lecture 14: endometrial receptivity and pregnancy Flashcards
What is shed at menses?
- the female endometrium
How do the sperm fertilise the egg?
- many sperm make light work
- digestion of complete holes through the zona pellucida by the acrosome reaction requires release of many acrosomes – initial sperm do the hard work for the others
- once one sperm has entered, hardening of the zona pellucida via intiation of the cortical response blocks polyspermy
What are stages of embryo development and hatching?
- ther fertilised oocyte is called a zygote
- the zygote undergoes continuous division (2-cell, 4-cell, 80cell) but does not grow
- once the cell number is greater than 16 the structure is called a morula
- by day 5 the structure is known as a blastocyst – it contains a fluid filled cavity, and different cell types:
- trophoblast – forms the placenta
- inner cell mass (embryoblast) - forms the baby
- the zona pellucida starts to break down
- caused by lytic factors from the uterine cavity and the blastocyst itself
- the embryo hatches from the zona pellucida and is now ready to implant into the uterus
What is implantation into the uterus?
- takes place ~6-7 days after fertilisation
- requires an appropriately receptive endometrium
- endometrium secretes proteins, lipids and other metabolites which can act as ‘nutrients’ for the blastocyst during implantation and later placentation
- endometrium produces ‘pro-implantation’ factors to enter into a dialogue and let the blastocyst know it is ready
- this process is absolutely critical for full pregnancy - many women lose the embryo at this point and never realise they are pregnant
- luminal epithelium lining the endometrium
What are the stages of blastocyst implantation?
- transport, orientation, hatching
- apposition
- not at full contact with the maternal endometrium at this point
- just floating above and thinking about implanting
- can enter into intense dialogue at this point
- maternal endometirum can alter expression of adhesion proteins at this point
- changed adhesion
- attachment
- invasion
- day after attachment
- there is also decidualisation of the endometrial lining
- presence of leukocytes, macrophages, natural killer cells
- trophoblast cells invading maternal endometrium and
What is needed between embryo and endometrium?
- synchrony of development
- when we have low embryonic quality (developed too far or not enough) then implantation is impossible
- endometrium also has to be adequately prepared
- their levels of preparedness need to be matched - both ready at the same time
- implantation is possible when egg is somewhat developed and endometrium kind of prepared
- but might not be proper or whatever
What is endometrial receptivity?
- gradually achieved over ~20 days of a 28 day menstrual cycle under the influence of oestrogen (follicular/proliferative phase) and progesterone (after ovulation, luteal/secretory phase)
- endometrium is receptive for around 4 days – known as the ‘window of implantation’
- spans days 20-24/LH+6-10
- associated with secretory transformation of endometrial cells and progressive decidual transformation of stromal cells
What are endometrial epithelial changes for receptivity and implantation?
- changes in endometrial epithelial cells
- under influence of oestrogen and progesterone glandular epithelial cells become secretory
- luminal epithelial cells alter adhesion molecules and integrity/tightness of junctions between cells
- epithelial surface becomes less polarised
- secreted factors enter into a communication with the blastocyst
- top picture: proliferative phase (endometrium not ready to receive the embryo)
- glandular epithelial cells are very small and compact, not secretory
- very small blood vessels
- stroma: tissue glue that holds it together is very tightly packed
- secretory phase
- endometrium ready to accept blastocyst
- presence of secretions within the uterine gland epithelial cells
- stromal cells become more oedematous
- starting to undergo decidual transformation under the influence of oestrogen and progesterone
- presence of highly enlarged blood vessels
- important to increase the vascularity of the tissue at this point and also allow influx of leukocytes
How can we be sure endometrial factors are required for implantation?
- uterine gland knockout sheep
- neonatal sheep administered progesterone
- no uterine glands
- retarted conceptus development
- no implantation
- no loss of adhesion molecules
- due to lack of secretory products
- knockout mice
- leukaemia inhibitory factor knockout
- implantation failure due to uterine defect
- blastocysts recovered from knockout implanted into wild type mother
- therefore LIF, an endometrial factor, was absolutely essential for implantation
- examination of factors with human uterine cavity
- presence of specific proteins can predict implantation of an embryo in an IVF cycle
- absence of certain factors in infertile women
- endometrial lavage
What happens to endometrial secreted factors?
- taken up by placental cells and support development
- endometrium is still 5-6 mm thick at 6 weeks of gestation, with highly active glands that discharge into the placenta
- endometrial secretions produced by the glands are taken up by the trophoblast cells of the placenta
- NB: no significant placental blood flow until around week 12 of gestation – nutrition must come from another source
What are the endometrial stromal changes for receptibity and implantation?
- progesterone dependent but also driven by certain cytokines (IL-11, activin)
- changed phenotype – take on many features of epithelial cells
- secrete many new proteins
- including chemokines, growth factors etc
- produce high levels of inhibitors of proteases (e.g. TIMPs, cystatin)
- decidualisation occurs into the mid-late secretory phase of a normal menstrual cycle
- in pregnancy these cells turn into the decidua of pregnancy
How do we know a decidualising endometrium is important for implantation?
- decidualising stromal cells act as sensors of embryo ‘health’
- culture of decidualising stromal cells with ‘bad’ embryos inhibited production of growth factors and cytokines
- decidualising stromal cells from fertile women will not migrate towards a bad embryo
- decidualising stromal cells from women with recurrent pregnancy loss WILL migrate
- cannot discriminate between a good and bad embryo
- reason for recurrent pregnancy loss – allow bad embryos to implant
- embryos cannot survive long because they are genetically abnormal, just long enough for a positive pregnancy test (hCG)
What are endometrial leukocyte (white blood cell) changes receptivity and implantation?
- leukocyte composition changes dramatically from non-receptive (proliferative) to receptive (mid-secretory) phase
- largest changes seen in uterine natural killer and macrophage numbers
- numbers change even more dramatically in first trimester of pregnancy
- uNK cells = 70% of total leukocytes
- macrophages = 20% of total leukocytes
What are the natural killer cells in the endometrium?
- phenotypically and functionally very different to natural killer cells in the circulation (peripheral NK cells)
- the presence of cell surface marker CD56 (CD56hi) and the absence of CD16 (CD19-) uterine NK cells (uNK)
- they represent 70% of leukocytes at the implantation site
- they may be regulated by signals from the maternal decidua
- during pregnancy they can respond to foetal signals
- they produce a range of soluble products, including angiogenic cytokines (such as angiopoeitin-2 and vascular endothelial growth factor C)
- they are found in close proximity to uterine spiral arteries (blood vessels) and facilitate remodelling during pregnancy
- may help in tolerance of foetal allograft
- mice deficient in uterine natural killer cells have placental defeects
What do signals produced by uterine NK cells influence?
- trophoblast function
- uterine/decidual natural killer cells isolated from first trimester of pregnancy endometrium/decidua
- measured factors produced by the natural killer cells
- invasive placental cells (trophoblast) isolated from first trimester of pregnancy samples
- stimulated with natural kill cell factors
- migration (mimicking placental invasion of the endometrium) examined
- IL-8, IP-10, VEGF, PLGF (last two important for remodelling blood vessels)