Early embryonic development Flashcards
What period is classified as the pre-embryonic period?
The first two weeks
What is ectopic pregnancy?
It is implantation at a site other than the uterine body (most commonly the fallopian tube). It can quickly become a life-threatening emergency.
What is placenta praevia?
It is implantation in the lower uterine segment. It can cause haemorrhaging in pregnancy. Requires C section delivery.
How many cells is the conceptus at implantation?
107 cells
What is gastrulation?
It is the process establishing the three germ layers and hence origin of all TOB.
Where is the primitive streak and node found?
On the dorsal surface of the epiblast.
What happens to the primitive streak and node with gastrulation?
It regresses
In gastrulation cell migrate through the streak forming the mesoderm except in two places. What are they?
The mouth and anus
How does the notochord emerge?
Prenotochordal cells of the epiblast migrate through the cranial part of the primitive pit.
What is situs inversus?
It is the mirror image of viscera resulting from immotile cilia
Left and right assymetry is controlled by what?
Cilliated cells at the node
What are monozygotic twins?
A single fertilised oocyte that gives rise to two identical kids
What are the two mechanisms whereby monozygotic twins are formed?
1) Inner cell mass is duplicated. Two embryos sharing a placenta
2) Embryo splits after first cleavage: two embryos, each its own placenta
What is teratogenesis?
It is the process through which normal embryonic period is disrupted. Thalidomide and other chemicals can act as teratogens.
What is neurolation?
It is the notochord-driven induction of ectoderm to form the nervous system.
What is the specialised overlying ectoderm called?
Neuroectoderm
How many somites are formed in total by the end?
31 pairs which leads to the 31 spinal nerves
Paraxial mesoderm goes on to form?
The axial skeleton, dermis, muscle
In which two planes does the embryonic folding takes place?
Cephalocaudal folding and lateral folding
What does embryonic folding achieve? (5)
- Creates a ventral body wall
- It pulls the aminotic membrane around the disk so the embryo is suspended within the amniotic sac
- It pulls the connecting stalk ventrally
- It puts the heart in the right place
What is the function of the zona pellucida?
It prevents polyspermy
What day does cleavage take place?
Day 2
The ……. is formed as a result of cleavage.
Morula
What are the two types of cells found in a blastocyst?
Trophoblast and embryoblast
