta (marginal attachment of the cord

4- Abnormalities according to the number: 1- Double placenta (bilobed or bidiscoid placenta). 2- Triple placenta (trilobed). 3- Accessory placenta (accessory one or more lobes). It may cause severe postpartum hemorrhage if the accessory lobe is retained in the uterus after labor. 5- Shape abnormalities: bi- or trilobed or horsehoe placenta

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