Placental growth, development and function

Question 1

Placental development

Answer 1

1. After fertilization the zygote is formed and by cleavage process morula is formed
   and which eventually forming the blastocyst is formed (cells encapsulating fluid
   cavity).
2. By day 7 blastocyst penetrates endometrium by trophoblastic invasion and a
   differentiation process resulting in syncytiotrophoblasts and cytotrophoblasts.
3. Decidual reaction when the endometrial stromal cells of the uterus come in
   contact with the syncytiotropholbsatic cells and become large and pale. Decidual
   cells nourish the growing placenta.
4. Formation of the villi begins continuous invasion of the trophoblast cells.

a. Primary villi – finger-like protrusions of the trophoblasts.
b. Secondary villi – external mesodermal cells forming the core inside the
protrusion.
c. Tertiary villi – formation of vasculature within the core

The villi will continue to grow and branch and eventually will form the villus tree.

5. Areas in relation to the developing placenta
   a. Deciduca basalis – decidua under the developing placenta
   b. Decidua capsularis – decidua in contact with the chorion

c. Decidua parietalis – the marginal decidua
d. Chorion frondosum – the area the develops to the placenta

e. Chorion laeve – outer layer of fetal membranes

6. Maternal vasculature pushes the villus tree to arrange into maternal cotyledons (functional unit of the placenta) which are separated by septa.
7. The final structure of the villus tree will contain a broad stem, free floating villi and anchoring
   villi. Initially there will be presence of 200 stem villi which will decrease in time to 40-50.

Question 2

Structure of the umbilical cord

Answer 2

1. Structure and function

a. Contains 2 arteries and 1 vein suspended within Warthon’s jelly covered by amniotic
epithelium.

b. Arteries contain deoxygenated blood.
c. Umbilical vein carries oxygenated blood to the fetus.

d. Cord length is about 30-90cm.

e. Vessels within the cord are helical which has protecting feature against twisting.
f. Arterial cord pressure is 70/60mmHg and venous pressure is 25mmHg.

g. Blood flow in the cord reaches 350mL/min

Question 3

Uteroplacental blood flow

Answer 3

Formation of the intervillous space begins with small lacunas of blood dispersed between the
decidual cells and eventually are dispersed between the syncytiotrophoblasts.

They are supplied by maternal spiral arteries.

Growing of the lacunas is due to their fusion with adjacent lacunasuntil the maternal blood “baths” the blastocyst allowing for further growth of the villi and until the formation of the cotyledon.

8. Trophoblastic invasion to maternal spiral arterioles leads to destruction of smooth muscle and
   formation of sinusoids which makes the blood circulation more efficient.
9. Uterine blood flow is between 500-750mL/min.
10. Factors influencing uterine blood flow
    a. Catecholamines
    b. Mechanical force
    c. Hemorrhages
    d. RAAS
    e. Vasodilators
11. Transfer of material is by simple diffusion, facilitated diffusion (passive and active) and
    pinocytosis.

Question 4

Placental functions

Answer 4

1. Gas exchange

a. Simple diffusion of oxygen.

b. What favors O2 uptake by the fetus and placenta? (right shift in the oxygen dissociation
curve – Bohr’s effect)
i. PO2 of maternal is 90-100mmHg which is higher than the fetal PO2.
ii. The period of high fetal metabolism produces high CO2 levels that promote O2
uptake from the mother.
iii. Fetal hemoglobin has higher affinity for oxygen.

c. The placenta utilizes high amount of oxygen and remaining goes to the fetus.

2. Nutrition and excretion
   a. Maternal glucose crosses the placenta and provides 90% of fetal energy.

b. Hormones regulating glucose metabolism do not cross the placenta – insulin, glucagon,
human placental lactogen and growth hormone.

c. Glycogen is stored in the placenta and also in the fetal liver which serves as energy
source for the period after pregnancy and for asphyxiated fetus.
d. Free fatty acids cross the placenta.

e. Proteins are synthesized by the fetus and can cross the placenta via active transporters.
f. Immunoglobulin are synthesized by the fetus and IgM appears first in the fetus by week
20.

g. IgG is the only one who can cross the placenta.

3. Hormones synthesis
   a. hCG produced by trophoblasts similar to (LH). It maintains corpus callosum which is
   responsible for progesterone until the placenta is able to produce it by itself.

b. hPL produced by syncytiotrophoblasts. It increases free fatty acids and insulin levels.
Used to assess placental function (low levels indicate placental failure).

c. Progesterone is formed by the placenta by week 17. About 350mg are formed by full
term.
d. Estrogen is mainly formed by the placenta and to lesser extent by the ovaries.