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Year 2 - T2 Theme 2 > L17 Fertilisation and placental development > Flashcards

L17 Fertilisation and placental development Flashcards

1
Q

Where do sperm and ovum meet

A
  • In fallopian tube(uterine tube) (usually ampulla) 12-24 hours after ovulation
  • Fusion occurs and 2nd meiotic division occurs
  • Acrosome reaction makes ovum impermeable to other sperm
  • End-zygote- has diploid (46 chromosomes)
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2
Q

Zygote to blastomere

A

Zygote –> 2 cell stage –> 4 cell stage –> 8 cell stage –> morula(72 hours) –> blastocyst(4 days)

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3
Q

What happens during days 4-5

A
  • The morula develops a cavity and becomes known as a blastocyst
  • Blastocyst thins out and becomes the trophoblast –> start of the placenta
  • The rest of the cells move(and are pushed up) to form the inner cell mass. This creates an embryonic pole
  • The blastocyst has now reached the uterine lumen and is ready for implantation
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4
Q

What happens during days 6-7(bilaminar disc of the embryo)

A
  • Inner cell mass differentiates into two layers –> epiblast and hypoblast
  • These two layers are in contact
  • Hypoblast forms extraembryonic membranes and the primary yolk sac
  • Epiblast forms embryo
  • Amniotic cavity develops within the epiblast mass
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5
Q

What happens after 16 days

A

Bilaminar disc develops further by forming 3 distinct layers(this process is known as gastrulation)

  • Initiated by primitive streak
  • The epiblast becomes known as ectoderm
  • The hypoblast is replaced by cells from the epiblast and becomes endoderm
  • The epiblast gives rise to the third layer, the mesoderm
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6
Q

Layers of the embryo

A
  • The hypoblast degenerates, the epiblast gives rise to all three germ layers - ectoderm, mesoderm and endoderm
  • The embryo folds to create the adult pattern
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7
Q

Describe the development of the placenta

A
  • Syncytiotrophoblast burrows into the myometrium of the uterus - the syncytiotrophoblasts invading the maternal spiral arteries and starting the formation of the primary/secondary and tertiary villi
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8
Q

Describe the formation of the placenta

A
  • Syncytiotrophoblast invades decidua (endometrium)
  • Cytotrophoblast cells erodes maternal spiral arteries and veins
  • Spaces (lacunae) between the fill up with maternal blood
  • Followed by mesoderm that develops into fetal vessels
  • Aiding the transfer of nutrients, O2, across a simple cellular barrier
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9
Q

What are cytotrophoblast cells(CTB)

A
  • Undifferentiated stem cells
  • Invade the maternal blood vessels and destroy the epithelium
  • Give rise to the syncytiotrophoblast cells (STB)
  • Reduce in number as pregnancy advances
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10
Q

What are syncytiotrophoblast cells (STB)

A
  • Fully differentiated cells
  • Direct contact with maternal blood
  • Produce placental hormones
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11
Q

Features of the placenta as an endocrine organ

A 
Human chorionic gonadotrophin (HCG) 
- Maintenance of corpus luteum of pregnancy 
- Progesterone and oestrogen 
Human placental lactogen HPL 
- Growth, lactation 
- Carbs and lipids
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12
Q

What is the maternal blood in the lacunae in direct contact with

A
  • Maternal blood in the lacunae is in direct contact with syncytiotrophoblasts
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13
Q

What does the placental barrier consist of

A
  • Mono layer of syncytiotrophoblast/cytotrophoblast/fetal capillary epithelium is all that separates the fetal and maternal blood
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14
Q

Changes in cytotrophoblast levels as pregnancy advances

A
  • Cytotrophoblasts decrease as the pregnancy advances (not needed)
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15
Q

Changes in placental barrier as pregnancy advances

A
  • The barrier thins as pregnancy advances leading to a greater surface area for exchange (over 10m2)
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16
Q

What substances are transferred across the placenta

A

Gases - oxygen and carbon dioxide by simple diffusion

Water and electrolytes

Steroid hormones

Proteins poor - only by pinocytosis

Transfer of maternal antibodies IgG - starts at 12 weeks - mainly after 34 weeks therefore lack of protection for premature infants

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17
Q

What is the decidua

A
  • The decidua is the modified mucosal lining of the uterus known as the endometrium that forms in preparation for pregnancy
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18
Q

Named parts of the decidua

A

Capsularis - overlying embryo and chorionic cavity

Parietalis - side uterus not occupied by embryo

Basalis - between uterine wall and chorionic villae

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19
Q

Location of basalis

A
  • Between the uterine wall and chorionic villae
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20
Q

Location of capsularis

A
  • Overlying embryo and chorionic cavity
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21
Q

Location of parietalis

A
  • Side uterus not occupied by embryo
22
Q

What is vasa praevia

A
  • Is a condition in which fetal blood vessels cross or run near the internal opening of the uterus
23
Q

What can cause vasa praevia

A
  • The fetal vessels within the umbilical cord pass over the internal os.
  • As the internal os dilates in labour, the vessels are stretched and exposed and can rupture leading to massive fetal blood loss and death
24
Q

How is vasa praevia diagnosed

A
  • Diagnosed on ultrasound using colour dopplers

- Management deliver by caesarean section when the fetus is above 34 weeks

25
Q

Describe the position of the placenta within the uterus

A
  • Mainly fundal (at the top)
  • Anterior or posterior (front wall or back wall)
  • ‘Low lying’ or placenta praevia (near to the cervical os)
26
Q

What is placenta praevia

A
  • Is when the placenta attaches to inside the uterus but near or over the cervical opening
27
Q

Symptoms of placenta praevia

A
  • Massive bleeding in pregnancy
  • Painless bleeding
  • Fetal death
  • Maternal death
28
Q

What can failure of trophoblastic invasion into maternal circulation cause

A
  • Poor maternal fetal mixing of blood
  • Lack of oxygen and nutrients to the fetus
  • Leads to fetal growth restriction
  • Pre-eclampsia (raised blood pressure)
29
Q

What is placenta accreta

A
  • Placenta accreta occurs when all or part of the placenta attaches abnormally to the myometrium(the muscular wall of the uterine wall)
30
Q

Difference between accreta, increta and percreta

A

Accreta - Chrionic villi attach to the myometrium, rather than being restricted within the decidua basalis

Increta - Chorionic villi invade into the myometrium

Percreta - chorionic villi invade through the perimetrium

31
Q

Consequence of placenta accreta

A
  • The placenta has invaded too deep into the myometrium and thus after birth, the placenta cannot separate and stays within the uterus
  • As a consequence, the uterus cannot contract down and massive bleeding can occur leading to the requirement to do a hysterectomy
32
Q

What can placental abruption cause

A
  • Massive bleeding in pregnancy(often concealed)
  • Extremely painful
  • Fetal death
  • Maternal death
33
Q

What can separation of the placenta during pregnancy cause

A
  • Leads to disruption of the blood to the fetus leading to fetal distress and death
  • Bleeding can be concealed ie not seen via the vagina but the uterus fills up with blood. Different to placenta praevia where the blood is seen from the vagina
34
Q

Placenta in multiple pregnancies

A

Morula –cleavage–> Dichorionic/diamniotic - Days 1-3

Blastocyst –cleavage–> monochorionic/diamniotic - Days 4-8

Implanted blastocyst –cleavage–> Monochorionic/monoamniotic - days 8-13

Formed embryonic disc –cleavage–> conjoined twins - days 13-15

35
Q

Development vs growth - embryo and fetal growth

A
  • First 12 weeks fetal development occurs - organs formed

- Then the baby needs to get bigger - fetal growth

36
Q

Two types of fetal growth problems - Definitions

A

Small for gestational age(SGA) < 5th centile
- Normal variant or growth restricted

Intra-uterine growth restriction (IUGR) < 5th centile
- Growth restricted (ie failure to achieve growth potential)

37
Q

Factors that can restrict fetal growth

A
  • Deficient placental invasion
  • Reduced placental reserve
  • Fetal need exceeds supply
  • IGUR
  • Hypoxia
  • Fetal vascular redistribution
  • Oliguria
  • Abnormal CTG
  • Fetal death
38
Q

Diagnosis of fetal growth restriction

A

Clinical suspicion - abdomen looks smaller

Clinical measurement of uterine size: Symphysis - fundal height (SFH)

SFH = weeks +/- cms

  • Abdominal examination in pregnancy-palpation
  • Ultrasound scan
39
Q

Effects of early fetal growth restriction

A
  • Symmetrical growth restriction - both head and abdominal growth affected
40
Q

Causes of early fetal growth restriction

A
  • Chromosomal anomaly (T21)
  • Viral infection (rubella, CMV)
  • Severe placental insufficiency
  • OR normal small baby (look at the parents)
41
Q

What is reduced in asymmetrical fetal growth restriction

A
  • In asymmetrical growth restriction, ONLY the abdominal head circumference is reduced
42
Q

Growth of what part of the body is affected during asymmetrical growth restriction

A
  • Just abdominal growth affected
43
Q

What is abdominal circumference an indication of

A
  • Abdominal circumference reflexts the size of the fetal liver
44
Q

Causes of fetal liver growth restriction

A

Placental insufficiency - no excess glycogen being deposited within the liver

45
Q

Consequences of hypoxia in the fetus

A
  • Blood flow(oxygen and nutrients) redirected to areas of greater importance such as the brain
  • Blood flow(oxygen and nutrients) redirected away from areas of lesser importance such as:
  • Gut(doesn’t eat)
  • Kidneys(placenta clears waste products)
  • Lungs(placenta brings O2)
46
Q

Ultrasound findings in IUGR

A
  • Small AC(small liver)
  • Decreased amniotic fluid(this is produced by the kidneys)
  • Increased blood flow to the brain(look at middle cerebral arteries in the brain - using the doppler effect scan)
47
Q

Clinical features of IUGR

A
  • SFH smaller than expected
  • Baby’s movements lessen to conserve energy
  • Fetal heart rate changes as hypoxia develops(as seen on CTG)
  • Fetal death
48
Q

Arguments for waiting - IGUR

A
  • Low chance of survival
  • To give steroids
  • Reduce need for C/S
49
Q

Arguments for delivering - IGUR

A
  • > or equal to 32 weeks
  • Doppler abnormality
  • Decreased movements
  • CTG abnormality
50
Q

Features of betamethasone/dexamethasone treatment

A
  • When given to the mother will cross the placenta and stimulate the alveoli cells to produce surfactant gene
  • Surfactant stops the collapse of the alveoli cells by coating the cells and reducing the surface tension
  • Helps prevent respiratory distress syndrome which leads to neonatal death in premature babies
  • Produced from 24-34 weeks and usually, the baby will have enough by 34 weeks in preparation for a term delivery
  • In premature babies, it is lacking
51
Q

Difference in ultrasound scans - normal pregnancy vs fetal growth restriction

A
  • In normal pregnancy, the scan would show normal blood flow in the middle cerebral artery in the brain
  • The peak corresponds to systole of the heart and during diastole, the flow is negative ie below the line
  • In growth restriction, the blood flow is maintained during both systole and diastole –> increasing blood flow

Year 2 - T2 Theme 2 (11 decks)

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