implantation

Question 1

hatching of the blastocyst requires

Answer 1

• proteolytic enzymes secreted from the trophectoderm and possibly from uterine secretions
• to dissolve a hole in the ZP for the blastocyst to squeeze out

Question 2

why is human implantation called “invasive”

Answer 2

conceptus breaks through the surface epithelium to invade underlying uterine stroma

Question 3

what is placenta praevia

Answer 3

(normally implants close to top)
- implants on/close to cervix
- restricts placental growth so is associated with antepartum haemorrhage

Question 4

can the embryo implant at any time?

Answer 4

• majority of time, uterine epithelium is non-receptive
• receptive during progesterone dominant phase

Question 5

role of oestrogen and progesterone in implantation

Answer 5

• both required to prime uterine lining
• oestrogen needed to prepare conceptus
  (known from rat experiments & ovariectomy after implantation)

Question 6

properties of non-receptive endometrium

Answer 6

• 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

Question 7

properties of receptive endometrium

Answer 7

• 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

Question 8

describe apposition

Answer 8

• 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

Question 9

describe LIF signalling

Answer 9

• 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

Question 10

describe message exchange in adhesion and invasion

Answer 10

• containment & control to keep trophoblast in check (TIMPs)
• incitement & encouragement to promote invasive events (MMPs)

Question 11

describe the decidual reaction

Answer 11

1. uterine epithelial cells start signalling to underlying stroma that invasion is imminent
2. endometrium becomes nutrient packed, highly vascular tissue called decidua - involves PGs
3. maternal stromal cells of endometrium > maternal decidual cells (spindly > large and round because lipids and glycogen)
4. decidual cells secrete cytokines, incl IGF binding proteins and prolactin, that aid placental growth
5. increased vascular permeability and angiogenesis

Question 12

three regions of the decidua

Answer 12

1. decidua basalis: closest to myometrium
2. capsularis: covers protruding side of conceptus; will disintegrate over time
3. parietalis: remainder of decidua

Question 13

why is the endometrial lining newly developed with each cycle

Answer 13

decidual cells cannot dedifferentiate into stromal cells

Question 14

how does the conceptus prevent shedding

Answer 14

• secretes hCG
• maintains corpus luteum so progesterone stays high & cannot start a new menstrual cycle
• by 10w, placenta secretes enough progesterone