Implantation and Early Pregnancy Flashcards
How does the blastocyst appearance change as it implants?
• It is within the stromal environment that the remaining 9 months of pregnancy occur, so very important that the embryo implants under the epithelium.
• At implantation, there is still a thin, spread out layer of TE cells on top of the ICM that has not yet been in contact with the maternal tissue
• Those that have interacted with the epithelium have undergone a change
− The cavity of the blastocyst is collapsed
− The pad of trophoblast has different sized nuclei and both mononuclear and syncytial trophoblast
− This is the invasive trophoblast
What is the embryonic/maternal clock?
• The embryonic clock is set at fertiliastion
− Embryos must develop to the blastocyst stage and hatch before they can implant
• The maternal clock is set at ovulation
− Ovulation occurs 36 hours after the LH surge
− Progesterone is produced by the corpus luteum → a week after ovulation, progesterone is at its peak in the absence of pregnancy. At this point, progesterone will fall and the endometrium will be shed.
− Doesn’t matter if you have fertilization or not, if you don’t have implantation, you will get mensruation
− If you do get pregnancy – progesterone levels continute to rise because trophoblast cells produce hCG → hCG rescues the corpus luteum.
What experiments defined the implantation window in mouse and humans?
In rodents:
• Embryos flushed from a donor oviduct and transferred to the uterus of a recipient female
• Allow the recipient female to mate with a vasectomized male (so she ovulates and produces the correct hormones)
• If you transfer the embryos 1-5 days after mating, you get no pregnancy rate
• Between days 5-6 they do implant – so there is a period of around 24 hours in which they can implant.
In humans
• If you don’t produce hCG at the right time, the corpus luteum will being to be lost → this is the closure of the implantation window
• In the period between the LH surge and the first time an embryo can implant (between 8-12 days after the LH surge), there is a change in the endometrium that makes it more receptive.
- In the lab, the blastocyst will adhere to any surface
- Unimplanted mouse embryos flushed after transfer to one recipient can implant into a second recipient if in her receptor phase
- This shows that the window is controlled by the mother, not the embryos → the embryos don’t implant because the mother isn’t receptive.
Explain how the endometrium goes from non-receptive to receptive in rodents
• Seems to be exerted by the endometrial epithelium
• Primarily driven by progesterone and estrogen
− In the mouse, despite the presence of adequate P, the uterus remains refractory to blastocyst attachment until the morning of day 5 of pregnancy, when the ovary secretes a secondary wave of E, termed nidatory E.
− This wave converts the nonreceptive epithelium (consisting of a single layer of epithelial cells lining the lumen) to a state favouring blastocyst implantation.
− Conversion of the epithelium allows both invasion by embryonic trophoblast, and inducing decidualisation of the storma
➢ primary function of nidatory E, in conjuction with the tumour suppressor p53, is to induce synthesis of LIF
➢ LIF is essential for the switch from non-receptive to receptive
➢ During the first hour, the JAK/STAT signaling pathway is activated and expression of 54 genes declines, primarily effecting epithelium cytoskeletal organization and chromatin organization
➢ Within 3 hours, expression of genes belonging to more than 25 biological processes was altered eg) IGF, VEGF, actin cytoskeleton, integrins…
What is the ‘barrier hypothesis’ to the implantation window?
• It is thought the apical epithelial glycocalyx may act as part of the barrier system for embryo selection
• Expression of MUC1 in endometrial epithelkum has been suggested to create a barrier that must be lifted
− In vitro, MUC1 was present at the surface of primary cultures of EECs
− The presence of a human blastocyst increases EEC MUC1 protein and mRNA compared to control EEC lacking a blastocyst
− When blastocysts attached to the EEC monolayer (ie, adhesion phase) – MUC1 was locally removed at the implantation site.
1. Progesterone combined with estradiol priming induces up-regulation of MUC1 at the receptive endometrium
2. During the apposition phase, presence of a human embryo increases EEC MUC1
3. At the adhesion phase, the embryo induces paracrine cleavage of MUC1 at the implantation site.
➢ → These findings strongly suggest that MUC1 may act as an endometrial anti-adhesive molecule that must be locally removed by the human blastocyst during the adhesion phase
How do you get stable attachment of the blastocyst to the epithelium?
• Embryo attaches to the top layer of the glycocalyx canopy
• You then get clearance of MUC1
• The embryo can then access adhesion receptors at the base of the canopy, which leads to firmer attachment.
− avb3 integrin seems to increase its expression in the area just surrounding blastocyst attachment.
− Osteopontin is a ligand for this integrin – it is expressed in endometrial epithelium in the implantation phase of the menstrual cycle and is increased by progesterone.
− Embryo attachment in vitro is destablised if either integrin avb3 or osteopontin is knocked down.
• When the embryo attaches, epithelial displacement occurs at the attachment sites
• Apoptosis occurs in the cells immediately beneath the attached embyros
How long does it take to get stable attachment?
• They start off weakly attached until 18 hours
• Then they stably attach for the most part – this occurs in the second day
• By 48 hours, about 2/3rds of the embryos have become stably attached and some of them have started to invade.
- This is important because you have to think about the ovary and corpus luteum → probably that the hCG doesn’t get into maternal blood until the embryo has got below the surface and has access to the maternal blood supply
- The quicker this happens, the less likely progesterone levels will fall to a point where the endometrium is shed
What is the updated implantation window hypothesis?
- An active embryonic stimulus is required to initiate implantation
- Receptivity reflects the ability of the epithelium to respond to this signal
Describe the features of decidualisation
- Decidua is a secretory tissue that helps the placenta get established
- In addition to having secretory glands, we have a change in the stromal cells in which they begin to enlarge and secrete
- Decidualisation is needed for successful gestation
- Decidualisation of the uterine lining fails in the absence of the progesterone receptor
What is histiotroph?
- Under the influence of progesterone, endometrial glands produce secretions during the first trimester
- Contains glucose oligomers and glycoproteins
- Known as uterine milk, or histiotroph
- Provides nutrient support to the embryo in early pregnancy
- After 11 weeks the glands regress, you then have the maternal blood supply for nutrition
What is the role of hCG
- Trophoblast produces the polypeptide hCG from the hatched blastocyst stage onwards
- Appears in measurable quantities in maternal blood and urine soon after implantation (used for pregnancy testing)
- hCG rescues the corpus luteum so progesterone continues to rise
- Lack of timely appearance of hCG probably accounts for the ‘closure’ of the receptive period
- The 48 hour window for invasion is critical for the trophoblast to access maternal capillaries and release hCG
Timing of implantation of the conceptus and loss of pregnancy
• In a study:
− Of 189 pregnancies, 141 lasted more than 6 weeks, 48 ended in early loss
− Among those that lasted > 6 weeks, the first appearance of hCG occurred 6-12 days after ovulation
− 118 women had implantation on day 8, 9 or 10
− The risk of early pregnancy loss increased with later implantation
− Pregnancies with late-implanting conceptuses may fail for several reasons:
➢ Receptivity of the endometrium decreases during the late luteal phase
➢ Corpus luteum is less responsive to hCG by 11 or 12 days after ovulation
➢ Factors intrinsic to the zygote could also be at work → unhealthy zygotes develop more slowly, or implantation may be abnormal resulting in later and weaker production of hCG.
What is the endocrine switch
- At 7-9 weeks the placenta takes over the production of E and P from the corpus luteum
- The placenta continues to produce large amounts of hCG
What are the landmarks of early pregnancy
Landmarks of early pregnancy: • Ovulation • Timing of fertilization • Development to blastocyst • hCG • Gestational sac • Embryonic heart (5 weeks) • Endocrine switch
