L13: Placenta

Question 1

What are the different types of placentas? What are the examples of animals?

Answer 1

• Discoid (human, rodent)
• Zonary (cat, dog)
• cotyledonary (cow, sheep)
• diffuse (horse)

Question 2

What are ‘caruncles’?

Answer 2

Implantation sites in sheep, blastocyst implants on these sites, doesn’t touch the whole surface just these parts

Question 3

What is cotyledon?

Answer 3

Implantation sites from fetal perspective, they implant on caruncles

Question 4

What is placentome?

Answer 4

Individual placental unit, a combination of caruncle and cotyledon

Question 5

What are the different types of placentation? What are their animal examples?

Answer 5

• Haemochorial - human, rodent (discoid)
• epitheliochorial - pig, horse (diffuse)
• synepitheliochorial - sheep, cow (cotyledonary)

Question 6

Which is the critical tissue for placenta development?

Answer 6

Trophoblast

Question 7

What are cytotrophoblast?

Answer 7

The trophoblast progenitor cells

Question 8

What is syncytiotrophoblast and how does it form?

Answer 8

Forms by fusion of cytotrophoblast cells to form a multinuclear tissue

Question 9

Which layer contains fetal vessels? How are the fetal and maternal blood separated?

Answer 9

In the human haemochorial placenta, it is the syncytiotrophoblastic layer with its underlying cytotrophoblast and mesodermal tissues containing fetal blood vessels that is interposed between the fetal and maternal circulations, and bathed in maternal blood.

Question 10

How do fetal membranes form?

Answer 10

Amnion and chorion form a sac surrounding the fetus

Question 11

What are the roles of placenta?

Answer 11

• protection of fetus from maternal immune system (trophoblast layer)
• placental transfer (trophoblast layer)
• endocrine organ

Question 12

How does protection of fetus from maternal immune system happen?

Answer 12

• protective mechanisms (immunosuppresion (e.g. progesterone, cytokines); self/non-self - HLA-G major histocompatibility complex (MHC on trophoblst - inhibit natural killer (NK) cells)
• ‘semi-allograft’ - trophoblast layer at interface and uterus specific

Question 13

How does placental transfer happen?

Answer 13

i) Diffusion O2, CO2, fatty acids, some electrolytes, fat soluble vitamins, therapeutic agents)
ii) lipophilic - transcellular and high permeability
iii) hydrophilic - diffusion limited. e.g. urea (don’t equilibrate between fetus and mother)
iv) facilitated diffusion - membrane bound carrier protein e.g. glucose transporter (Glut-1)
v) active transport - Na+ dependent and independent - e.g. Fe, Ca, amino acids
vi) endocytosis - lipoproteins and immunoglobulins (passive immunity)

Question 14

How does placenta work as an endocrine organ?

Answer 14

• human chorionic gonadotrophin (hCG)
  i) homology with gonadotrophins
  ii) rescue corpus luteum / maternal recognition
  iii) pregnancy test and marker for Down Syndrome
• progesterone
  i) immune suppression
  ii) suppression of uterine activity
• oestrogens
  i) blood volume and vascular tone
  ii) parturition
• other peptide hormones
  i) placental lactogen (PL) - 96% homology with GH
  ii) corticotrophin-releasing hormone (CRH) - feto-placental interaction and fetal maturation

Question 15

Where does fetal and placental steroidogenesis take place in sheep?

Answer 15

in placental trophoblast cells to maintain pregnancy

Question 16

How is fetal growth assessed?

Answer 16

Ultrasound scanning
i) Early pregnancy (8-12 weeks): viability, gestational age by crown rump length; fetal number; serious pathology
ii) Detailed anomaly scan (18-20 weeks)
iii) Placenta and amniotic fluid (fetal >20 weeks): polyhydramnios (too much amniotic fluid) and oligohydramnios (too little amniotic fluid) (can be signs of kidneys action, might be something wrong, as it is formed from baby’s urine)
iv) Fetal growth and assessment
v) Invasive procedures (amniocentesis, chorionic villus [placental] sampling, fetal surgery)

Question 17

What are the factors affecting prenatal growth?

Answer 17

• Fetal and maternal genome and genetic factors – regulates responses to factors defining ‘size’ (e.g. growth factors) – small dad, small mom – small baby
• Hormones and growth factors – complex series of interactions between mother-placenta-fetus
  i) Insulin – diabetics and gestational diabetics have big babies
  ii) Growth hormone
  iii) Placental lactogen?
  iv) Thyroxine
  v) Growth factors – IGFs, TGFs, FGF, etc
• Maternal nutrition and health
  i) Polytocous animals – undernutrition has affect but often needs to be extreme
  ii) Less of an effect in man (?)
• Environmental factors
  i) Can affect nutrition and health, litter size
• Litter size and duration of pregnancy
  i) Small animals rapid growing and relatively heavier litters compared to larger animals
• Placental size and function. Fetal growth may be restricted by:
  i) Abnormal differentiation or maternal nutrition limitations: use as model for IUGR; may have effect on chronic diseases in adulthood
• Pathophysiological reduction in blood flow
  i) Pregnancy induced hypertension / pre-eclampsia