Placenta Amniotic Fluid and Umbilical Cord

I. PLACENTA (Figure 5-1)

A. Components

1. The maternal component of the placenta consists of a portion of the endometrium called the decidua basalis.

2. The fetal component of the placenta consists of tertiary chorionic villi, which are collectively called the villous chorion.

B. Afterbirth appearance of the placenta

1. The maternal surface of the placenta is characterized by 15-20 cotyledons that impart a cobblestone appearance. The surface is dark red and oozes blood after birth owing to torn maternal blood vessels.

2. The fetal surface of the placenta is characterized by the chorionic blood vessels. It appears smooth and shiny because the amnion covers this surface.

C. Clinical correlations

1. Velamentous placenta occurs when the umbilical vessels abnormally travel through the amniochorionic membrane before reaching the placenta proper. If the umbilical vessels cross the internal os, a serious condition called vasa previa exists. In vasa previa, if one of the umbilical vessels ruptures during pregnancy, labor, or delivery, the fetus will bleed to death.

2. Placenta previa occurs when the placenta attaches in the lower part of the uterus, covering the internal os. (The placenta normally implants in the posterior superior wall of the uterus.) Uterine vessels rupture during the later part of pregnancy as the uterus begins to gradually dilate. The mother may bleed to death, and the fetus will also be placed in jeopardy because of the compromised blood supply.

a. Placenta previa is clinically associated with repeated episodes of bright red vaginal bleeding.

b- Because the placenta blocks the cervical opening, delivery is usually accomplished by cesarean section.

3. Placenta as an allograft. The fetal component of the placenta inherits both paternal and maternal genes and, therefore, may be considered as an allograft with respect to the mother. However, the placenta is not rejected in most cases. The two factors responsible for the lack of rejection are:

Smooth Chorion

Smooth chorion capsularis parietalis

Intervillous space

Decidua

Villous chorion

Amnion

Intervillous space

Decidua basalis

Villous chorion

Spiral artery

Chorionic

Fetal Placental Amniotic Villi

Amnion

Decidua parietalis

Smooth chorion arteries (2)

Umbilical vein (1)

Figure 5-1. (A) Relationship of the fetus, uterus, and placenta in the early fetal period. The outer arrows indicate that as the fetus grows within the uterine wall the decidua capsularis expands and fuses with the decidua parietalis, thereby obliterating the uterine cavity. The inner arrows indicate that as the fetus grows, the amnion expands toward the smooth chorion, thus obliterating the chorionic cavity. (B) Diagram of the placenta. This diagram of the placenta is oriented in the same direction as (A) for comparison. Note the relationship of the villous chorion (fetal component) to the decidua basalis (maternal component). Maternal blood enters the intervillous space (curved arrow) via the spiral arteries and bathes the villi in maternal blood. The villi contain fetal capillaries and thus maternal and fetal blood cxchangc occurs.

Intervillous space

Decidua basalis

Villous chorion

Spiral artery

Chorionic

Amnion

Smooth chorion

Decidua parietalis arteries (2)

Umbilical vein (1)

Figure 5-1. (A) Relationship of the fetus, uterus, and placenta in the early fetal period. The outer arrows indicate that as the fetus grows within the uterine wall the decidua capsularis expands and fuses with the decidua parietalis, thereby obliterating the uterine cavity. The inner arrows indicate that as the fetus grows, the amnion expands toward the smooth chorion, thus obliterating the chorionic cavity. (B) Diagram of the placenta. This diagram of the placenta is oriented in the same direction as (A) for comparison. Note the relationship of the villous chorion (fetal component) to the decidua basalis (maternal component). Maternal blood enters the intervillous space (curved arrow) via the spiral arteries and bathes the villi in maternal blood. The villi contain fetal capillaries and thus maternal and fetal blood cxchangc occurs.

a. Syncytiotrophoblast cells lining the villous chorion lack major histocompatibility complex (MHC) antigens and thus do nor evoke an immune response, b- Decidual cells within the endometrial stroma secrete prostaglandin E2, which inhibits T lymphocyte activation.

4. Preeclampsia and eclampsia. Preeclampsia refers to the sudden development of maternal hypertension, edema, and proteinuria usually after week 32 of gestation. Eclampsia includes the additional symptom of convulsions.

a. Risk factors include nulliparity, diabetes, hypertension, renal disease, twin gestation, or hydatidiform mole (produces first trimester preeclampsia), b- The cause is abnormal placentation producing a mechanical or functional obstruction of the spiral arteries of the uterus. C. Treatment consists in delivery of the baby as soon as possible.

5. Twinning (Figure 5-2)

a. Dizygotic (fraternal) twins develop from two zygotes. The fetuses have two placentas, two chorions, and two amniotic sacs.

b. Monozygotic (identical) twins develop from one zygote. In 65% of cases, the fetuses have one placenta, one chorion, and two amniotic sacs. In the remaining 35% of cases, the fetuses have two placentas (separate or fused), two chorions, and two amniotic sacs.

II. PLACENTAL MEMBRANE

A. Layers

1. In early pregnancy, the placental membrane consists of the syncytiotrophoblast, cytotrophoblast (Langerhans cells), connective tissue, and endothelium of the fetal capillaries. Hofbauer cells are found in the connective tissue and are most likely macrophages.

2. In late pregnancy, the cytotrophoblast degenerates and the connective tissue is displaced by the growth of fetal capillaries, leaving the syncytiotrophoblast and the fetal capillary endothelium.

Chorion Monozygotic Twins

Figure 5-2. Arrangement of the placenta, chorion, and amniotic sac of (A) dizygotic twins and 35% of monozygotic twins and (B) 65% of monozygotic twins. In general, dizygotic twins can be distinguished from monozygotic twins by an inspection of the afterbirth. However, please note that in approximately 35% of cases the determination will be in error.

Figure 5-2. Arrangement of the placenta, chorion, and amniotic sac of (A) dizygotic twins and 35% of monozygotic twins and (B) 65% of monozygotic twins. In general, dizygotic twins can be distinguished from monozygotic twins by an inspection of the afterbirth. However, please note that in approximately 35% of cases the determination will be in error.

B. Function. The placental membrane separates maternal blood from fetal blood. Some substances (both beneficial and harmful to the. fetus) cross the placental membrane freely, whereas it is impermeable to others (Table 5^1).

C. Clinical correlation. Erythroblastosis fetalis occurs when Rh~positive fetal red blood cells (RBCs) cross the placental membrane into the maternal circulation of an Rh-negative mother. The mother forms anti-Rh antibodies that cross the placental membrane and destroy fetal RBCs, which leads to:

1. The release of large amounts of bilirubin (a breakdown product of hemoglobin) that may cause brain damage

2. Severe hemolytic disease whereby the fetus is severely anemic and demonstrates

Table 5-1

Substances That Cross or Do Not Cross the Placental Membrane

BENEFICIAL SUBSTANCES THAT CROSS THE PLACENTAL MEMBRANE

• Oxygen, carbon dioxide

• Glucose, amino acids, free fatty acids, vitamins

• Water, sodium, potassium, chloride, calcium, phosphate

• Urea, uric acid, bilirubin

• Fetal and maternal RBCs

• Maternal serum proteins, a-fetoprotein

• Steroid hormones (unconjugated)

• IgG (confers passive immunity)

HARMFUL SUBSTANCES THAT CROSS THE PLACENTAL MEMBRANE*

• Viruses—e.g., rubella, cytomegalovirus, herpes simplex type 2, varicella zoster, Coxsackie, variola, measles, poliomyelitis

• Category X Drugs (absolute contraindication in pregnancy)—e.g., thalidomide, aminopterin, methotrexate, busulfan (Myleran), chlorambucil (Leukeran), cyclophosphamide (Cytoxan), Phenytoin (Dilantin), triazolam (Halcion), estazolam (ProSom), warfarin (Coumadin), isotretinoin (Accutane), clomiphene (Clomid), diethylstilbestrol (DES), ethisterone, norethisterone, megestrcl (Mcgace), oral contraceptives (Ovcon, Levlen, Norinyl), nicotine, alcohol

• Category D Drugs (definite evidence of risk to fetus)—e.g., tetracycline (Achromycin), doxycycline (Vibramycin), streptomycin, Amikacin, tobramycin (Nebcin), phénobarbital (Donnatal), pentobarbital (Nembutal), valproic acid (Depakene), diazepam (Valium), chlordiazepoxide (Librium), alprazolam (Xanax), lorazepam (Ativan), lithium, chlorothiazide (Diuril)

• Carbon monoxide

• Organic mercury, lead, polychlorinated biphenyls (PCBs), potassium iodide,

• Toxoplasma gondii, Treponema palladium

• Rubella virus vaccine

• Anti-Rh antibodies

SUBSTANCES THAT DO NOT CROSS THE PLACENTAL MEMBRANE

• Maternally-derived cholesterol, triglycerides, and phospholipids

• Drugs (e.g., succinylcholine, curarc, heparin, methyldopa, drugs similar to amino acids)

♦See Chapter 26

total body edema (hydrops fetalis), which may lead to death. In these cases, an intrauterine transfusion is indicated.

III. AMNIOTIC FLUID is basically water that contains carbohydrates, lipids, proteins (e.g., hormones, enzymes, a-fetoprotein), desquamated fetal cells, and fetal urine.

A. Production. Amniotic fluid is produced by dialysis of maternal and fetal blood through blood vessels in the placenta and by excretion of fetal urine into the amniotic sac.

B. Resorption. After being swallowed by the fetus, the amniotic fluid is removed by the placenta and passed into the maternal blood.

C. Clinical correlations

1. Oligohydramnios occurs when there is a low amount of amniotic fluid (< 400 ml in late pregnancy). Oligohydramnios may be associated with the inability of the fetus to excrete urine into the amniotic sac due to renal agenesis. This results in many fetal deformities (Potter's syndrome) and hypoplastic lungs due to increased pressure on the fetal thorax.

2. Polyhydramnios occurs when the level of amniotic fluid is high (>2000 ml in late pregnancy). Polyhydramnios may be associated with the inability of the fetus to swallow due to anencephaly or esophageal atresia. Polyhydramnios is commonly associated with maternal diabetes.

3. a-Fetoprotein (AFP) is "fetal albumin," which is produced by fetal hepatocytes. It is routinely assayed in amniotic fluid and maternal serum between weeks 14 and 18 of gestation. AFP levels change with gestational age so that proper interpretation of AFP levels depends on an accurate gestational age.

a. Elevated AFP levels are associated with neural tube defects (e.g., spina bifida or anencephaly), omphalocele (allows fetal serum to leak into the amniotic fluid), esophageal and duodenal atresia (which interfere with fetal swallowing).

b. Reduced AFP levels are associated with Down syndrome.

4. Premature rupture of the amniochorionic membrane is the most common cause of premature labor and oligohydramnios. (Rupture of the amniochorionic membrane is commonly referred to as "breaking of the water bag.")

5. Amniotic band syndrome occurs when bands of amniotic membrane encircle and constrict parts of the fetus causing limb amputations and craniofacial anomalies.

IV. UMBILICAL CORD. The definitive umbilical cord contains the right and left umbilical arteries, left umbilical vein, and mucous connective tissue.

A. The umbilical arteries carry deoxygenated blood from the fetus to the placenta.

B. The left umbilical vein carries oxygenated blood from the placenta to the fetus.

C. Clinical correlations

1. The presence of only one umbilical artery within the umbilical cord is an abnormal finding that suggests cardiovascular abnormalities. (Normally, two umbilical arteries are present.)

2. Physical inspection of the umbilicus in a newborn infant may reveal:

a. A light gray shining sac indicating an omphalocele (see Chapter 7).

b. Fecal (meconium) discharge indicating a vitelline fistula (see Chapter 7). C. Urine discharge indicating a urachal fistula (see Chapter 8).

V. VASCULOGENESIS (de novo Blood Vessel Formation)

A. Mesoderm differentiates into angioblasts that form angiogenic cell clusters.

B. Angioblasts around the periphery of the angiogenic cell clusters give rise to the endothelium of blood vessels.

C. Vasculogenesis occurs initially in extraembryonic visceral mesoderm around the yolk sac on day 17 and later in mesoderm within the fetus.

A. Mesoderm differentiates into angioblasts, which form angiogenic cell clusters.

B. Angioblasts within the center of angiogenic cell clusters give rise to primitive blood

C. Hematopoiesis occurs initially in extraembryonic visceral mesoderm around the yolk sac during week 3.

D. Beginning in week 5, hemaropoiesis is taken over by a sequence of embryonic organs: liver, spleen, thymus, and bone marrow.

E. Several types of hemoglobin are produced during hematopoiesis.

1. During the period of yolk sac hematopoiesis, the earliest embryonic form of hemoglobin, called hemoglobin 52€2, is synthesized.

2. During the period of liver hematopoiesis, the fetal form of hemoglobin, called hemoglobin <x272> is synthesized. Hemoglobin ot272 is the predominant form of hemoglobin during pregnancy because it has a higher affinity for oxygen than does the adult form of hemoglobin (hemoglobin a2(32) and thus "pulls" oxygen from the maternal blood into fetal blood.

3. During the period of bone marrow hematopoiesis (approximately week 30), the adult form of hemoglobin, called hemoglobin cx202, is synthesized and gradually replaces hemoglobin a2y2.

F. Clinical correlation. Thalassemia syndromes constitute a heterogeneous group of genetic defects characterized by the lack of or decreased synthesis of either the a- globin chain («-thalassemia) or (3-globin chain (P-thalassemia) of hemoglobin a232-

1. Hydrops fetalis is the most severe form of a-thalassemia. It causes severe pallor,

VI. HEMATOPOIESIS (Blood Cell Formation; Figure 5-3)

cells.

Yolk sac

Bone marrow

Yolk sac

Bone marrow

Hematopoiesis Fetal And Adult Diagram

10 12

20 Weeks

10 12

20 Weeks

Birth

Figure 5-3. Diagram showing the contribution of various organs to hematopoiesis during embryonic and fetal development.

Shunts:

Inferior vena cava

Left umbilical vein (f02) (Adult remnant: ligamentum teres)

Shunts:

Inferior vena cava

Left umbilical vein (f02) (Adult remnant: ligamentum teres)

Umbilical Cord Formation

@ Ductus arteriosus (Adult remnant: ligamentum arteriosum)

- (2) Foramen ovale {Adult remnant: fossa ovale)

©Ductus venosus (Adult remnant: ligamentum venosum)

Figure 5-4. Fetal circulation. Note the three shunts and the changes that occur after birth (remnants).

Right and left umbilical arteries (|02) {Adult remnant: medial umbilical ligaments)

@ Ductus arteriosus (Adult remnant: ligamentum arteriosum)

- (2) Foramen ovale {Adult remnant: fossa ovale)

©Ductus venosus (Adult remnant: ligamentum venosum)

Figure 5-4. Fetal circulation. Note the three shunts and the changes that occur after birth (remnants).

Table 5-2

Remnants Created by Closure of Fetal Circulatory Structures

Fetal Structure

Adult Remnant

Right and left umbilical arteries Left umbilical vein Ductus venosus Foramen ovale Ductus arteriosus

Medial umbilical ligaments Ligamentum teres Ligamentum venosum Fossa ovale

Ligamentum arteriosum generalized edema, and massive hepatosplenomegaly and leads invariably to intrauterine fetal death.

2. p-Thalassemia major is the most severe form of ^-thalassemia, It causes a severe, transfusion-dependent anemia. It is most common in Mediterranean countries and in parts of Africa and Southeast Asia.

VII. FETAL CIRCULATION involves three shunts: (1) ductus venosus, (2) foramen ovale, and (3) ductus arteriosus (Figure 5-4). Several changes occur in the neonatal circulation when right atrial pressure decreases owing to occlusion of placental circulation and when left atrial pressure increases due to increased pulmonary venous return from the lungs. Table 5-2 summarizes the remnants that result from closure of the fetal structures.

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

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Responses

  • Joel Gr
    Does sodium pentobarbital cross placental membranes?
    5 years ago
  • MILO
    Does the amnion cover the maternal surface of the placeta?
    5 years ago
  • Tegan
    Does sodium pentabarbital cross the placenta?
    5 years ago
  • azzeza
    What separates the amniotic sac and placenta?
    5 years ago
  • Nadine
    What is turtle surface of placenta?
    4 years ago

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