L6: Placenta and twins

Question 1

What is the definition of the placenta?

Answer 1

the primary site of nutrient and gas exchange between mother and fetus.

Question 2

What is the structure of the placenta?

Answer 2

✓ Fetal part: develops from chorion frandosum.

✓ Maternal part: derived from the endometrium (decidua basalis and maternal blood in intervillous space).

Question 3

What are the surfaces of the placenta like?

Answer 3

✓ Fetal surface of the placenta:
• Smooth-shiny and covered by amnion.
• Umbilical cord attached near its center.

✓ The maternal surface of the placenta:
• Shows cotyledons with Cobblestone appearance
produced by bulging villi.

Question 4

What is the gross appearance of the placenta?

Answer 4

✓ Shape: discoid.
✓ Diameter: 15-20cm.
✓ Thickness: 2-3cm.
✓ Wight: 500-600gm (about 1\6 weight of fetus).

Question 5

What structures could be seen by the microscope in the placenta?

Answer 5

1) Intervillous space

2) Placenta barrier or membrane

Question 6

What are intervillous spaces?

Answer 6

❖ Large blood-filled space results from coalescence and enlargement of lacunar networks.

❖ Contain maternal blood derived from lacuna developed in syncytiotrophoblast during 2nd week of development.

Question 7

What is the definition of the Placenta barrier or membrane?

Answer 7

the membrane separating maternal from fetal blood.

Question 8

What is the structure of the placenta barrier or membrane?

Answer 8

Until 20 weeks
✓ 4 layers:
1) Syncytiotrophoblast.
2) Cytotrophoblast.
3) Extra embryonic mesoderm.
4) Endothelium of fetal capillaries.

After 20 weeks
✓ cytotrophoblast is lost so the barrier becomes 3 layers: 1) Syncytiotrophoblast.
2) Extra embryonic mesoderm
3) Endothelium of fetal capillaries.

Late in pregnancy:
✓ extraembryonic mesoderm is lost, the barrier becomes formed of 2 layers:
1) Syncytiotrophoblast.
2) Enodothelium of fetal capillaries

Question 9

What is the function of the placenta?

Answer 9

“REEM PN”

1) Respiratory function: by simple diffusion

2) Nutritive function: by simple and selective diffusion

3) Excretory function: by simple and selective diffusion

4) Protective function

5) Metabolism

6) Endocrine function

Question 10

What is the protective function of the placenta?

Answer 10

 Transfer maternal antibodies as for diphtheria, smallpox, and measles.

 Barrier against transmission of some infection from mother to fetus but some viruses as German measles and syphilis pass through placental membrane and
cause fetal infection.

Question 11

What is the endocrine function of the placenta?

Answer 11

 Chorionic gonadotrophin hormones (early in pregnancy)
• Trophoblast produces chorionic gonadotropin hormone which maintains corpus luteum and stimulates corpus luteum to secret progesterone.

 Progesterone (later in pregnancy > 4th month)
• Trophoblast produces progesterone from the 4th month which is essential for the maintenance of pregnancy.

 Estrogen (near labor)
• Produced by syncytiotrophoblast which increases the sensitivity of the myometrium to oxytocin which has a role in delivery.

Question 12

What are the anomalies of the placenta?

Answer 12

• Position:
  ❖ Placenta previa.
  ❖ Placenta increta & percreta: chorionic villi penetrate myometrium.
• attachment of umbilical cord to the placenta.
• shape:
  ❖ Zonary placenta: empty in middle.
  ❖ Bipartite placenta (bilobed): with one umbilical cord.
  ❖ Tripartite placenta: with one umbilical cord attached to it.
  ❖ Irregular placenta.
  ❖ Membranous placenta: thin and membranous.
• number:
  ❖ Placenta succentoriata: There is one or more small accessory placenta in addition to the main placenta which are retained in the uterus after delivery and cause postpartum hemorrhage

Question 13

What is the maternal circulation in the placenta?

Answer 13

Maternal blood enters the intervillous spaces from the spiral arteries of the decidua and after gas exchange leaves the spaces via numerous, decidual Veins to the uterine vein.

Question 14

What is the fetal circulation in the placenta?

Answer 14

❖ The deoxygenated blood reaches the chorionic villi by two umbilical arteries.

❖ Exchange of gasses, nutrients, and waste products occurs through the placental
membrane and the oxygenated blood returns to the fetus by the umbilical vein.

Question 15

What is the definition of multiple pregnancies?

Answer 15

Delivery of more than one fetus:
• Two fetuses (twins).
• Three fetuses (triplets).
• Four fetuses (quadruplets).
• Five fetuses (quintuplets).

Question 16

What is the incidence of multiple pregnancies?

Answer 16

✓ Twins: once every 90 pregnancies.
✓ Triplets: once every 8100 pregnancies.
✓ Quadruplets and others are rare

Question 17

What are the causes of multiple pregnancies?

Answer 17

✓ Hereditary.

✓ Medical causes: due to administration of exogenous gonadotrophin to stimulate ovulation in women with ovulatory failure and also In vitro fertilization.

Question 18

Where are multiple pregnancies more common?

Answer 18

• Race: most common in Negroes
• Age: Increased maternal age
• Parity: more common in multipara
• Heredity: family history of multifetal gestation
• Nutritional status: well-nourished women

Question 19

• The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.

• Placenta grows throughout pregnancy.

• Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy (approximately 12– 13 weeks).

Answer 19

…

Question 20

What are the types of multiple pregnancies?

Answer 20

✓ Monozygotic twins. ✓ Dizygotic twins.

Question 21

What is the definition of monozygotic (identical - true - uniovular) twins?

Answer 21

fertilization of one oocyte (develops from one zygote).

Question 22

What is the cause of monozygotic twins?

Answer 22

results from splitting of the zygote at various stages of development

Question 23

what are the incidences of monozygotic twins?

Answer 23

30% of twins

Question 24

What are the characters of monozygotic twins?

Answer 24

❖ Of the same sex, genetically identical, similar in physical character and the difference occur only by the environment.

❖ Of the same fingerprint.

Question 25

what are the various stages of development at which the separation may take place?

Answer 25

• Separation in two cell stages (up to 3 days)
• Separation in inner mass stage (between 4th & 7th day)
• Separation in the bilaminar disc (between 8th & 12th day)

Question 26

What happens if separation takes place in two cell stages (up to 3 days)?

Answer 26

❖ Two fetuses have strong resemblance as regard blood group, fingerprint, sex, the color of the eye, and hair.

❖ Have 2 amnion, 2 placentas and 2 chorions.

Question 27

What happens if separation takes place in the inner mass stage (between the 4th & 7th day)?

Answer 27

❖ Results from the division of inner cell mass into 2 embryos

❖ Have 2 amnion, 1 placenta, and 1 chorion

Question 28

What happens if separation takes place in the bilaminar disc (between the 8th & 12th day)?

Answer 28

❖ Cells of bilaminar disc divide into equal groups to give rise to equal embryos or unequal groups to give one large fetus and the other is small.

❖ Have 1 amnion, 1 placenta, and 1 chorion.

Question 29

What are the causes of conjoined monozygotic twins? (Monsters or Siamese)

Answer 29

the embryonic disc does not divide completely.

Question 30

What are the characters of conjoined monozygotic twins?

Answer 30

United in Ventral:
1) Omphalopagus
2) Thoracopagus (in thoracic)
3) Cephalopagus (in the head)
4) Caudal/ ischiopagus

Lateral: Parapagus

Dorsal:
1)Craniopagus
2)Pygopagus

Question 31

What is the cause of dizygotic (in identical, biovular, and fraternal) twins?

Answer 31

fertilization of two oocytes by different sperms to form zygotes

Question 32

What is the incidence of dizygotic twins?

Answer 32

70% of twins.

Question 33

What are the characters of dizygotic twins?

Answer 33

 Same sex, or different.
 Vary in physical appearance.
 Each has 1 amnion, 1 chorion, 1 placenta but the chorion and placenta may be fused