Unified Lec 11 Placenta PDF

Unified Lecture 11 THE PLACENTA HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA Learning Outcomes By the end of this lecture the student must be able to: 1. Differentiate between the fetal and the maternal component of the placenta 2. Discuss the formation of the villi 3. Describe the b...

Unified Lecture 11 THE PLACENTA HIHD-211 TERM-3 Basic Science Department COSHP, KSAU-HS, KSA Learning Outcomes By the end of this lecture the student must be able to: 1. Differentiate between the fetal and the maternal component of the placenta 2. Discuss the formation of the villi 3. Describe the blood-placenta barrier 4. Describe placental circulation 5. List the functions of the placenta STRUCTURE OF THE PLACENTA Placenta is a feto-maternal organ which is the primary site of nutrient and gas exchange between the fetus and the mother. 2. At full term: 1. The shape is disc like Weight- 500-600 gm Diameter- 15-25 cm) 3. Placenta is expelled off after the birth. LO-1 STRUCTURE OF THE PLACENTA Placenta has two surfaces: 1. Maternal surface is covered by thin layer of decidua basalis and has 1530 slightly bulging areas called cotyledons separated by decidual septa. 2. Fetal surface is covered chorionic plate which is in turn covered by amnion. Chorionic vessels converge towards the umbilical cord. Umbilical cord is attached to this surface. LO-1 Fetal surface Maternal surface FUNCTIONS OF THE PLACENTA 1. GAS EXCHANGE: Exchange of carbon dioxide, carbon monoxide and oxygen by simple diffusion 2. METABOLIC TRANSFER: Nutrients and electrolytes such as free fatty acids, amino acids, carbohydrates and vitamins 3. HORMONE SECRETION: 1) Progesterone 2) Estrogenic hormones predominantly estradiol 3) Human chorionic gonadotropin (HCG) during first two months of pregnancy 4) Somatomammotropin 4. FETAL PROTECTION: Passive immunity by maternal immunoglobulin G (IgG) LO-5 PLACENTAL DEVELOPMENT Components Placental development takes place from both the chorion and the decidua Therefore, placenta is a feto-maternal organ with two components: 1. Fetal component is derived from the trophoblast and extraembryonic mesoderm (the chorionic plate) 2. Maternal component is derived from the endometrium (Decidua basalis) LO-3 THE CHORION The chorion is the name given to the trophoblast after the formation of a layer of extraembryonic mesoderm on its inner surface due to the formation of chorionic cavity. Types of Chorion: 1. Chorion leave (smooth): It takes no share in the formation of the placenta, As the chorion grows, It comes in contact with the decidua parietalis and these layers fuse. Forms the protective shell surrounding the developing fetus. 2. Chorion Frondosum (rough): The bushy embryonic pole where the villi grow and expand The placenta develops from the chorion frondosum LO-2 THE DECIDUA The endometrium (lining of the uterus) is known as the DECIDUA, consisting of three regions named by location Regions Description 1. Decidua basalis Region between the blastocyst & the myometrium 2. Decidua capsularis Endometrium that covers the implanted blastocyst 3. Decidua parietalis All the remaining endometrium LO-2 DEVELOPMENTAL CHANGES IN PLACENTA Endovascular Invasion Maternal blood is delivered to the placenta by spiral arteries in the uterus. Erosion of these maternal vessels to release blood into intervillous spaces is accomplished by endovascular invasion by cytotrophoblast cells. Invasion of the spiral arteries by cytotrophoblast cells transforms these vessels from small-diameter, highresistance vessels to larger-diameter low resistance vessels that can provide increased quantities of maternal blood to intervillous sacs LO-2 DEVELOPMENTAL CHANGES IN PLACENTA By the beginning of the third week the trophoblast is characterized by primary villi that consist of a Cytotrophoblastic core covered by a syncytial layer. Secondary villi are formed when the mesodermal cells penetrate the core of the primary villi and grow towards the decidua. By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system. The villus is now known as a tertiary villus or a definitive placental villus. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA Capillaries in tertiary villi make contact with capillaries developing in the mesoderm of the chorionic plate and in the connecting stalk. These vessels, in turn, establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA When the heart begins to beat in the fourth week of development, the villous system is ready to supply the embryo proper with essential nutrients and oxygen. Cytotrophoblastic cells in the villi penetrate progressively into the overlying syncytium until they reach the maternal endometrium. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA Here they establish contact with similar extensions of neighboring villous stems, forming a thin outer cytotrophoblast shell. This shell gradually surrounds the trophoblast entirely and attaches the chorionic sac firmly to the maternal endometrial tissue. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA Villi that extend from the chorionic plate to the decidua basalis (decidual plate: the part of the endometrium where the placenta will form) are called stem or anchoring villi. Those that branch from the sides of stem villi are free (terminal) villi, through which exchange of nutrients and other factors will occur. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA The newly formed free villi are primitive, but by the beginning of the fourth month, cytotrophoblastic cells and some connective tissue cells disappear. The syncytium and endothelial wall of the blood vessels are then the only layers that separate the maternal and fetal circulations. Disappearance of cytotrophoblastic cells progresses from the smaller to larger villi. LO-2 DEVELOPMENTAL CHANGES IN PLACENTA The chorionic cavity, meanwhile, becomes larger, and by the 19th or the 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk. The connecting stalk later develops into the umbilical cord, which forms the connection between the placenta and embryo LO-2 Connecting Stalk DEVELOPMENTAL CHANGES IN PLACENTA During the fourth and fifth months, the decidua forms a number of decidual or placental 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 a number of compartments, or cotyledons. Because the decidual septa do not reach the chorionic plate, contact between intervillous spaces in the various cotyledons is maintained. LO-2 1. 80 –100 spiral endometrial arteries discharge into the intervillous space. 2. The blood is propelled in jet like fountains by the maternal blood pressure. Now the pressure of this entering blood is higher than that in the intervillous space. It forms a roof of the space. 3. As the pressure dissipates, the blood flows slowly around the branch villi. Exchange of metabolites and gases with the fetal blood. 4. As the pressure decreases, the blood flows back from the chorionic plate and enter the endometrial veins to the maternal circulation. Placental circulation Fetal placental circulation Two Umbilical Arteries: Carry poorly oxygenated blood from the fetus to the placenta. Within the branch chorionic villi, they form: Arterio-capillary venous network: It brings the fetal blood extremely close to the maternal blood. The well oxygenated fetal blood in the capillaries passes into veins accompanying the chorionic arteries. At the umbilical cord, they form the One Umbilical Vein. ❸ ❷ Reductions of uteroplacental circulation result in fetal hypoxia and intrauterine growth restriction (IUGR). Severe reductions of circulation may result in embryo/fetal death 1 8 ❹ ❶ Maternal placental circulation Placental Membrane It is a composite thin membrane of extra fetal tissues which separates the fetal and maternal bloods. Upto(20) weeks it is composed of four layers: At full term it becomes thinner and composed of three layers only: 1 Syncytiotrophoblast 1 Syncytiotrophoblast 2 Cytotrophoblast. 2 Connective tissue of the villus 3 Connective tissue of the villus 3 Endothelium of fetal capillaries 4 Endothelium of fetal capillaries At some sites, the syncytio comes in direct contact with the endothelium of the capillaries and forms Vasculo syncytial placental membrane. 1 9 20