AQA Biology Placenta and Umbilical Cord PDF

40 The Placenta And Umbilical Cord ILOs By the end of this lecture, students will be able to 1. Differentiate between the origin and structure of the placenta & umbilical cord. 2. Interpret the causes of congenital anomalies of the placenta & umbilical cord. 3. Correlate postna...

40 The Placenta And Umbilical Cord ILOs By the end of this lecture, students will be able to 1. Differentiate between the origin and structure of the placenta & umbilical cord. 2. Interpret the causes of congenital anomalies of the placenta & umbilical cord. 3. Correlate postnatal changes of umbilical cord to its congenital anomalies. 4. Value the structural importance of placental barrier and correlate that with clinical conditions that develop when breeched. BLACENTA Origin of placenta: By the beginning of the fourth month, the placenta has two components: (a) A fetal portion, formed by the chorion frondosum; and (b) A maternal portion, formed by the decidua basalis On the fetal side, the placenta is bordered by the chorionic plate; on its maternal side, it is bordered by the decidua basalis, of which the decidual plate is most intimately incorporated. Development of placenta: Between the chorionic and decidual plates are the intervillous spaces, which are filled with maternal blood. During the fourth and fifth months the decidua forms a number of decidual septa, which project into intervillous spaces but do not reach the chorionic plate. These septa have a core of maternal tissue, but their surface is covered by a layer of syncytial cells, so that at all times a syncytial layer separates maternal blood in intervillous lakes from fetal tissue of the villi. As a result of this septum formation, the placenta is divided into compartments, or cotyledons. Since the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained. Origin and development of placenta 1  Structure of placenta Full term placenta: At full term, the placenta is discoid with a diameter of 15 to 25 cm, is 3 cm thick, and weighs about 500 to 600 g. At birth, it is torn from the uterine wall and, approximately 30 minutes after birth of the child, is expelled from the uterine cavity. After birth, when the placenta is viewed from the maternal surface, 15 to 20 cotyledons, are clearly recognizable and it is reddish and rough. Grooves between the cotyledons are formed by decidual septa. The fetal surface of the placenta is whitish and smooth because it is covered by the amnion. Attachment of the umbilical cord is usually central or eccentric. Placental circulation: It is divided into: 1- Maternal circulation: Cotyledons receive their blood through 80 to 100 spiral arteries that pierce the decidual plate and enter the intervillous spaces. Oxygen and nutrients from maternal blood passes through placental barrier into umbilical vein to the fetus. At the same time, CO2 and waste products passes from the umbilical artery of the fetus to the placenta through the barrier. 2- Fetal circulation: The two umbilical arteries carry fetal blood containing CO2 and waste products which passes to the placenta through placental barrier. At the same time, oxygen, nutrients pass from maternal blood into umbilical vein to the fetus. Functions of placenta: 1- Nutrition: Nutrients pass from the mother to the fetus. 2- Excretion: Waste products pass from the fetus to the mother. 3- Respiration: Oxygen and CO2 are exchanged between the mother and fetus. 4- It Prevents toxins, drugs and microorganisms to pass from the mother to fetus. 2  5- Production of hormones like progesterone, estrogen and HCG. Placental circulation and exchange of materials between maternal and fetal blood Placental barrier (membrane): Definition: They are the structures separating fetal blood from maternal blood (wall of tertiary villi). They prevent mixing of fetal and maternal blood and allow exchange of gases and passage of nutrients and waste products. Structure: 1- The placental membrane is initially composed of four layers: (a) the endothelial lining of fetal vessels; (b) the primary mesoderm; (c) the cytotrophoblast; and (d) the syncytiotrophoblast. 2- From the fourth month on, the placental membrane thins, since the endothelial lining of the vessels comes in intimate contact with the syncytial membrane, greatly increasing the rate of exchange. What are the layers forming the barrier? 3- At the end of pregnancy, permeability must decrease, so, fibrinoid material is deposited on the outer surface. What are the layers forming the barrier? ▪ The placental membrane is not a true barrier, since many substances pass through it freely. Placental barrier at the different stages of pregnancy 3  UMBILICAL CORD Full term umbilical cord: ▪ At birth, the umbilical cord is approximately 2 cm in diameter and 50 to 60 cm long. It is tortuous, causing false knots. ▪ Normally there are two arteries and one vein in the umbilical cord pathed in Warton’ jelly). What is it? Transverse section of a definitive umbilical cord Development of umbilical cord: 1- Primitive umbilical cord: It is formed during folding. It contains: a. Body stalk containing allantois and umbilical blood vessels. b. Yolk stalk and vitelline blood vessels. c. The remaining part of extra embryonic coelom. 2- Definitive umbilical cord: Extra embryonic coelom and allantois obliterate Why? and yolk stalk degenertates. So, it will contain Umbilical blood vessels and Wharton’s jelly covered by amniotic membrane. Structures passing through the primitive umbilical ring and umbilical cord 4 Abnormalities (CLINICAL CORRELATION) of placenta and umbilical cord: 1- Bilobed or triloped placenta. 2- Accessory placenta. 3- Placenta praevia: It may be; a. Centralis b. Marginalis c. Parietalis which leads to antepartum haemorrhage. So, delivery must be by ceasarian section. 4- An extremely long cord may encircle the neck of the fetus, usually without increased risk, whereas a short one may cause difficulties during delivery by pulling the placenta from its attachment in the uterus. What is the result of this anomaly? 5- Presence one artery only Why? These babies have approximately a 20% chance of having cardiac and other vascular defects. 6- Anomalies in the attachment to placenta: marginal attachment or insertion into the chorionic membranes outside the placenta (velamentous insertion). 7- Exomphalos (Omphalocele): Due to failure of a loop of the intestine to return to the abdominal cavity after herniation. The protruded part is covered by amniotic membrane. Abnormalities of placenta and umbilical cord 5