Placenta Development/Function

Question 1

Placental functions

Answer 1

• Metabolism
  
  • Synthesizes glycogen, cholesterol, and fatty acids
  • Detoxifies
  • Produces enzymes
• Transport/transfer
• Hormone production
• Immunological

Question 2

Name two classes of hormones produced by the placenta and some examples of them

Answer 2

Steroid hormones

• Progesterone
  
  • Maintains pregnancy, inhibits myometrial activity, supports mammary growth
• Estrogen
  
  • Increases uterine growth and vascular supply to decidua and myometrium, increases metabolism and placental enzyme systems
• Androgens, corticosteroids

Peptide Hormones

• Human Chorionic Gonadotropin (HCG)
  
  • supports initial embryonic growth/development
• Human Placental Lactogen
  
  • competes with insulin, factor in GDM
• Human chorionic adenocorticotropic hormone
• CTRH
• Relaxin
• Inhibin

Question 3

Immonological functions of placenta

Answer 3

• Trophoblast does not express target antigens
• Maternal immune response –> “tolerance”
• Maternal – fetal “cellular traffic”

Question 4

Types of transport across placenta

Answer 4

• Simple - diffusion across gradients
  
  • Gases, drugs, steroids, electrolytes
• Facilitated diffusion - need carrier molecule but no energy
  
  • Glucose, cholesterol
• Active transport
  
  • Amino acids, vitamins, transferrin, calcium, iodine
• Pinocytosis
  
  • IgG

Question 5

Amniotic fluid: origin

Answer 5

• Begins from maternal plasma in 1st trimester
• Mostly fetal urine and some fetal lung fluid by second trimester

Question 6

Amniotic fluid functions

Answer 6

• Protects baby from maternal trauma, cushions umbilical cord
• Provides space and growth factors for movement and limb development. Also lungs and GI
• Antibacterial properties that provide some protection from infection
• Reservoir of fluid and nutrients for the fetus

Question 7

Amniotic fluid composition

Answer 7

1st half of pregnancy: derived from fetal and possibly maternal compartments. Water and solutes freely traverse fetal skin and may diffuse through the amnion and chorion as well. Thus amniotic fluid in early gestation is a dialysate that is identical to the fetal and maternal plasma, but with a lower protein concentration

2nd half of pregnancy: mostly fetal urine (hypotonic) that contains increasing concentrations of urea, uric acid, and creatinine as the fetal kidneys mature. Also lung fluid, oronasal fluid, transmembranous and intermembranous flow.

Question 8

Umbilical cord anatomy

Answer 8

• 2 arteries – come from fetus to placenta with deoxygenated blood
• 1 vein – comes from placenta to fetus carrying oxygen. Thinner walls than arteries (also larger).
• Placental capillaries – mixed deoxy and oxy
• Wharton’s jelly protects umbilical cord vessels → helps prevent compression
• Veins collapse first and then arteries.

Question 9

Placenta formation

Answer 9

• Late luteal phase: stromal cells of endometrium get larger, mature
• Ovulation → fertilization → blastocyst formation as it moves along falopian tube → embeds into primed endometrium
• Secretory endometrium undergoes decidual reaction - protected space from maternal immune system. 3 layers of decidua form under direction of pregnancy hormones
  
  • Decidua basalis – spongy, arteries and dilated veins
  • Decidua capsalaris – outer capsule that overlies trophoblast
  • Decidua parietalis - goes around the whole uterine cavity
• Trophoblast cells surrounding blastocyst → project into decidua basalis
• Trophoblast cells differentiate into 2 layers:
  
  • Syncytiotrophoblast (outer) - multinucleated protoplasm mass sends finger-like projections into endometrium, taking in uterine cells along the way. Secretes steroid and peptide hormones necessary for pregnancy.
  • Cytotrophoblast – inner layer, also secretes important peptide hormones → develops into chorionic villi (project into syncytiotrophoblast to form anchor villi, some arborize to form lots of branches → area of gas exchange)
• Extavillous trophoblasts → migrate into the decidua and myometrium and also penetrate maternal vasculature and remodel spiral arteries
  
  • altered in PEC and IUGR
• Syncytiotrophoblast grows → lacunae (spaces) that will fill with maternal blood from spiral arteries as they are remodeled into low-resistance/pressure, continuous flow.
• Lacunae – uterine vessel communication begins uteroplacental circulation
• Fetal blood cells circulate by Day 21

Question 10

When does initial implantation occur?

Answer 10

6 or 7 days post fertilization

Question 11

Amnion development

Answer 11

• Early during implantation, a space develops between the embryonic cell mass and adjacent trophoblasts.
• Small cells that line this inner surface of trophoblasts (amniogenic cells) → amnionic epithelium.
• Identifiable on 7th or 8th day of embryo development
• Iinitially a minute vesicle, then develops into a small sac that covers the dorsal embryo surface.
• As the amnion enlarges, it gradually engulfs the growing embryo, which prolapses into its cavity

Question 12

Chorion development

Answer 12

• As the blastocyst with its surrounding trophoblasts grows and expands into the decidua, one pole form the placenta from villous trophoblasts and anchoring cytotrophoblasts. The other pole faces the endometrial cavity.
• As growth of embryonic and extraembryonic tissues continues, the blood supply to the chorion facing the endometrial cavity is restricted → villi in contact with the decidua capsularis cease to grow → degenerate → avascular fetal membrane that abuts the decidua parietalis, that is, the chorion laeve (smooth chorion).
• This smooth chorion is composed of cytotrophoblasts and fetal mesodermal mesenchyme that survives in a relatively low-oxygen atmosphere.

Question 13

What is the nitabuch layer and what is the clinical significance?

Answer 13

• 1 of the 3 layers of the decidua basalis
• Can see it on U/S → missing nitabuch layer indicates placenta accreda