Placenta and Fetal Membranes Flashcards
Implantation is sometimes viewed as a “controlled invasion.” Implantation initiates approximately day [] after fertilization and begins with attachment of the blastocyst to the uterine wall. The embryonic pole of the blastocyst adheres to the uterine epithelium. This adhesion appears to be mediated by [] and [], making the adhesion similar to that of cells of the immune system adhering to capillary walls. Implantation also requires the secretion of [] and [], as well as [] which erode the uterine tissue and facilitate invasion of the trophoblastic cells.
Implantation is sometimes viewed as a “controlled invasion.” Implantation initiates approximately day 7 after fertilization and begins with attachment of the blastocyst to the uterine wall. The embryonic pole of the blastocyst adheres to the uterine epithelium. This adhesion appears to be mediated by integrins and L-selectin, making the adhesion similar to that of cells of the immune system adhering to capillary walls. Implantation also requires the secretion of microRNAs and exosomes, as well as proteases which erode the uterine tissue and facilitate invasion of the trophoblastic cells.
The cells of the embryo (which, due to paternal contribution, are partially immunologically foreign) invade through the uterine epithelium and the entire conceptus becomes embedded in the uterine wall. Why the conceptus is not “rejected” remains incompletely understood but is thought be the result of modulation of the maternal immune system by [] and [], including [], [], and []. In addition, [] are modified in the endometrium and play an important role in maintenance of the pregnancy.
The cells of the embryo (which, due to paternal contribution, are partially immunologically foreign) invade through the uterine epithelium and the entire conceptus becomes embedded in the uterine wall. Why the conceptus is not “rejected” remains incompletely understood but is thought be the result of modulation of the maternal immune system by cytokines and hormones, including progesterone, chorionic gonadotropin, and Leukemia Inhibitory Factor. In addition, Natural Killer cells are modified in the endometrium and play an important role in maintenance of the pregnancy.
Blastocyst
- Inner cell mass- will form the embryo
- Trophoblast- will form the placenta
a. Cytotrophoblast - individual cells derived from fetal tissue
b. Syncytiotrophoblast - multinucleate cells formed from fusion of cytotrophoblastic cells. Will form the layer of cells in contact with maternal blood.
Blastocyst: Inner Cell Mass will form the…
…embryo.
Blastocyst: Trophblast will form the…
…placenta.
Blastocyst: Cytotrophoblast is made up of…
…individual cells derived from fetal tissue.
Blastocyst: Syncytiotrophoblast is made of…
…multinucleate cells formed from fusion of cytotrophoblastic cells. Will form the layer of cells in contact with maternal blood.
Amnion
a. Derived from epiblast cells at the superior pole of the embryonic disk.
b. Provides environment for symmetrical growth, free movement.
c. Because of embryonic folding, it envelopes the embryo, covers umbilical cord and the fetal surface of the placental disk (chorionic plate).
Yolk Sac
- The primary yolk sac forms on about day [] as cells of the hypoblast migrate over the inner surface of the cytotrophoblast.
- A second wave of proliferation on day [] results in the formation of secondary (definitive) yolk sac.
a. Site of [] (in the extraembryonic mesoderm), beginning 3rd week of gestation
b. The [] is derived, in part, from yolk sac endoderm
c. It is the origin of [].
Yolk Sac
- The primary yolk sac forms on about day 8 as cells of the hypoblast migrate over the inner surface of the cytotrophoblast.
- A second wave of proliferation on day 12 results in the formation of secondary (definitive) yolk sac.
a. Site of blood cell formation (in the extraembryonic mesoderm), beginning 3rd week of gestation
b. The gut is derived, in part, from yolk sac endoderm
c. It is the origin of primordial germ cells.
Rarely, the yolk sac may persist in the adult intestine as the [].
Rarely, the yolk sac may persist in the adult intestine as the Meckel’s diverticulum.
Allantois
a. out-pocketing of the hindgut
b. source of umbilical vessels
When the conceptus has completely invaded the uterine wall, the [] begin to form [], which are the beginning of the placental circulatory system. As implantation continues, the [] erodes the walls of the maternal uterine vessels, and the [] fill with maternal blood.
When the conceptus has completely invaded the uterine wall, the syncytiotrophoblast begin to form lacunae, which are the beginning of the placental circulatory system. As implantation continues, the syncytiotrophoblast erodes the walls of the maternal uterine vessels, and the lacunae fill with maternal blood.
FORMATION OF PLACENTAL VILLI
At the cellular level:
- Primary villi form in the beginning of the [] week
• Proliferation of [] within the [].
- Secondary villi
• Primary villi [] with [].
- Tertiary villi form by the end of []week
• Secondary villi that have formed [].
FORMATION OF PLACENTAL VILLI
At the cellular level:
- Primary villi form in the beginning of the 3rd week
• Proliferation of cytotrophoblast columns within the syncytiotrophoblast.
- Secondary villi
• Primary villi (cytotrophoblast + syncytiotrophoblast) with connective tissue core.
- Tertiary villi form by the end of 3rd week
• Secondary villi that have formed fetal blood vessels within the connective tissue core.
Pre-eclampsia is associated with…
…shallow trophoblastic invasion. In addition, shallow invasion is correlated with fetal growth retardation due to malformation of the fetoplacental vascular bed and impaired transport of nutrients.
Preeclampsia: New onset hypertension with either proteinuria or end-organ dysfunction after 20 weeks gestational age.
Eclampsia: Pre-eclampsia+maternal seizures. Can result in maternal death due to stroke, intracranial hemorrhage, or acute respiratory distress.
Placentation - Formation of the Placental Disk
- Initially, villi are present over nearly the entire surface of the conceptus. However, the blood supply to the villi adjacent to the endometrium is superior and the villi in this region proliferate to form the chorion frondosum that will eventually form the definitive placenta.
- As growth continues, the villi that face the uterine lumen regress, resulting in a smooth membrane called the smooth chorion (chorion leave). The amnionand smooth chorion fuse. This leaves the definitive placenta as a disk occupying about 30% of the uterine surface.
The chorionic plate is composed of [] and faces the developing fetus. The chorionic plate is the side into which the umbilical cord inserts. The major umbilical vessels can be seen radiating out across the surface of the chorionic plate.
The chorionic plate is composed of fetal connective tissue and faces the developing fetus. The chorionic plate is the side into which the umbilical cord inserts. The major umbilical vessels can be seen radiating out across the surface of the chorionic plate.
The decidua is []. The area of contact between the decidua of the uterus and the cytotrophoblast is called the []. Decidual septa project through the [], forming [] on the maternal surface.
During implantation, the endometrium of the uterus undergoes the decidual reaction, in which the stromal cells of the endometrium become loaded with lipids and glycogen, and the tissue becomes edematous. The decidual cells are highly synthetic (peptide hormones, steroids, prostaglandins), and are thought to be involved in hormonal “cross-talk” between the maternal and fetal tissues.
The decidua is maternal tissue. The area of contact between the decidua of the uterus and the cytotrophoblast is called the basal plate. Decidual septa project through the basal plate, forming cotyledons on the maternal surface.
- Smooth chorion (chorion laeve). As mentioned above, the smooth chorion is the portion of the chorion where the villi degenerate during fetal growth. By term the smooth chorion (together with the decidua capsularis) has fused with the decidua parietalis. Cells from both the smooth chorion and the decidua are highly synthetic, and hormonal “cross-talk” is thought to occur between these two tissues.
- Amnion. The amnion envelops the fetus and the contained amniotic fluid. The amnion expands at a faster rate than the smooth chorion, so eventually the amnion fuses with the chorion. Amniotic cells are also highly synthetic, and make a variety of hormones.
Monozygotic Twinning
Monozygotic twinning is the result of splitting of the fertilized egg or inner cell mass. Depending upon the timing of the split, the fetal membranes may form one or two placental discs and one or two amniotic membranes. In contrast, dizygotic twinning (two separate oocytes) always results in two placental discs and amnions.
The placental villi extend from the chorionic plate into the intervillous space, branching to form somewhat of a “tree-like” structure. [] span the placental disk and branch to form [] which in turn branch to form[]. [] are “the exchangers”. Their major function is in maternal-fetal exchange of gases, nutrients, antibodies, etc. Each contains a dilated fetal capillary.
The placental villi extend from the chorionic plate into the intervillous space, branching to form somewhat of a “tree-like” structure. Anchoring villi span the placental disk and branch to form intermediate villi which in turn branch to form terminal villi. Terminal villi are “the exchangers”. Their major function is in maternal-fetal exchange of gases, nutrients, antibodies, etc. Each contains a dilated fetal capillary.
The syncytiotrophoblast is an unusual epithelium. It is derived from the fusion of cytotrophoblastic cells, therefore it is multinucleate with no lateral cell membranes. It has an extensive microvillous border, greatly enhancing the area of exchange.
Roles of the Synctiotrophoblast: Regulation of Exchange
a. Oxygen and carbon dioxide
b. Nutrients (amino acids, glucose)
c. Antibodies (IgG for passive immunity)
d. waste products
Late in gestation the syncytiotrophoblast becomes very thin on the terminal villi, making the distance for exchange minimal.
A. Simple diffusion (lipophilic substances, gases)
C. facilitated diffusion
D. co-transport
E. exchange
F. active transport
G. ion-channels
H. vesicular transport
B. Paracellular transport is depicted but is a minor factor in placental transport.
The high exchange capacity between the maternal and fetal circulations also puts the fetus at risk for any toxins or pathogens present in the maternal bloodstream. These include, but are not limited to: [], [], [] and [], and []. The factors that control transport include [], [], or t[].
The high exchange capacity between the maternal and fetal circulations also puts the fetus at risk for any toxins or pathogens present in the maternal bloodstream. These include, but are not limited to: viruses (cytomegalovirus, rubella, HIV), medications, alcohol and illicit drugs, and environmental pollutants. The factors that control transport include size, hydrophobicity, or the ability to exploit normal transport pathways (viruses).
Placental Abnormalities: Placenta Accreta, Placenta Increta and Placenta Percreta
- Placenta Accreta: Abnormal adherence to the uterine wall with no clear decidua basalis.
- Placenta Increta: Invasion into the myometrium.
- Placenta Percreta: Invasion through the myometrium to the uterine serosa.
Placental Abnormalities: Placenta Previa
•The placenta overlies the cervical os of the uterus.
Placental Abnormalities: Velamentous Insertion of the Cord
•The umbilical cord inserts into the chorion laeve away from the placental disc.
