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Fetal Membranes and Placenta
Ass. prof. Dr. Malak A. Al-yawer
Changes in the Trophoblast at the beginning of the second month of development
the trophoblast is characterized by a great number of secondary and tertiary villi, which give it a radial appearance At the embryonic pole, villi are numerous and well formed at the abembryonic pole, they are few in number and poorly developed.
Changes in the Trophoblast
Clinical Correlates Preeclampsia
The condition appears to be a trophoblastic disorder related to failed or incomplete differentiation of cytotrophoblast cells, many of which do not undergo their normal epithelial to endothelial transformation.
During the following months, numerous small extensions grow out from existing stem villi and extend as free villi into the surrounding lacunar or intervillous spaces.
Decidua
Before the fertilized ovum reaches the uterus , the mucous membrane of the body of the uterus undergoes important changes
Decidua
Decidua capsularis part of decidua which covers the fertilized ovum Decidua basalis part of decidua intervening between the ovum and uterine wall Decidua parietalis part of decidua which lines the remainder of the body of the uterus
Decidua
Chorion
Chorion
Structure of the 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
Decidual septa
Because the maternal blood in the intervillous spaces is separated from the fetal blood by a chorionic derivative, the human placenta is considered to be of the hemochorial type.
The placental membrane
Structure of villi at various stages of development placental membrane 4th week 4th month
Drawings of sections through a branch villus at 10 weeks and full term.
Note that the placental membrane becomes very thin at full term. Hofbauer cells are thought to be phagocytic cells.
Schematic drawing of a transverse section through a full-term placenta
Placental Changes at the end of Pregnancy
Placental Abnormalities
Separation of the placenta
The fetal surface of the placenta ( facing the fetus )
maternal side of the placenta
Is textured and spongy looking Is divided by a series of fissures into lobules or cotyledons The fissures contain the remains of septae which extended between the maternal and fetal portions
Examination of the placenta
prenatally by ultrasonography or magnetic resonance imaging. or postnatally by gross and microscopic study
may provide clinical information about the causes of IUGR, placental dysfunction, fetal distress and death, and neonatal illness.
Placental studies
can also determine whether the placenta is complete. Retention of a cotyledon or an accessory placenta in the uterus may cause severe uterine hemorrhage.
Function of the Placenta
Metabolism (e.g., synthesis of glycogen) Transport of gases and nutrients Endocrine secretion
1. Placental Metabolism
The placenta, particularly during early pregnancy, synthesizes glycogen, cholesterol, and fatty acids, which serve as sources of nutrients and energy for the embryo/fetus.
2. Placental Transfer
The transport of substances in both directions between the fetal and maternal blood is facilitated by the great surface area of the placental membrane. free surface of syncytiotrophoblast has many microvilli, more than 1 billion/cm2 at term
Almost all materials are transported across the placental membrane by one of the following four main transport mechanisms simple diffusion, facilitated diffusion, active transport, and pinocytosis.
Other Placental Transport Mechanisms
Transport of gases and nutrients
Transfer of Gases Oxygen, carbon dioxide, and carbon monoxide cross the placental membrane by simple diffusion.
Nutritional Substances
Hormones
Protein hormones do not reach the embryo or fetus in significant amounts, except for a slow transfer of thyroxine and triiodothyronine. Unconjugated steroid hormones cross the placental membrane rather freely.
Electrolytes
These compounds are freely exchanged across the placental membrane in significant quantities, each at its own rate.
Transmission of Maternal Antibodies
Waste Products
Urea and uric acid pass through the placental membrane by simple diffusion. Conjugated bilirubin (which is fat soluble) is easily transported by the placenta for rapid clearance.
Drugs and Drug Metabolites
3. production of hormones
The placenta ( syncytiotrophoblast ) is classified as an endocrine organ . It produces both protein and steroid hormones 1. human Chorionic Gonadotropin ( hCG ) 2. estrogen 3. Progestrone 4. human Chorionic Somatomammotropin (hCS )
human Chorionic Gonadotropin ( hCG )
estrogen
Progestrone
human Chorionic Somatomammotropin (hCS )
Formly known as human placental lactogen ( hPL ) Similar to growth hormone This protein hormone influencing (1) the preparations of breasts for lactation and (2) lipid CHO metabolism
The primitive umbilical cord
Foetal Membranes
Thin layers or tissues which surround the embryo or foetus and provide for its nutrition, excretion and protection
Foetal Membranes are
allantosis yolk sac amnion and chorion
Amnion
Is a transparent greyish membrane which lines the chorion .It covers the foetal surface of the placenta and the umblical cord. The amniotic sac contains the foetus swimming in the liquor amni
Amniotic fluid (liquor amni) Nature
Is a clear ,pale, slightly alkaline pH 7.2 fluid.
The amount of fluid increases 30 mL at 10 weeks of gestation 450 mL at 20 weeks 800 to 1,000 mL at 37 weeks. Then decreases later on to be scanty in post-term pregnancy
Amniotic fluid composition
Circulation of amniotic fluid
The amniotic fluid is not in a static state but is in a continuous turnover,500 ml of it are replaced each hour
Origin of amniotic fluid
Foetal (1)active secretion from the amniotic epithelium (2) transudation from the foetal circulation (3)foetal urine The foetal origin contributes more in production of amniotic fluid.
Maternal transudation from maternal circulation
Uptake of amniotic fluid is by
Absorption through the amnion to the maternal circulation Foetal swallowing
From the beginning of the fifth month, (1) the fetus swallows its own amniotic fluid, (2) Fetal urine is added daily to the amniotic fluid
Functions During pregnancy
During labour
The fore bag of water helps the dilatation of the cervix during labour It acts as an antiseptic for birth canal after rupture of the membranes.
Clinical Correlates Amniotic Fluid
Chorion
This the outer membrane It forms a large portion of the connective tissue thickness of the placenta on its foetal side It is the structure in and through which the major branching umblical vessels travel on the surface of the placenta
Fetal Membranes in Twins
Arrangement of fetal membranes in twins , depending on the a. type of twins b. the time of separation of Monozygotic twins.
Dizygotic Twins
Normally, each embryo has its own amnion, chorion, and placenta but sometimes the placentas are fused
Possible relations of fetal membranes in monozygotic twins
. A. Splitting occurs at the two-cell stage, each embryo has its own placenta, amniotic cavity, and chorionic cavity.
Possible relations of fetal membranes in monozygotic twins
B. Splitting of the inner cell mass into two completely separated groups. The two embryos have a common placenta and a common chorionic sac but separate amniotic cavities
Possible relations of fetal membranes in monozygotic twins
Parturition (Birth)
For the first 34 to 38 weeks of gestation, the uterine myometrium does not respond to signals for parturition (birth). However, during the last 2 to 4 weeks of pregnancy, this tissue undergoes a transitional phase in preparation for the onset of labor.
Labor itself is divided into three stages
(1) effacement (thinning and shortening) and dilatation of the cervix (this stage ends when the cervix is fully dilated), (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes
Parturition (childbirth)
Clinical Correlates Preterm Birth
Thank you
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