St A - Placentation and Implantation

Question 1

Explain the formation of the placenta

Answer 1

• Embryonic portion of placenta supplied from outermost layers of trophoblast cells (chorion) while maternal portion by endometrium underlying chorion.
  Choronic villi extend from chorion to endometrium. Endometrium around villi changes by enzymes and paracrine agents so each villi surrounded by pool of maternal blood.

Question 2

Describe the placenta blood supply

Answer 2

Maternal blood - enters sinuses via uterine artery and exits via uterine vein.
- Foetus blood flows into capillaries of choronic villi.
Umbilical cord connects foetus to placenta

Question 3

Describe the implantation and placentation time line

Answer 3

Fertilisation (24 hours post ovulation),
Transport to uterus (3-4 days),
Formation of blastocyst (4.5 days)
Attachment (7-9 days)

Question 4

Explain what occurs at the end of week 1 when attachment and implantation occurs

Answer 4

• Day 6/7 the blastocyte leaves zona pelucida and is bather in uterine secretions for 2 days (progesterone prepares supportive environment and oestrodiol required to release glandular secretion)
• Attachment and implantation (complex interaction)

Question 5

What occurs at day 13 (when woman expects nect period)

Answer 5

• Syncytiotrophoblast cells erode walls of maternal capillaries which bleed into spaces (primitive placental circulation - Nutrition still depends on uterine secretion). Breakthrough bleeding may occur

Question 6

Chorionic villi are formed from what? and describe features of villi

Answer 6

Syncytiotrophoblasts. In the core of villi is a foetal capillary loop, eventually the villi become localised at embryonic pole presenting large surface area for exchange of O2, nutrients and waste.

Question 7

Describe the supply of nutrients in the first trimester

Answer 7

• Limited embryonic growth, nutrition is largely based on uterine secretion and tissue

Question 8

How thick does endometrium need to be for successful implantation?

Answer 8

8mm

Question 9

What is the function of hCG and what is it produced from?

Answer 9

Syncytiotrophoblasts secrete hCG soon after implantation. Its function is to maintain progesterone secretion from the corpus luteum until the placenta can synthesis its own progesterone

Question 10

What are three main functions of progesterone in pregnancy?

Answer 10

• Suppression of follicular growth and ovulation,
• Suppression of the immune response,
• Maintenance of endometrium
  (cholesterol from maternal circulation is substrate for progesterone)

Question 11

Describe the function of oestrogen in pregnancy?

Answer 11

• Stimulates continuous growth of uterine myometrium,
• Stimulates growth of ductal tissue of breast,
• Relaxes and softens maternal pelvic ligaments and symphysis pubis of pelvic bones (expansion of uterus),
• Stimulates LDL cholesterol uptake and activity of P450 enzymes

Question 12

monitoring oestrogen levels is useful for?

Answer 12

Measuring foetal well being and placental function

Question 13

Briefly describe the placenta in the 1st, 2nd and 3rd month

Answer 13

1st - Villus formation,
2nd - Increasing surface area and circulation,
3rd - Growing and becoming increasingly more efficient.

Question 14

What are the placental functions?

Answer 14

Adopts functions of GI (supplying nutrients), respiratory (exchanging CO2 and O2), and renal systems (Regulates fluid volumes and disposal of waste metabolites). It acts as an endocrine gland, synthesising steroids and proteins that affect both maternal and foetal metabolism. Most molecules can pass through the membrane.

Question 15

Describe the nutrient exchange across the placenta

Answer 15

• Water and electrolytes diffuse freely,
• Glucose passes via facilitated diffusion,
• Amino acids are actively transported for foetal growth,
• Lipids cross as FFA

Question 16

Describe the gas exchange across the placenta

Answer 16

• Simple diffusion of gases across membrane is close to the efficiency of the lungs.
• Foetal haemoglobin has a greater affinity for O2 than adult haemoglobin. Towards the end of pregnancy, exchange capacity decreases and placenta is less able to meet demands of baby.