Early Embryology

Question 1

Where does fertilisation occur?

Answer 1

Ampulla of uterine tube

Question 2

What type of cells are the ovum and sperm?

Answer 2

Haploid cells

Question 3

How does the sperm get into the ovum?

Answer 3

The sperm has a specialised lysosome in its head which dissolves the acrosomal membrane, the sperm’s protective shell around its head

It undergoes capacitation (maturation) and the acrosome reaction (described above) to be able to enter the maternal cytoplasm

Question 4

What are the two layers surrounding the ovum?

Answer 4

Zona pellucida and Corona radiata

Question 5

What happens in the ovum once the sperm enters?

Answer 5

• The sperm decondenses and forms a pronuclei
• The male and female pronuclei fuse together
• Maternal and paternal chromosomes line up on spindle equator
• Chromatids separate, forming two separate cells

Question 6

What changes inside the embryo during the first 3-4 days?

Answer 6

Cell numbers increase but cell size decreases

Question 7

When does cleavage occur?

Answer 7

36 hours after fertilisation

Question 8

What is compaction and when does it occur?

Answer 8

Individual cells become less visible, cells start to communicate between each other through gap junctions - anterior and posterior axis of embryo is estabilished
Begins at 8 cells

Question 9

What is the morula stage?

Answer 9

• Reached after 3/4 days, becomes a solid ball of 16+ cells
• The morula is transported along the uterine tube and arrives at the uterus approximately 3/4 days after fertilisation (30-40 cells)
• Zona pellucida disappears before implanting
• Outer cells divide to become trophectoderm and inner cells, combining into the inner cell mass

Question 10

What happens 4.5+ days after fertilisation?

Answer 10

Blastocyst stage

• Cavities (blastocoel) form in inner cell mass
• Blastocyst is made up of the trophoblast and inner cell mass
• Embryo is still free and unattached in the uterus
• Cells are totipotent until the blastocyst stage

Question 11

What are blastomeres?

Answer 11

Cells of the embryo at the cleavage stage

Question 12

What is the process of cleavage after fertilisation?

Answer 12

Cell division without growth

Question 13

Where do they remove cells from for genetic testing?

What are they looking for?

Answer 13

• 5-6 days old trophectoderm
• Safer than blastomere or inner cell mass sampling
• They are looking for single gene disorders or chromosome abnormalities

Question 14

What happens 5.5-6 days after fertilisation?

Answer 14

Binding at the uterus epithelium by the embryonic pole

• Epithelium of the uterus becomes more sticky during this period since expression of MUC-1 (that makes it non-sticky) is turned off
• Binding is via selectins of the embryo to the glyco-component on epithelial cells of the uterus
• Similar mechanism to while blood cell adhesion to blood vessel walls
• Integrins, lamina and fibronectins are involved in initial penetration of implantation

Question 15

What happens 6-7 days after fertilisation?

Answer 15

Implantation of blastocyst begins

• Trophoblast splits into syncytiotrophoblast and cytotrophoblast
• Trophoblast becomes the “invasive”, pushes through the uterus epithelium wall to create a network of cytoplasm with nuclei in it a.k.a. syncytiotrophoblast multinucleates syncitium invasion via metalloproteases
• Immunosuppression of host/graft reactions, so embryo is tolerated

Question 16

What is syncyntiotrophoblast?

Answer 16

Bits of trophoblast fused together; contains lots of nuclei without cell walls

Question 17

What happens 8 days after fertilisation?

Answer 17

• Two layers form in the embryo: epiblast and hypoblast, which will eventually develop into the ectoderm and endoterm
• Amniotic cavity begins to form as a space within the epiblast

Question 18

What happens 9 days after fertilisation?

Answer 18

• Hypoblast cells coat the blastocyst cavity to form the primary yolk sac
• Spaces develop within the syncytiotrophoblast
• Amniotic cavity is formed
• Extraembryonic mesoderm slowly starts to form below trophoblast (syncytritrophoblast and cytotrophoblast)

Question 19

What happens 11-12 days after fertilisation?

Answer 19

• Extraembryonic mesoderm forms with spaces, that will eventually form the chorionic cavity
• Embryonic disc has two layers bilayered
• Syncytiotrophoblast (SCT) cells erode through the walls of maternal capillaries which bleed into the spaces of SCT

Question 20

What happens at the end of week 2 after fertilisation?

Answer 20

2 layers develop in the trophoblast

• Syncytiotrophoblast
• Cytotrophoblast

2 layers form in the inner cell mass

• Epiblast - ectoderm
• Hypoblast - endoderm

2 cavities form

• Amniotic cavity
• Chorionic cavity

The remnant of the primary yolk sac can sometimes hang around in an adult

Question 21

What is preeclampsia?

Answer 21

Defect in the invasion process

• Cytotrophoblast tried to invade spiral arteries of the mother but can’t - defective
• Spiral arteries keep their muscular walls and act as high resistance vessels, increasing blood pressure

Question 22

What is ectopic implantation?

Answer 22

Embryo leaves the ampulla and implants into the peritonial cavity
- Most common site is ampulla due to tube rupture

Ectopics more likely in inflammatory disease of pelvis

Question 23

What are the causes of congenital malformations?

Answer 23

Genetic - chromosome/single mutation/deletions (20-25%)
Multifactorial or unknown (65-75%)
Environmental
- Drugs/chemicals (nicotine, alcohol, chemotherapy)
- Infectious agents (rubella, toxoplasma)
- Ionising radiation

Question 24

How does large alcohol consumption affect the developing embryo?

Answer 24

The neurotubes/neuro crest cells don’t work as well, resulting in:

• Growth retardation
• Central nervous system
• Effects on facial development

Question 25

How are the axes determined in the embryo?

Answer 25

Dorsal/ventral
- determined in the oocyte

Left/right
- determined by cilia movement in the node

Rostral/caudal