Gametes - Multiples Flashcards by Michelle Doody

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

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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

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Cleavage via mitosis forms

2 cell stage

2 blastomeres with no cellular debris around it

roughly the same size

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4 cell stage

NO CHANGE IN OVERALL SIZE YET

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8 cell stage

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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

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effect of compaction on embryo

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Role of E-Cadherin molecules in compaction of human embryos (markers on morula)

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Precompaction

Low biosynthetic activity

Quiescent - low QO₂(inactivity/dormancy)

Oviod mitochondria (egg shaped)

pyruvate = preferred nutrient

maternal genome

individual cells

identical cells

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Post compaction

high biosynthetic activity

highly active - high QO2

elongated mitochondria

glucose = preferred nutrient

embryonic genome

transported epithelium

cell differentiation - inner cell mass, trophectoderm

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MPN

Stops extrusion of 2nd polar body

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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

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trophoblasts form

Invading placenta

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inner cell mass cells

form embryo

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Blastocyst structure

function of ZP

ZP2 and ZP3 - crucial for sperm binding

ZP1 - support (premature hatching without it)

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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