Implantation and the Placenta Flashcards

1
Q

Summarise the events of ovulation - fertilisation - implantation.

State where they occur.

A
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2
Q

What happens to the conceptus after it is formed?

A
  • Cell division to ~32 cells.
  • Conceptus ‘held’ in fallopian tube as oestrogen maintains contraction of smooth muscle near where fallopian tube enters the wall of the uterus.
  • Conceptus undergoes a number of mitotic cell divisions (cleavage) and a morula is formed.
    • Divisions are unusual as no cell growth occurs before each division, therefore the conceptus reaching uterus is the same size as original fertilised egg.
  • Cells are totipotent.
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3
Q

What are totipotent cells?

A
  • Totipotent cells can form all the cell types in a body, plus the extraembryonic, or placental, cells.
  • Embryonic cells within the first couple of cell divisions after fertilisation are the only cells that are totipotent.
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4
Q

What are pluripotent cells?

A
  • Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent.
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5
Q

What are multipotent cells?

A
  • Multipotent cells can develop into more than one cell type, but are more limited than pluripotent cells.
  • Adult stem cells and cord blood stem cells are considered multipotent.
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6
Q

Which kind of cell is used for genetic screening and why?

A
  • Blastocyst is pluripotent, so a cell can be removed for testing without damage to the embryo.
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7
Q

What happens when the blastocyst enters the uterine cavity?

A
  • Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes and conceptus passes into uterus.
  • Approximately 4-5 days after fertilisation, cavities develop between the cells.
  • For ~3 days, blastocyst lies free in the uterine cavity supported by uterine secretions, receiving nutrients from it.
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8
Q

What gives rise to the placenta?

A

Trophoblast (part of blastocyst)

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9
Q

What gives rise to the embryo?

A

Inner cell mass (part of the blastocyst)

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10
Q

What happens to the blastocyst on day 6?

A
  • Day 6 - blastocyst attaches to the endometrium adjacent to the inner cell mass (embryonic pole).
  • Trophoblast differentiates into:
    • Inner cytotrophoblast
    • Outer syncytiotrophoblast (loses cell boundaries)
  • Fingers of syncytiotrophoblast invade the endometrium.
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11
Q

Summarise the events of migration of the conceptus from fertilisation to implantation.

A
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12
Q

What is the sequence of events in the mother-fetus link development?

A
  • Invasion of conceptus to endometrium
  • Decidualisation - endometrial remodeling including secretory transformation of the uterine glands, influx of specialised uterine natural killer cells, and vascular remodeling.
  • Placentation - placenta formation.
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13
Q

Describe the attachment and implantation of the blastocyst.

State when this happens.

A
  • 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.
    • Oestrodiol 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 to ‘flow’ into the endometrium, causing oedema, glycogen synthesis and increased vascularisation (decidualisation). The pregnant endometrium is now termed the decidua.
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14
Q

Describe the events of day 13 (implantation).

A
  • This is when the woman would expect her next period.
  • 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 disc is slow and it remains very small (0.1-0.2mm).
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15
Q

Describe the development of the placenta.

A
  • 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 become 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.
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16
Q

What is the syncytiotrophoblast bathed in?

A

Maternal blood

17
Q

Summarise the formation of the placenta.

A
  • Embryonic portion of placenta supplied from outermost layers of trophoblast cells (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 and paracrine agents so each villi is surrounded by a pool/sinus of maternal blood.
18
Q

Describe the placental blood supply.

A
  • Maternal blood
    • Enters placental sinuses / pools via uterine artery
    • Flows through sinuses
    • Exits via uterine veins
  • Fetal blood
    • Flows into capillaries of chorionic villi via umbilical arteries
    • And back to fetus via umbilical vein
  • Umbilical cord connects fetus to placenta
19
Q

What is the role of hCG?

A
  • 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 day 7-8 post-conception.
  • 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. Commercial kits pick it up in urine.
  • 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.
20
Q

What are the functions of the placenta with respect to the fetal organs?

A
  • Fetal gut - supplying nutrients.
  • Fetal lung - exchanging O2 and CO2.
  • Fetal kidney - regulating fluid volumes and disposing of waste metabolites.
  • Endocrine gland - synthesis of steroids and proteins which affect both maternal and fetal metabolism.
21
Q

What are the functions of the placenta in the first 3 months?

A
  • 1st month - villus formation.
  • 2nd month - increasing surface area and circulation.
  • 3rd month - growing, becoming increasingly efficient.
  • Surface area of the diffusion membrane is huge. Most molecules can pass through the membrane - after 20 weeks, placental membrane thins even more with the loss of cytotrophoblast.
  • 3rd trimester syncytiotrophoblastic cells may be lost into the maternal blood.
22
Q

Describe nutrient exchange across the placenta throughout pregnancy.

A
  • Nutrient exchange is rapid and increases as pregnancy advances.
  • Water and electrolytes diffuse freely.
  • Glucose passes via facilitated diffusion. Fetus has little capacity for gluconeogenesis.
    • Babies of diabetic mothers are heavier than normal range.
    • Storage of glycogen in liver for postnatal requirements.
  • Amino acids are actively transported for fetal growth.
  • Lipids cross as free fatty acids.
  • Vitamins.
23
Q

Describe the gas exchange which occurs across the placenta.

A
  • 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 fetus is flow limited.
  • Fetal Hb has a greater affinity for O2 than adult Hb.
  • Towards the end of pregnancy, exchange capacity decreases and placenta is less able to meet the demands of the fetus.