IMMS 3: Embryology

Question 1

How many spermatozoa reach the fallopian/uterine tube?

Answer 1

200-500 spermatozoa.

Question 2

How long can spermatozoa survive in the female reproductive tract versus ovum?

Answer 2

Up to 5 days for spermatozoa. Ovum can only survive 24 hours if not fertilised.

Question 3

Are spermatozoa moved more by their tails or uterine contractions?

Answer 3

Uterine contractions.

Question 4

What happens when sperm penetrates the zona pellucida?

Answer 4

The zona pellucida changes so no more sperm enter the oocyte - prevents polyspermy.

Question 5

What are the two types of molar pregnancy?

Answer 5

Complete mole and partial mole.

Question 6

What is a complete molar pregnancy?

Answer 6

A single sperm fertilises egg that doesn’t contain any genetic material. Abnormal trophoblast develops, which causes placenta to form, but no embryo develops. Gives positive pregnancy test.

Question 7

What is a partial molar pregnancy?

Answer 7

Normal egg cell fertilised by 2 spermatozoa. Embryo starts to develop but cannot survive -> miscarriage. Gives positive pregnancy test.

Question 8

What are the names of the stages of development from fertilisation to foetus?

Answer 8

Zygote -> morula (4 days post fertilisation) -> blastocyst (5-9 days) -> embryo (10-12 days) -> foetus (by 9 weeks).

Question 9

What are the similarities and differences between a zygote and a morula?

Answer 9

Zygote has <16 cells. Morula has 16-32 cells. Both are surrounded by the zona pellucida and cells are tightly compacted.

Question 10

What is the zona pellucida?

Answer 10

Glycoprotein coat surrounding the ovum which remains until blastocyst hatches from zona pellucida at 5-6 days after fertilisation, prior to implantation.

Question 11

What cells make up the morula?

Answer 11

Blastomeres.

Question 12

What two structures do the cells of the morula differentiate into?

Answer 12

Inner cell mass (ICM); will eventually develop into the embryo.
Trophoblast; outer cells which will develop into the placenta.

Question 13

What are the 3 stages of implantation?

Answer 13

Begins around day 6 after fertilisation.
1. Appopsition.
2. Adhesion.
3. Invasion.

Question 14

How does maternal/foetal gas exchange occur?

Answer 14

Placental villi, containing foetal blood, are bathed in maternal blood which fills the the lacunae.

Question 15

When does the dorsal ventral axis develop and what are the two layers?

Answer 15

In the blastocyst, inner cell mass flattens into bilaminar disc made of epiblast and hypoblast, forming the dorsal ventral axis.

Question 16

Where does the amniotic cavity form?

Answer 16

Between the epiblast and the trophoblast.

Question 17

Why might implantation be unsuccessful/problematic?

Answer 17

Blastocyst is abnormal (e.g. chromosomal abnormality).
Blastocyst doesn’t reach uterus.
Endometrium is not receptive.
Implantation may be ectopic (most commonly in fallopian tube, can also occur in ovary, abdo cavity if implants too far through endometrium, or cervix).

Question 18

What happens in invasive placentation?

Answer 18

Placenta invades too far into endometrium. Placenta then doesn’t come away easily after birth, can result in severe haemorrhage.
Placenta accreta = too deep in endometrium.
Placenta increta = deeper, reaches myometrium.
Placenta percreta = even deeper, through myometrium reaching to other organs e.g. bladder.

Question 19

What is placenta previa?

Answer 19

Placenta covers the cervix causing problems with delivery, almost always results in c-section.

Question 20

What happens in gastrulation?

Answer 20

Week 3: bilaminar disc develops into trilaminar disc, establishing the three germ layers. Body axes anterior-posterior develops.

Question 21

What is the primitive streak?

Answer 21

Groove at caudal end of epiblast which epiblasts then migrate and invaginate through. Three layers are formed from the epiblasts: ectoderm, mesoderm, endoderm.

Question 22

Which germ layer is formed during gastrulation?

Answer 22

Mesoderm.

Question 23

When might situs inversus or dextrocardia occur?

Answer 23

During gastrulation.

Question 24

When does neurulation occur?

Answer 24

Week 4

Question 25

What is the key process that occurs during neurulation?

Answer 25

Formation of the neural tube.

Question 26

How is the neural tube formed during neurulation?

Answer 26

Ectoderm thickens into neural plate.
Plate bends to form a groove.
Two sides of the groove fuse to form the neural tube.
Neural tube detaches from ectoderm.
Cranial end expands, will become the brain. Caudal end remains tubular, will become the spinal cord.

Question 27

When does the embryo fold from a flat disc into a cylinder?

Answer 27

Week 4

Question 28

What two planes does folding occur in?

Answer 28

Lateral - edges curve anteriorly and fuse, form abdominal walls.
Craniocaudal - head and tail ends curve.

Question 29

How is the primitive gut formed and in which week?

Answer 29

Fusion of the lateral folds pinches off most of the yolk sac which becomes the primitive gut tube.
Week 4.

Question 30

When are organ systems formed?

Answer 30

Weeks 3-8 of gestation.

Question 31

When might a neural tube defect form?

Answer 31

In week 4 when the neural tube closes along its length, starting in the middle.

Question 32

What is anencephaly?

Answer 32

A neural tube defect which occurs when the cranial end of the neural tube doesn’t close properly.

Question 33

Where do the progenitor heart cells initially reside?

Answer 33

Epiblast, then settle in mesoderm.

Question 34

In limb development, during which week do the hand and foot plates appear?

Answer 34

Week 6.

Question 35

When do limb buds appear?

Answer 35

Week 4

Question 36

When does the heart tube form?

Answer 36

End of week 3, cardiac looping occurs in week 4.

Question 37

Why is retinoic acid (vitamin A) concentration important during embryonic development?

Answer 37

Retinoic acid concentration is low by oesophagus with higher concentrations at other end of gut, thought to provide positioning information for gut cells on which cells to differentiate into. Vitamin A supplements in pregnancy can disrupt normal retinoic acid signalling, therefore is teratogenic.

Question 38

How and when are digits formed in the limb buds?

Answer 38

Through apoptosis. Formed by week 8.

Question 39

What do the hypoblasts become?

Answer 39

The yolk sac.