Gametes - Multiples Flashcards
When does cleavage begin
how does it progress
Begins 12 hrs post-fertilisation
zygote divides into 2 cells (mitosis)
2 cell → 4 cell (24 - 36 hrs)
4 cell → 8 cell (36 - 72 hrs)
16 cell
Morula
Blastocyst
for 1st 5 days of life post fertilisation what is the embryo surrounded by
Floats around in uterine and oviductal fluid - has huge effect on epigenome - what factors are present
Cleavage via mitosis forms
2 cell stage
2 blastomeres with no cellular debris around it
roughly the same size
4 cell stage
NO CHANGE IN OVERALL SIZE YET
8 cell stage
When does morula develop
when does it enter the uterus
structure
72 hrs (3 days) from fertilisation
morula enters uterus after 3 days in oviduct
solid sphere of cells - includes zona pellucida
NO ENLARGEMENT
compaction - formation of tight junctions between blastomeres
totipotency - becomes embryonic stem cells
markers = ecad molecules
effect of compaction on embryo
Role of E-Cadherin molecules in compaction of human embryos (markers on morula)
Precompaction
Low biosynthetic activity
Quiescent - low QO2 (inactivity/dormancy)
Oviod mitochondria (egg shaped)
pyruvate = preferred nutrient
maternal genome
individual cells
identical cells
Post compaction
high biosynthetic activity
highly active - high QO2
elongated mitochondria
glucose = preferred nutrient
embryonic genome
transported epithelium
cell differentiation - inner cell mass, trophectoderm
MPN
Stops extrusion of 2nd polar body
Blastocyst
how does it form
what characteristic is lost
morula enters the uterine cavity - floats freely
- accumulates fluid and forms a cavity between its cells*
- once the cavity appears - now called a blastocyst*
loss of totipotency - trophoectoderm, inner cell mass, blastocoele cavity
trophoblasts - will form invading placenta
inner cell mass cells - will form embryo
trophoblasts form
Invading placenta
inner cell mass cells
form embryo
Blastocyst structure
function of ZP
ZP2 and ZP3 - crucial for sperm binding
ZP1 - support (premature hatching without it)
Hatching of blastocyst
Series of contractions and relaxations that push the embryo out
Twins definition
Those born at the same time or of the same pregnancy
types of twins
how is this determined
fraternal “identical”
conjoined
incidence of twins
1 in 250
incidence of triplets
1 in 10,000
incidence of quadruplets
1 in 700,000
what factors increase incidence of dizygotic twins
race
age
heredity
prior pregnancy
complications for mother for a multiple pregnancy
Anaemia
Hydramnios (increase in amniotic fluid)
Preeclampsia (increase in BP)
Preterm labour
Postpartum haemorrhage
Caesarean delivery
complications for foetus with a multiple pregnancy
Abortion
Malpresentation - foetus may be in breach
Placenta praevia - implantation very low in uterus which blocks birth canal - more common with multiples - more common with mothers who have had 1+ children
abruptio placenta
PROM (premature rupture of membranes)
Prematurity
umbilical cord prolapse
IUGR
congenital abnormalities
Average GA at delivery for twins
36 weeks
average GA at delivery for triplets
33 weeks
Incidence of preterm delivery in twins pregnancy
50%
When is surfactant produced
week 28
how are “identical” monozygotic twins formed
single fertilised zygote splits into 2 separate individuals
offspring have “identical” genetic info, however
In contrast to stable genome the epigenome, including various DNA modifications such as DNA methylation, is dynamic and interchangeable in response to various environmental and stochastic events (random)
Using WGS, specific rare somatic mutations - SNPs may occur in one but not the other MZ twin
Dichorionic diamniotic
when & where does this occur
incidence
Embryo splits before cells begin to differentiate @ 4 days
monozygotic twins will implant as 2 separate blastocysts
separate chorion and separate amnion
in this case they are travelling through the oviduct when they separate
occurs in 1 in ever 4 twin sets
decreased risk of entanglement and twin-twin syndrome that occurs with greater freq in monochorionic twinning
Monochorionic diamniotic
when does this occur
associated risks
incidence
Embryo splits between days 4-8
twins whill share a chorion (placenta) but have separate amniotic sacs
70% of twins
if the placenta abrupts, both twins will be lost
Twin to twin transfusion syndrome (TTTS)
Shared placenta so BVs often go between the 2
imbalance of blood flow
1 twin often much smaller
extra blood flow to the other may lead to heart failure
physiological processes taking place in the recipient and donor twin in TTTS
monochorionic monoamniotic
when does it occur
incidence
associated risks
embryo split between day 8-13 - they were implanted in the endometrium as 1 and THEN split
share chorion and amnion
only 1-2% of MZ twins occur this way
increased risk of entanglement of umbilical cords - foetal HR is often tested daily to check for this
risk decreases as twins mature and there is less room and less movement
conjoined twins
how does this occur
on day 13 the embryonic disc (bilayer of epiblast and endoderm) begins to differentiate
if the split occurs AFTER day 13, the twins will share a chorion and amnion
they will also share body parts => conjoined
only monozygotic twins can be conjoined
Dizygotic (fraternal) twins
how does this occur
multiple sperm fertilised multiple eggs
each offspring is unique in their genetic makeup (no more closely related than any other 2 siblings)
2/3rds of cases
may be of different sex
each foetus has its own placenta and amniotic sac
placentae may be separate or fused
factors affecting incidence of DZ twins
- Induction of ovulation - 10% with clomide and 30% with gonadotrophins (in the 1st 1/2 of cycle - GnRH released - FSH and LH released - High levels of progesterone are gone)
- increased maternal age due to increased gonadotrophin production (Towards the end (perimenopausal) - multiple follicles released)
- Increases with parity (multiple children)
- familial - usually on maternal side
