Placentation and Implantation

Question 1

In the earliest stages of pregnancy the anatomical link between mother and foetus develops through a series of phases.

Answer 1

• Invasion of conceptus to endometrium
• Decidualisation ie endometrial remodeling including secretory
transformation of the uterine glands, influx of specialised uterine natural killer cells, and vascular remodeling.
• Placentation ie placenta formation

Question 2

Formation of the placenta

Answer 2

• Embryonic portion of placenta supplied from outermost layers of trophoblast cells (ie the chorion).
• Maternal portion by endometrium underlying the chorion.
• Choronic villi * extend from chorion to endometrium.
• Villi have network of capillaries – part of embryo’s circulatory system.
• Endometrium around villi is changed by enzymes & paracrine agents so each villi is surrounded by a pool/sinus of maternal blood.

Question 3

Maternal blood

Answer 3

– Enters placental sinuses/pools via
uterine artery.
– Flows through sinuses.
– Exits via uterine veins

Question 4

Foetus blood

Answer 4

– Flows into capillaries of choronic villi
via umbilical afteries
– And back to foetus via umbilical vein
• Umbilical cord connects foetus to placenta

Question 5

Implantation and placentation Timeline

Answer 5

• Fertilisation occurs 24 hours post ovulation
• Transport to uterus 3 – 4 days
• Formation of blastocyst 4.5 days
• Attachment 7-9days

Question 6

• Day 6/7 the blastocyst leaves the zona pelucida and is bathed by uterine secretions for 2 days:

Answer 6

– progesterone prepares supportive uterine environment increasing glandular tissue
– oestrodiol is required to release the glandular secretion

Question 7

Attachment and Implantation:

Answer 7

– very limited time window:
– complex interactions between trophoblast and maternal epithelial tissue
• causes syncytiotrophoblast cells ‘flow’ into the endometrium
• causing oedema, glycogen synthesis and increased vascularisation (decidualisation). The pregnant endometrium is now termed the decidua.

Question 8

Day 13 Implantation

Answer 8

Syncytiotrophoblast cells erode through the walls of large maternal capillaries which then bleed into the spaces - primitive placental circulation
Nutrition still depends on uterine secretion and tissues
Breakthrough bleeding may occur
Growth in the embryonic disk is slow and it remains very small (0.1 - 0.2 mm)

Question 9

Placental development

Answer 9

• Syncytiotrophoblast forms villi that
project into the blood filled spaces
(chorionic villi). In the core of the villus is a fetal capillary loop - dilated at the tip (slow flow rate)
• Embryonic placental structure develops over several weeks. The villi eventually becoming localised at the embryonic pole and presenting a huge surface area for exchange of O2, nutrients and
waste products
• Maternal side of the placental circulation is restricted and is not functional until 10 – 12 weeks
• First trimester embryo largely dependant on uterine tissues for nutrients and O2

Question 10

Chorionic villi

Answer 10

Maternal and fetal circulations are separated by the placental membrane. There is no mixing of maternal and fetal blood

Question 11

1st trimester:

Answer 11

limited embryonic growth (complex differentiation) - nutrition of the embryo is largely based on uterine secretion and tissues.
Lack of appropriate hormonal support (luteal phase defect) may account for early pregnancy losses.
Endometrium should be at least 8mm thick for successful implantation

Question 12

hCG

Human chorionic gonadotropin

Answer 12

Maintains progesterone secretion from the corpus luteum until the placenta can synthesise its own progesterone

Syncytiotrophoblasts secrete hCG soon after implantation (peaks ~8-10 weeks of gestation)
Measurable by d 7-8 postconception

Question 13

Role / functions of hCG

Answer 13

• In a non-fertile cycle the CL will fail after 10 days and menstruation will occur
• An implanting embryo must prevent menstruation. The syncytiotrophoblast
secretes Human Chorionic Gonadotropin (hCG)
• From day 6-7 after fertilisation hCG can be detected in maternal blood by immunoassay**
• hCG mimics the action of LH and supports the steroid synthesis of the corpus luteum, and therefore prevents both menstruation and any further follicular development
• hCG stimulates the Leydig cells of male fetuses to produce testosterone - important for development of the male duct system

Question 14

Progesterone in pregnancy

Answer 14

• Suppression of. follicular growth and ovulation
• Suppression of the immune response
• Maintenance of endometrium

Question 15

progesterone in pregnancy part 2

Answer 15

• After 4-5 weeks placenta is secreting all steroid hormones required for pregnancy (CL not required after 5 weeks)
• Foetus is self sufficient.
• Cholesterol from the maternal circulation is the substrate for progesterone production.

Question 16

Oestrogen in pregnancy

Answer 16

1. Oestriol is the main oestrogen in pregnancy
2. Foetus and placenta (feto-placental unit) cooperate to secrete oestrogens (mainly oestriol)
3. Stimulate continuous growth of uterine myometrium
4. Stimulates growth (with progesterone) of ductal tissue of breast
5. Along with relaxin, relaxes and softens maternal pelvic ligaments and symphysis pubis of pelvic bones – allows expansion of uterus
6. Stimulate LDL cholesterol uptake and activity of P450 enzymes – Contribute to progesterone synthesis
7. Foetal well being and placental function can be measured by monitoring oestrogen levels.

Question 17

PLACENTAL FUNCTIONS

Answer 17

• 1st month - villus formation
• 2nd month - increasing surface area and circulation
• 3rd month - growing, becoming increasingly efficient
• Adopts the functions of the GI, respiratory and renal systems
• Surface area of the diffusion membrane is huge
• Most molecules can pass through the membrane - after 20 weeks placental membrane thins even more with loss of cytotrophoblast
• 3rd trimester syncytiotrophoblastic cells may be lost into the maternal blood

Question 18

Placenta functions for foetus

Answer 18

1. Foetal “gut”: supplying nutrients.
2. Foetal “lung”: exchanging O2 and CO2.
3. Foetal “kidney”: regulates fluid volumes and disposing of waste metabolites.
4. Endocrine gland: synthesises steroids and proteins that affect both maternal and foetal metabolism.

Question 19

Nutrient exchange across the placenta

Answer 19

• Nutrient exchange is rapid and increases as pregnancy advances
• Water and electrolytes diffuse freely
• Glucose passes via facilitated diffusion. Foetus has little capacity for gluconeogenesis
• Babies of diabetic mothers are heavier than normal range • Storageofglycogeninliverforpostnatalrequirements
• Amino acids are actively transported for fetal growth
• Lipids cross as free fatty acids
• Vitamins

Question 20

Gas exchange

Answer 20

• Simple diffusion of gases across the membrane is close to the efficiency of the lungs.
• Concentration gradients are influenced by blood flow rates.
• Quantity of O2 reaching the foetus is flow limited.
• Fetal haemoglobin has a greater affinity for O2 than adult haemoglobin.
• Towards end of pregnancy exchange capacity decreases and placenta is less able to meet the demands of the foetus.