12 Embryology

Question 1

From fertilisation to birth. What are the steps and time spans?

Answer 1

pre-embryonic: first 2 weeks
embryonic period: 3-8 weeks. when all the body structures are built.
fetal period: 9th week to birth. maturation and growth

Question 2

How is birth calculated?

Answer 2

calculate from date of LMP so 38 weeks onwards +/-2 weeks

Question 3

Pre-embryonic period

What happens? What are the stages?

Answer 3

1 Cleavage: formation of the morula
2 Compaction: formation of blastocyst
3 Implantation begins

Question 4

Fertilisation (brief)

process

Answer 4

oocyte is released from the ovary and travels along the fallopian tube and is fertilised by the sperm in the ampulla. egg+sperm=ZYGOTE

Question 5

What is the perfect site of implantation of the zygote?

Answer 5

The posterior uterine wall

Question 6

Viability of oocyte?

Of sperm?

Answer 6

1 day

3 days

Question 7

What happens in week 1 of gestation? What is the very first step? whats protecting it?

Answer 7

first step is mitotic division into two daughter cells. normally 30 hours after fertilisation. the two daughter cells are equal and are called blastomeres.

The coat outside the cells is called the zona pellucida. This is a glycoprotein shell that protects the zygote from further fertilisation by other sperm.

Question 8

What happens in week 1 of gestation?

Whats a morula? Whats PGD?

Answer 8

blastomeres continue to divide but get small as confined to the shell. the clump of cells is called a morula. these cells are totipotent so can therefore have capacity to develop into any cell type.

PGD: pre-implantation genetic diagnosis. This is when a cell can be safely removed from the morula and tested for serious heritable conditions prior to transfer of the embryo into the mother.

Question 9

What happens in week 1 of gestation?

Compaction what happens?

Answer 9

Cells are pluripotent after compaction as can become many cell types but not all.

Formation of the first cavity called the blastocyst cavity. the outercell mass is called the trophoblast and inner cell mass is called the embryoblast.
–> the blastocyst

Question 10

What happens in week 1 of gestation?

Hatching

Answer 10

the blastocyst hatches from the zona pellucida so is no longer constrained to the shell

Question 11

What happens in week 1 of gestation?

Beginning of implantation. how many cells do we have at this point?

Answer 11

approx 100 cells. about 8 will give us the embryo and the remainder begin the development of the fetal membranes. allow the outer cell mass to make contact with the/ into endometrial lining of the uterus. its celled the conceptus

Question 12

What happens in week 2 of gestation?

overview

Answer 12

Differentiation into two distinct layers:

• Outer cell layer: syncytiotrophoblast (multi nucleated sheet of epithelial cells, good for transport)and cytotrophoblast (essentially stem cell layer to provide for the syn layer)
• Inner cell layer becomes the bilaminar disk: epiblast and hypoblast.

Question 13

What happens in week 2 of gestation? By the end of this week?

Answer 13

The conceptus has implanted on the lining. the embryo has two cavities: amniotic cavity and the yolk sac. these are suspended by the connecting stalk within the supporting sac of the chorionic cavity

Question 14

What happens in week 2 of gestation?

Implantation what are the benefits

Answer 14

It is intestitial as the uterine epithelium is breached and the conceptus implants within the uterine stroma. It establishes maternal blood flow within the placenta so the embryo has support form maternal circulation. implantation also allows the basic structural unit of materno-fetal exchange (the chorionic villus)

Question 15

what are conditions linked to implantation defects?

there’s four listed

Answer 15

IUGR
Pre-eclampsia
ectopic pregnancy
placenta praevia

Question 16

Ectopic pregnancy

Answer 16

happens normally when implantation happens within the fallopian tube. can be a life threatening emergancy.

Question 17

placenta praevia

Answer 17

implantation happens in the lower uterine segment and can cause haemorrhage as placenta grows across the opening of the reproductive tract. will require a c-section.

Question 18

What happens in week 2 of gestation? Day 9

Answer 18

Embryonic pole: rapid development of syncytiotrophoblast

abembryonic pole: primitive yolk sac formed that is within contact with the cytotrophoblast layer

Question 19

What happens in week 2 of gestation? Day 11

Answer 19

start to separate yolk sac membrane away from the cytotrophoblast layer forming extraembryonic mesoderm between them

Question 20

What happens in week 2 of gestation? Day 12

Answer 20

Maternal sinosoids invaded by syncytiotrophoblast –> the beginning of uteroplacental circulation.

Question 21

What happens in week 2 of gestation? Day 13

Answer 21

Formation of the secondary yolk sac aka the definitive yolk sac. the primitive yolk sac pinches off

Question 22

What happens in week 2 of gestation? Day 14

Answer 22

The spaces within the extraembryonic mesoderm merge to form the chorionic cavity. the embryo and its cavities are suspended by the connecting stalk which is a column of mesoderm which will later be the umbilical cord.

can be bleeding around this time.