Embryology Flashcards
Fertilisation - Day 1
- Sperm and Ovum meet in Uterine Tube (usually ampulla) 12-24 hours after ovulation.
- Penetration of Corona radiate and Zona pellucida
- Fusion and 2nd meiotic division
- Acrosome reaction makes ovum impermeable to other sperm
- End- Zygote- has diploid (46 chromosomes)
Fertilisation - Days 2-3
- Cleavage is the rapid process of mitotic divisions
- First mitotic division is around 30 hours post fertilization.
- By day 3 16 cell embryo
- Each cell is known as a blastomere.
- Solid sphere is known as a morula.
Fertilisation - Days 4-5
- Morula develops a cavity and becomes known as a blastocyst.
- The outer layer of the blastocyst thins out and becomes the trophoblast this helps form the placenta
- The rest of the cells move (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.
Fertilisation steps days 1-5
fertilisation 1st cleavage 4 cell stage advanced morula (3-4) blastocyst (4-5) inner cell mass
Fertilisation - Days 6-7
Bilaminar Disc- As the embryo starts to implant it forms two layers.
Inner cell mass differentiates into two layers: epiblast and hypoblast.
These two layers are in contact.
Hypoblast forms extraembryonic membranes
Epiblast forms embryo
Amniotic cavity develops within the epiblast mass
Primary Yolk Sac Day 6-8
- Derived from the hypoblast is the exocoelomic membrane.
(Also known as: Extraembryonic hypoblast/membrane) - This creates a cavity known as the Yolk Sac
- The Yolk Sac contains nutrients that supply the embryo before the placenta functions.
Week 2
4 stages of implantation
- Blastocyst has reached the uterine cavity
- Need for oxygen, nutrients and removal of waste
- Corpus luteum produces progesterone to maintain endometrium
- 4 Stages of implantation
- Shredding/Hatching of the zona pellucida
- Apposition to ensure embryonic pole is in contact
- Adhesion via molecular communication
- Invasion
- Implantation initiates a decidual reaction which causes the maternal cells to contribute to the placenta
Basic blastocyst describe
trophoblast surrounds, inner cell mass splits to epiblast and hypoblast
What two layers form the bilaminar embryonic disc.
what happens to these layers?
epiblast:
-Within the epiblast a cavity develops- the amniotic cavity, the epiblasts that line the cavity become amnioblasts
Hypoblast:
- exocoelomic membrane develops, later becomes yolk sac
Trophoblast
where is it derived from?
what does it become?
- Outer layer of cells of the blastocyst become trophoblast
- Trophoblast differentiates into cytotrophoblast and the syncytiotrophoblast.
- The cytotrophoblast is a single layer of cells and is the inner part.
- The syncytiotrophoblast is the outer layer and where it invades becomes known as the syncytium. Syncytiotrophoblasts secrete HCG
Week 3 plus
Gastrulation
At the start of week 3 the embryo (bilaminar disc) develops further by forming 3 distinct layers (this process is known as gastrulation).
- Initiated by primitive streak.
- Two layers have already formed (epiblast and hypoblast).
- 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.
- The hypoblast degenerates. The epiblast gives rise to all three germ layers.
Week 4 plus
- By 4th week the flat disc has to fold into 2 directions
+ Longitudinal (cephalocaudal) (day 21) begins so that head and tail are brought closer.
+ Lateral (transverse) (day 18) brings the amniotic cavity down, creating the future gut tube inside the peritoneal cavity.
Ectoderm - what does it become?
nervous system
skin, eyes, ears
Mesoderm - what does it become?
skeletal system
muscular system
renal system
cardiovascular (as well as endoderm)
Endoderm - what does it become?
cardiovascular (as well as mesoderm)
Gastrointestinal system
