Fertilisation occurs where?
Ampulla of the fallopian tube
First cleavage of zygote
Occurs at day one
- In the fallopian tube
2-cell stage
Occurs at day 2
- In fallopian tube
4 cell stage
Occurs at day 2
Blastocyst
Forms at day 4, after fertilisation
Formed from the blastula which arranges into:
- Inner cell mass–> embryoblast
- Lined by trophoblast
- Contained blastocele cavity
Morula
Ball of cells from fertilised ovum, forms blastocyst.
- >8 cells
Day 4-5
- Develops a cavity–> blastocyst
Trophoblast
Thin outer layer of the blastocyst
- Develops into placenta
Day 6-7 of the embryp
Inner cell mass differentiates
- Epiblast
- Hypoblast
Hypoblast forms extraembryonic membrane and primary yolk sac
Epiblast forms embryo
Amniotic cavity originates from within epiblast.
The epiblast and hypoblast cells in contact forms the bilaminar disc of the embryo.
Day 16+ of embryo
Bilaminar disc differentiation–> Gastrulation
Primitive streak initiates germ layer formation
- Epiblast into ectoderm
- Invading epiblast replace hypoblast–> endoderm
- Epiblast also forms mesoderm
Epiblast forms all three germ cells cells
Syncytiotrophoblast
Layer of cells external to the blastocyst
- Full differentiated cells
- Outer layer of the trophoblast
- Invades the decidua (endometrium)
Directly in contact with maternal blood.
Produces placental hormones
Cytotrophoblast
Cells that line inner layer of trophoblast
- Undifferentiated stem cells
Erodes maternal spinal vessel epithelium
- Gives rise to syncytiotrophoblast
Reduces as pregnancy advances
Formation of placenta
Syncytiotrophoblast invades the decidua
Cytotrophoblast erode maternal spiral vessels
The lacunae spaces fill with maternal blood
- Mesoderm form fetal blood vessels
Hormones produced by the placenta
hCG
- Maintans corpus luteum
- Stimulates oestrogen and progesterone release
Human placental lactogen (HPL)
- Helps growth
- Stimulates lactation
Placental barrier
Composed of a monolayer of:
- Syncytiotrophoblast (in direct contact with maternal blood)
- Cyrotrophoblast
- Fetal capillary epithelium
The barrier things as pregnancy advances= greater SA.
Components that are transferred across the placenta
Gases
- Simple diffusion
Water and electrocytes
Steroid hormones
Proteins- pinocytosis
Maternal antibodies IgG
- beings at week 12
- Peaks after week 34
Decidua
Modified endometrium
Three layers
- Capsularis
- Parietalis
- Basali–> between uterine wall and chorionic villae
Types of umbilical insertions
Central
- Normal
Marginal
Succenturiate
Circumvallate
Velamentous
Velamentous cord insertion
When the umbilical cord inserts and transverses fetal membrane before entering fetus.
Vasa praevia
When fetal blood vessel runs across the internal os opening
Complications
- Massive, painless bleeding
- Fetal death
- Maternal death
Positions of the placenta
anterior or posterior wall?
At the top? fundal
Placental praevia
Failure of trophoblastic invasion into maternal circulation at 12-18 weeks
Poor mixing of blood leads to
- Lack of o2 and nutrients
- IUGR
- Pre-clamspia
Placenta accreta
When the placenta attaches too deeply into the uterine wall.
Accreta- Invasion into myometrium, not constricted by basalis
Increta
- Invasion into myometrium
Percreta
- Invasion into perimetrium
Complications
- Severe bleeding
- Treated with hysterectomy.
Placental abruption
Premature separation of the placenta from the uterus
Complications
- Very painful
- Fetal/ maternal death
- Massive bleeding
DCDA
Either from dizygotic twins or monozygotic twins that split from between days 1-3.
