Fertilisation and placental development Flashcards
Fertilisation and beyong
Sperm and ovum meet in fallopian tube 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 )
Stages from zygote to blastomere
Zyogte
2 cell stage
4 cell stage
8 cell stage
Morula (72 hours)
Blastocyst (4 days)
Days 4-5
The morula develops cavity and becomes blastocyst
Blastocysts thins out and becomes the trophoblast- start of the placenta
The rest of the cells move for form the inner cell mass- creates embryonic pole
The blastocyst has now reached the uterine lumen and is ready for implantation
Day 6-7 Bilaminer 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
Dasy 16+
Bilaminar disc develops further forming 3 distinct layers (gastrulation)
Initiated by primitive streak
The epiblast becomes known as ecyoderm
The hypoblast is replaced by cells from the epiblast and becomes endoderm
The epiblast gives rise to the third layer the mesoderm
Embryo- the 3 germ layers
The hypoblast degenerates
The epiblast gives risk to all three germ layers
The embryo folds to create the adult pattern
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
Formation of the placenta
Syncytiotrophoblast invades decidua (endometrium)
Cytotrophoblast cells erodes maternal spiral arteries and veins
Spaces (lacunae) between ten fill up with maternal blood
Followed by mesoderm that develops into foetal vessels
Aiding the transfer of nutrients, O2 across a simple cellular barrier
Cytotrophoblast cells
Undifferentiated stem cells
Invade the maternal blood vessels and destroy the epithelium
Give risk to syncytiotrophoblast cells
Reduce in number as pregnancy advances
Syncytiotrophoblast cells
Fully differentiated cells
Direct contact with maternal blood
Produce placental hormones
The placenta as an endocrine organ
Human chorionic gonadotrophin
- maintenance of corpus luteum of pregnancy
- progesterone and oestrogen
Human placental lactogen
- growth, lactation
- carbohydrate and lipid
Placental barrier
Maternal blood in the lacunar in direct contact with cyncytiotrophoblasts
Mono layer of syncytioptrpphoblast/ cytotrophoblast/ foetal capillary epithelium is all that separates the foetal and maternal blood
Cytotrophoblasts decrease as the pregnancy advances
The barrier thins as pregnancy advances leading to a greater surface area for exchange
Transfer across the placenta
Gases- oxygen and CO2 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
Topographical names of parts of the decidua
Capsularis- overlying embryo and chorionic cavity
Parietalis- side uterus not occupied by
Basalis- between uterine wall and chorionic villae
Position of the placenta within the uterus
Mainly fundal
Anterior or posterior
Low lying or placenta praevia (near to the cervical os)
