Embryology Flashcards
what is the pre-embryonic period?
From fertilisation to the trilaminar disc stage
1-3 weeks
what is the embryonic period?
From the trilaminar disc stage to the foetal stage
4-8 weeks
rapid growth and differentiation
organogenesis
period where defects can occur and the foetus is susceptible to pharmacological change
what is the foetal period?
The consolidation and growth of the baby
9 weeks - term
what is the difference between fertilisation age and menstrual age?
Menstrual age: time since the first day of mothers last period
Fertilisation age: time at which the ovum was actually fertilised
Menstrual age is two weeks more than the fertilisation age
Describe the structure of the egg
Cumulus: outside layer of cells which the nourish and support the egg as it develops
Zona pellucida: special membrane
Oocyte plasma membrane: surrounds the egg and has specific receptors for sperm
How does fertilisation occur?
- acrosome full of enzymes bursts in head of sperm upon contact with egg
- sperm penetrates zona pellucida
- binds to receptors on oocyte membrane so egg and sperm fuse
- zona pellucid thickens to prevent polyspermy
- receptors withdraw
- fusion of nuclei- 46 chromosomes
how many cells make a morula?
16
cell types within a blastocyst?
embryoblasts (inner cell mass)
trophoblasts
what happens if the zona pellucida is damaged at the blastocyst stage?
The cells separate but share the same placenta so you get monozygotic twins
when does the blastocyst implant into the endometrium?
day 6
How does the blastocyst receive nutrients from the mother?
diffusion
what is an ectopic pregnancy?
implantation in an abnormal site
The inner cell mass differentiates into..?
Epiblasts and hypoblasts
the trophoblasts become..?
syncitiotrophoblasts which grow into the endometrium and cytotrophoblasts
where is the amniotic cavity found?
in the middle of the epiblasts cells
what is the coagulation plug?
once the embryoblast has fully implanted into the endometrium, the coagulation plug holds it to the wall to prevent it from moving back out into the uterus.
what membrane is formed when the hypoblast cells grow back towards the coagulation plug?
what is formed when there is a second wave of hypoblasts cell growth?
Haussers membrane- within is the primary yolk sac
secondary yolk sac and the primary yolk sac dissipates
when does gastrulation occur?
end of week 2
day 13-14
what is the role of the trophoblasts?
The trophoblast layer produces HCG
It stimulates the corpus luteum to produce oestrogen and progesterone to maintain the endometrium
why do the mothers WBC not attack the ‘foreign’ embryo?
the syncitiotrophoblasts form a barrier which prevent the WBC from passing. keeping the maternal and foetal blood flow separate. They switch off genes which would trigger an immune response also.
how does the mesoderm develop?
primitive streak appears on the ectoderm (epiblast cells). These cells migrate down and out between the ectoderm and endoderm. forming another layer called the mesoderm. This creates a trilaminar disc.
Describe the steps of neurolation
Underneath the primitive streak, mesoderm cells differentiate to form the notochord.
Induces the thickening of the above ectoderm to form the neural plate.
The neural plate sinks into the mesoderm to form the neural tube (week 3)
What do the neural crest cells go on to become?
melanocytes, dorsal root ganglion, adrenal medulla
Name three neural tube defects
- Hydrocephalus: water in the brain
Cerebral hemispheres swell up
Impaired development of cerebral cortex - Microcephaly: small brain
- Anencephaly: most of the brain is absent
failure of front end of neural tube to close
when does the embryo fold craniocaudally and laterally?
end of week 3
what do the three germ layers become
Endoderm: respiratory tract and GI tract
Paraxial mesoderm: muscle, skin, skeleton apart from cranium (vertebrae), connective tissue
Intermediate mesoderm: urogenital system
Lateral mesoderm: abdominal wall
Ectoderm: skin, nervous system
3 degrees of spina bifida
Occulta
Meningocoel
Myelocoel
At what point can sexing of the baby happen?
fifth month
At what point can a heartbeat be heard?
day 22
