Placentation and Implantation

Question 1

Mother-Foetus Link Development

Answer 1

In the earliest stages of pregnancy the anatomical link between mother and foetus develops through a series of phases: Invasion (of conceptus to endometrium), Decidualisation (i.e., endometrial remodelling including secretory transformation of the uterine glands, influx of specialised uterine natural killer cells, and vascular remodelling), Placentation (placenta formation).

Question 2

Placentation

Answer 2

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

Question 3

Placental Blood Supply

Answer 3

• Maternal blood - enters placental sinuses/pools via uterine artery. Flows through sinuses. Exits via uterine veins.
• Foetus blood - flows into capillaries of chorionic villi via umbilical arteries. And back to foetus via umbilical vein.
• Umbilical cord connects foetus to placenta.

Question 4

Attachment and Implantation

Answer 4

• Day 6/7 the blastocyst leaves the zona pellucida and is bathed by uterine secretions for 2 days: progesterone prepares supportive uterine environment increasing glandular tissue, oestradiol is required to release the glandular secretion.
• Attachment and Implantation - very limited time window: complex interactions between trophoblast and maternal epithelial tissue causes syncytiotrophoblast cells flow into the endometrium and also causes oedema, glycogen synthesis and increased vascularisation (decidualisation). The pregnant endometrium is not termed the decidua.

Question 5

Implantation

Answer 5

• 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 (0,1-0.2mm)

Question 6

Placental Development

Answer 6

• Syncytiotrophoblast forms villi that project into the blood filled spaces (chorionic villi). In the core of the villus is a foetal 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 dependent on uterine tissues for nutrients and O2.

Question 7

Chorionic Villi

Answer 7

• Maternal and foetal circulations are separated by the placental membrane. There is no mixing of maternal and foetal blood.
• Syncytiotrophoblast is bathed in maternal blood.
• 1st Trimester - 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 8

Human Chorionic Gonadotropin (hCG)

Answer 8

Maintains progesterone secretion from the corpus luteum until the placenta can synthesise its own progesterone. Syncytiotrophoblasts secrete hCG soon after implantation (peaks at approx 8-10 weeks of gestation). Measurable by day 7/8 after conception.

Question 9

Roles of hCG

Answer 9

• In a non-fertile cycle the Corpus Luteum 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 foetuses to produce testosterone - important for development of the male duct system.

Question 10

Progesterone’s Role in Pregnancy

Answer 10

• must have progesterone for the maintenance of pregnancy - increases throughout pregnancy. Suppresses follicular growth and ovulation, suppresses immune response and maintenance of endometrium.
• After 4-5weeks placenta is secreting all steroid hormones required for pregnancy (corpus luteum not required after 5 weeks).
• Foetus is self sufficient.
• Cholesterol from the maternalcirculation is the substrate for progesterone production.

Question 11

Oestrogen’s Role in Pregnancy

Answer 11

• oestradiol is the main oestrogen in pregnancy.
• foetus and placenta (foeta-placental unit) cooperate to secrete oestrogens (mainly oestriol).
• stimulate continuous growth of uterine myometrium.
• stimulate growth (with progesterone) of ductal tissue of breast.
• along with relaxin, relaxes and softens maternal pelvic ligaments and symphysis pubis of pelvic bones - allows expansion of uterus.
• Stimulate LDL cholesterol uptake and activity of P450 enzymes - contribute to progesterone synthesis.
• Foetal well being and placental function can be measured by monitoring oestrogen levels.

Question 12

Placental Functions

Answer 12

• 1st month - villus formation
• 2nd month - increasing surface area and circulation
• 3rd month - growing, becoming increasingly efficient.
• Adopts the functions of the GI (supplies nutrients), respiratory (exchanges O2 and CO2), and renal (regulates fluid volumes and disposing of waste metabolites) 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.
• Synthesises steroids and proteins that affect both maternal and foetal metabolisms.

Question 13

Nutrient Exchange Across Placenta

Answer 13

• 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 and storage of glycogen in liver for postnatal requirements).
• Amino acids are actively transported for foetal growth.
• Lipids cross as free fatty acids.
• Vitamins.

Question 14

Gas Exchange of Placenta

Answer 14

• 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.
• Foetal haemaglobin 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.