L17 Fertilisation and placental development Flashcards
Where do sperm and ovum meet
- 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)
Zygote to blastomere
Zygote –> 2 cell stage –> 4 cell stage –> 8 cell stage –> morula(72 hours) –> blastocyst(4 days)
What happens during days 4-5
- 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
What happens during days 6-7(bilaminar disc of the embryo)
- 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
What happens after 16 days
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
Layers of the embryo
- The hypoblast degenerates, the epiblast gives rise to all three germ layers - ectoderm, mesoderm and endoderm
- The embryo folds to create the adult pattern
Describe the development of the placenta
- 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
Describe the formation of the placenta
- 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
What are cytotrophoblast cells(CTB)
- 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
What are syncytiotrophoblast cells (STB)
- Fully differentiated cells
- Direct contact with maternal blood
- Produce placental hormones
Features of the placenta as an endocrine organ
Human chorionic gonadotrophin (HCG) - Maintenance of corpus luteum of pregnancy - Progesterone and oestrogen Human placental lactogen HPL - Growth, lactation - Carbs and lipids
What is the maternal blood in the lacunae in direct contact with
- Maternal blood in the lacunae is in direct contact with syncytiotrophoblasts
What does the placental barrier consist of
- Mono layer of syncytiotrophoblast/cytotrophoblast/fetal capillary epithelium is all that separates the fetal and maternal blood
Changes in cytotrophoblast levels as pregnancy advances
- Cytotrophoblasts decrease as the pregnancy advances (not needed)
Changes in placental barrier as pregnancy advances
- The barrier thins as pregnancy advances leading to a greater surface area for exchange (over 10m2)
What substances are transferred across the placenta
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
What is the decidua
- The decidua is the modified mucosal lining of the uterus known as the endometrium that forms in preparation for pregnancy
Named parts of the decidua
Capsularis - overlying embryo and chorionic cavity
Parietalis - side uterus not occupied by embryo
Basalis - between uterine wall and chorionic villae
Location of basalis
- Between the uterine wall and chorionic villae
Location of capsularis
- Overlying embryo and chorionic cavity
Location of parietalis
- Side uterus not occupied by embryo
What is vasa praevia
- Is a condition in which fetal blood vessels cross or run near the internal opening of the uterus
What can cause vasa praevia
- 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
How is vasa praevia diagnosed
- Diagnosed on ultrasound using colour dopplers
- Management deliver by caesarean section when the fetus is above 34 weeks
Describe the position of the placenta within the uterus
- Mainly fundal (at the top)
- Anterior or posterior (front wall or back wall)
- ‘Low lying’ or placenta praevia (near to the cervical os)
What is placenta praevia
- Is when the placenta attaches to inside the uterus but near or over the cervical opening
Symptoms of placenta praevia
- Massive bleeding in pregnancy
- Painless bleeding
- Fetal death
- Maternal death
What can failure of trophoblastic invasion into maternal circulation cause
- Poor maternal fetal mixing of blood
- Lack of oxygen and nutrients to the fetus
- Leads to fetal growth restriction
- Pre-eclampsia (raised blood pressure)
What is placenta accreta
- Placenta accreta occurs when all or part of the placenta attaches abnormally to the myometrium(the muscular wall of the uterine wall)
Difference between accreta, increta and percreta
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
Consequence of placenta accreta
- 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
What can placental abruption cause
- Massive bleeding in pregnancy(often concealed)
- Extremely painful
- Fetal death
- Maternal death
What can separation of the placenta during pregnancy cause
- 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
Placenta in multiple pregnancies
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
Development vs growth - embryo and fetal growth
- First 12 weeks fetal development occurs - organs formed
- Then the baby needs to get bigger - fetal growth
Two types of fetal growth problems - Definitions
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)
Factors that can restrict fetal growth
- Deficient placental invasion
- Reduced placental reserve
- Fetal need exceeds supply
- IGUR
- Hypoxia
- Fetal vascular redistribution
- Oliguria
- Abnormal CTG
- Fetal death
Diagnosis of fetal growth restriction
Clinical suspicion - abdomen looks smaller
Clinical measurement of uterine size: Symphysis - fundal height (SFH)
SFH = weeks +/- cms
- Abdominal examination in pregnancy-palpation
- Ultrasound scan
Effects of early fetal growth restriction
- Symmetrical growth restriction - both head and abdominal growth affected
Causes of early fetal growth restriction
- Chromosomal anomaly (T21)
- Viral infection (rubella, CMV)
- Severe placental insufficiency
- OR normal small baby (look at the parents)
What is reduced in asymmetrical fetal growth restriction
- In asymmetrical growth restriction, ONLY the abdominal head circumference is reduced
Growth of what part of the body is affected during asymmetrical growth restriction
- Just abdominal growth affected
What is abdominal circumference an indication of
- Abdominal circumference reflexts the size of the fetal liver
Causes of fetal liver growth restriction
Placental insufficiency - no excess glycogen being deposited within the liver
Consequences of hypoxia in the fetus
- 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)
Ultrasound findings in IUGR
- 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)
Clinical features of IUGR
- SFH smaller than expected
- Baby’s movements lessen to conserve energy
- Fetal heart rate changes as hypoxia develops(as seen on CTG)
- Fetal death
Arguments for waiting - IGUR
- Low chance of survival
- To give steroids
- Reduce need for C/S
Arguments for delivering - IGUR
- > or equal to 32 weeks
- Doppler abnormality
- Decreased movements
- CTG abnormality
Features of betamethasone/dexamethasone treatment
- 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
Difference in ultrasound scans - normal pregnancy vs fetal growth restriction
- 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
