Development of the placenta Dr Hiba Ahmed Suhail M. B. Ch. B. /F. I. B Development of the placenta Dr Hiba Ahmed Suhail M.B. Ch. B./F.I.B.O.G. Dep. Of gynecology & obstetrics College of medicine University of Mosul

Development of the placenta It is a functioning feto maternal organ originated from both the trophoblastic chorionic plate (fetal part ) and the decidual plate ( maternal part ) inclosing a space containing maternal blood.

Formation of the villi By the second weeks of the development the trophoblast erodes deeply into the maternal decidua, so that the endothelium of the maternal blood vessels is destroyed & maternal blood is in direct contact with the (trophoblast),without the intervention of any decidual , the uteroplacental circulation established .

The trophoblast is charctarized by the villous structures The primary villi : it consist of columns of cytotrophoblast surrounded by syncytiotrophoblast(syncytium) it appears by the beginning of the third weeks. Secondary villi : the embryonic mesoderm appears it penetrates the core of the primary villi forming the secondary villi. The tertiary villi : the mesodermal cells in the core of the villus begin to differentiate in to blood vessels forming the villus capillary system by about the end of third week. Terminal villi : after the 20th week the cytotrophoblast disappear & finally only thin layer of syncytium remains only within the villus and together with the maternal capillary endothelium will separate the maternal and fetal blood to facilitate the exchange function of the placenta.

The functional unit of the placenta is fetal cotyledon & the mature placenta has about 120 cotyledons , which are grouped into visible lobes . Each cotyledon contains a primary villus stem arising from the chorionic plate, which is divided to form secondary & tertiary stems from which arise the terminal villi , where the fetal-maternal exchange takes place.

After implantation the decidualized endometrium differentiate in to : Decidua basalis is the part of the decidua present on the embryonic pole and it will connect to the chorionic villi forming the placenta. Decidua capsularis is the part of the decidua that present on the abembryonic pole which will fuse with decidua parietalis. Decidua parietalis is the part of the decidua that lining the remaining part of the uterine cavity. At first the villi are formed over all the surface of the gestational sac ( at 4th week ) covering the entire surface of the chorion Between 12th -16th week the villi on the decidea capsularis surface degenerate & become smooth called the chorion leave . In compensation the villi on the decidua basalis undergo great hypertrophy called chorion frondosum & its matted into solid disc which is the fully developed placenta.

The placenta at term At term it is a discoid spongy structure ,it is circular in shape with diameter of 15-25 cm and 3cm thickness , it weighs about 500-600 gm and it s weight directly correlated with the fetal weight , the placenta usually attached to the uterine wall near the fundus, to either the anterior or posterior surface. Following the delivery of the baby in the 2nd stage of labour the placenta is torn from the uterine wall and expelled in the 3rd stage of labour On inspection it consist of : 1-Maternal surface a 15-20 visible rough lobe covered by a thin layer of the decidua. 2-Fetal surface smooth area covered entirely with chorionic plate then amnion and the chorionic vessels converge toward the umbilical cord The umbilical cord usually attached to the placenta eccentricly .

Function of placenta: 1- Enables the fetus to take oxygen and nutrients from the maternal blood. 2. Excretory function when carbon dioxide and other waste products pass from the fetus to the maternal circulation. 3. Barrier against the transfer of infection to the fetus. 4. Secrete hormones like human chorionic gonadotropin ( HCG ) , oestrogen , progesterone and human placental lactogen (hPL).

Normal placentation The maternal flow to the placenta increases throughout pregnancy from 50mL/min in the 1st trimester to 600mL/min at term . This increase in perfusion can only be accomplished by the anatomical conversion of the maternal spiral arteries by trophoblastic invasion , from narrow tortuous thick muscular vessels to wide-bored flaccid vessels. In the first 12weeks the decidual segment of the spiral artery, is invaded by trophoblast . Following this, the trophoblast invasion of the intramyometrial segment of the spiral arteries ,lack of smooth muscle within the myometrial arteries make them less respond to the vaso active substance which further reduces resistance to blood flow to the placenta. By 20 weeks this changes usually completed.

Abnormal placentation Pre-eclampsia and chronic hypertention Intrauterine growth restriction(IUGR). Abruptio placentae. Collagen vascular disease. Antiphospholipid syndrome. Sever D.M. These disorders result from total or patchy failure of trophoblast invasion of the myometrial segments of the spiral arteries.

Placental abnormalities : Infraction : represents an area of ischemic necrosis of cotyledon resulting from spiral artery occlusion , usually by thrombosis. Basal haematomas : consist of a mass of blood in the center of the cotyledon due to the rupture of the damaged spiral artery these pathological condition associated with increased perinatal mortality. Calcification of the placenta : deposition of calcium within the placental tissue (of no clinical importance ). Anomalies in weight: increase in cases of diabetes and haemolytic disease of the newborn and reduced in pre-eclampsia and severe diabetes.

Hydropic placenta : In sever cases of isoimmunization ( hydrops fetalis ) the placenta show an increased weight, enlarged, pale and oedematous . Bilobate and trilobate placenta : instead of a single placenta, it may consist of 2 or 3 lobes partly fused ( it has no effect on the pregnancy out come ).

Placenta succenturiata : one or more accessory lobes of placenta are found on the chorion at a distance from the edge of the main placenta , this lobe is of clinical importance because its liable to be retained in the uterus after the placenta has been expelled causing post partum hemorrhage ( PPH ) this abnormality should be discovered after inspection of placenta & membrane, when a round defect seen in the membrane. Placenta circumvallata: where the original area of attachment of the chorionic plate to the uterine wall is small & placental growth has continued beyond its margin , a fibrous ring is seen on the fetal surface of the placenta.

Morbid adherence of the placenta : In 3rd stage of labour the placenta normally separates from the maternal decidua ( the superficial part of the decidua comes away with the placenta and the deeper part remains on the uterine wall ). It results from increased penetration of the decidua and myometrium by the chorionic villi. The severity is determined by the depth of invasion. 1. Placenta accrete the placenta is partially or completely adherent to the uterus with penetration of villi into the superficial part of the myometrium. 2. Placenta increta the villi penetrate deeply through the decidua into the myometrium. 3. Placenta percreta penetration can even be seen on the serosal surface of the uterus and may invade the bladder .

Placenta previa and scared uterus is a risk factors. It occur 1 in 500 pregnancies and presents as prolonged 3rd stage of labour with postpartum hemorrhage. When an attempt to remove the placenta manually there is no line of cleavage ( separation ) makes it impossible to remove the placenta completely thus exposing the mother to risk of sever postpartum hemorrhage and we may need to do hysterectomy.

Placenta previa: in about 1 in 250 pregnancies the placenta is implanted wholly or partially on the lower uterine segment of the uterus . This is a serious condition which may cause severe haemorrahge in pregnancy or labour. Scared uterus, multiple pregnancy, multiparity and smoking is a risk factors. Tumours of the placenta : such as choriocarcinoma , tumours ( less common) and vascular tumours called haemangiomas ( rare).

The umbilical cord At term the umbilical cord is about 50cm length , 2cm in diameter , it is tortuous causing false knots it contain 2 arteries and 1 vein covered with jelly of Wharton and amnion. Abnormal length: Excessive length predispose to prolapse of the cord, formation of loops round some part of the fetus may cause IUD in very rare cases. Short cord predispose to delay in 2nd stage of labour , premature separation of the placenta, inversion of uterus are rare complication . Knots in the cord : These may be formed by fetal movement , knots are rarely tight enough to obstruct the circulation , but they do occasionally cause IUD.

Abnormal insertion of the cord: The cord usually attached to the placenta slightly in an eccentric position, The cord attached to the edge of placenta ( marginal placenta) it is of no clinical significant. Velamentous insertion of the cord rarely the cord is attached to the membrane at some distance from the edge of the placenta , ( outside the placenta ) at this point the vessel branches which run on the membrane before reaching the placenta and if vessel pass across part of the membrane that lies in front of the presenting part called ( vasa previa ) , this branch may be damaged when the membrane rupture, leading to fetal blood loss. Single umbilical artery: This is uncommon, may be associated with other abnormalities of the fetus, mainly the renal , cardiac and other vascular defect .