implantation Flashcards
hatching of the blastocyst requires
- proteolytic enzymes secreted from the trophectoderm and possibly from uterine secretions
- to dissolve a hole in the ZP for the blastocyst to squeeze out
why is human implantation called “invasive”
conceptus breaks through the surface epithelium to invade underlying uterine stroma
what is placenta praevia
(normally implants close to top)
- implants on/close to cervix
- restricts placental growth so is associated with antepartum haemorrhage
can the embryo implant at any time?
- majority of time, uterine epithelium is non-receptive
- receptive during progesterone dominant phase
role of oestrogen and progesterone in implantation
- both required to prime uterine lining
- oestrogen needed to prepare conceptus
(known from rat experiments & ovariectomy after implantation)
properties of non-receptive endometrium
- epithelial cells resistant to embryo adhesion
- cells have a thick mucin (MUC1) glycoprotein coat, long microvilli & negative surface charge
- high level of PG receptor expression to prepare for blastocyst binding
properties of receptive endometrium
- blastocysts adhere to thinner mucin coat, no negative charge, LIF = “come here”, shorted microvilli
- pinopodes allows absorption of uterine fluid so less uterine cavity vol so adhere to uterus
- decrease in PR receptors
describe apposition
- blastocyst approaches and interacts with uterine epithelium
- L-selectin binds to ligand in epithelium, which promotes secretion of hCG by trophoblast, which signals to epithelium to secrete trophinin and break down MUC1 glycocalyx (so trophoblast can interact better)
- allows LIF signalling
describe LIF signalling
- causes upregulation of HB-EGF on uterine epithelium which binds to HSPG on trophoectoderm and ErB on the trophoblast > release of prostaglandins, decidualisation
- induces expression of adhesion molecules, e.g. integrin, on both surfaces: OPN induces integrin on trophoblast
describe message exchange in adhesion and invasion
- containment & control to keep trophoblast in check (TIMPs)
- incitement & encouragement to promote invasive events (MMPs)
describe the decidual reaction
- uterine epithelial cells start signalling to underlying stroma that invasion is imminent
- endometrium becomes nutrient packed, highly vascular tissue called decidua - involves PGs
- maternal stromal cells of endometrium > maternal decidual cells (spindly > large and round because lipids and glycogen)
- decidual cells secrete cytokines, incl IGF binding proteins and prolactin, that aid placental growth
- increased vascular permeability and angiogenesis
three regions of the decidua
- decidua basalis: closest to myometrium
- capsularis: covers protruding side of conceptus; will disintegrate over time
- parietalis: remainder of decidua
why is the endometrial lining newly developed with each cycle
decidual cells cannot dedifferentiate into stromal cells
how does the conceptus prevent shedding
- secretes hCG
- maintains corpus luteum so progesterone stays high & cannot start a new menstrual cycle
- by 10w, placenta secretes enough progesterone