Implantation and the Placenta Flashcards
Summarise the events of ovulation - fertilisation - implantation.
State where they occur.
What happens to the conceptus after it is formed?
- 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.
What are totipotent cells?
- 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.
What are pluripotent cells?
- Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent.
What are multipotent cells?
- 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.
Which kind of cell is used for genetic screening and why?
- Blastocyst is pluripotent, so a cell can be removed for testing without damage to the embryo.
What happens when the blastocyst enters the uterine cavity?
- 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.
What gives rise to the placenta?
Trophoblast (part of blastocyst)
What gives rise to the embryo?
Inner cell mass (part of the blastocyst)
What happens to the blastocyst on day 6?
- 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.
Summarise the events of migration of the conceptus from fertilisation to implantation.
What is the sequence of events in the mother-fetus link development?
- 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.
Describe the attachment and implantation of the blastocyst.
State when this happens.
- 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.
Describe the events of day 13 (implantation).
- 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).
Describe the development of the placenta.
- 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.