Basic Embryology

Question 1

Primary oocytes are held at what stage until sexual maturity?

Answer 1

Prophase I - specifically diplotene (the name of this rest phase is dictyotene).

Question 2

What happens when the oocyte matures?

Answer 2

Meiosis resumes and completes 1st meiotic division.
Cell enters 2nd meiotic division but rests in metaphase pending ovulation.
Meiosis 2 completed if fertilised by a sperm.

Question 3

What does cleavage refer to?

What is the morulla?

Answer 3

Series of mitotic divisions.

Morulla- collection of ~30 cells, at around 96 hours.

Question 4

What do the placenta and fetal membranes develop from?

Answer 4

The outer cell mass. Trophoblast.

Question 5

From what cells does the embryo form?

Answer 5

The 2-4 inner most cells of the preceding morulla e.g. the inner cell mass - aka embryoblast.

Question 6

How many germ layers does the developing embryo have and what are they called?
What is the process called that establishes these layers?

Answer 6

Three layers. They are the Ectoderm, the Endoderm and the Mesoderm.
The process that establishes them is Gastrulation.

Question 7

What are the consequence of the neuropores failing to close properly?
Describe what they are and what can be done by the woman to prevent them from occurring?

Answer 7

Neural tube defects e.g. Anencephaly and Spina Bifida.
Anencephaly - failure of the neural tube to close at the cranial/anterior end and results in most of the brain failing to form.
Spina Bifida - failure of the neural tube to close from the mid section (cervical) through to the tail (caudal), most commonly occurs in the lumbosacral region suggesting that this area is more susceptible to genetic/environmental factors.
Rates of neural tube defects are significantly lower where Folic Acid is given as a supplement prior to conceiving and for the first 3 months of pregnancy, it is not fully understood why.

Question 8

What cells originate from the endoderm? Give some examples.

Answer 8

The epithelium of the gastrointestinal tract and the respiratory tract. This includes epithelium of;
Pharyngeal derivatives,
Lungs,
Trachea and bronchus,
Liver,
Pancreas,
Urinary bladder,
Urachus.

Question 9

What cells originate from the ectoderm?
What process is specifically associated with ectodermal development?
What can the ectoderm be split into?

Answer 9

The ectoderm gives rise to:

• central nervous system (brain and spinal cord)
• peripheral nervous system
• sensory epithelia of the eye, ear and nose
• the epidermis and its appendages (nails and hair)
• mammary glands
• pituitary gland
• subcutaneous glands
• tooth enamel

Ectodermal development of the nervous tissue is called Neurulation.

Ectoderm can be split into Surface Ectoderm and Neuroectoderm.

Question 10

What happens if cell division goes wrong at any point during embryogenesis?

Answer 10

Can end up with abnormalities in:

• placenta only (CPM)
• embryo only
• placenta and embro.

Question 11

What cells originate from the mesoderm? (state some examples)

Answer 11

The mesoderm gives rise to:

• connective tissue cartilage and bone
• dermis (the lower layer of the skin)
• urogenital system
• blood and lymph cells
• blood and lymph vessels
• serous membranes such as the pericardium (heart), peritoneum (lungs) and the pleura (lungs)
• adrenal cortex

Question 12

What is amniotic fluid?

Answer 12

It is 99% water. It is a clear, yellowy fluid produced in part by amniotic cells but primarily derived from maternal blood.

Question 13

Name some functions of amniotic fluid (include at least one mechanical and one physiological function).

Answer 13

Mechanical:
1 absorbs jolts / cushions the fetus,
2 enables symmetrical growth,
3 allows free fetal movements
4 maintains constant fetal temperature
5 prevents fetus from adhering to the amnion.

Physiological:
1 helps musculoskeletal development
2 essential for proper development of the respiratory system

Question 14

What is hydramnios/polyhydramnios and name something it can be caused by?

Answer 14

Too much amniotic fluid. Can be caused by maternal diabetes (25%), central nervous system disorders and gastrointestinal defects that stop the fetus swallowing the fluid. Can also be idiopathic(35%).

Question 15

What is oligohydramnios and what can it be caused by?

Also what can it cause?

Answer 15

A lack of amniotic fluid. Can be caused by premature rupture of membranes and urinary tract abnormalities/renal agenesis - less fluid due to lack of fetal urine.
It can cause IUGR and limb abnormalities.
Pulmonary hyperplasia and facial defects due to compression by the uterine wall.

Question 16

What cells are present in amniotic fluid?

Answer 16

Fetal skin cells and the epithelium surfaces of the fetal:

• respiratory tract
• gastrointestinal tract
• genitourinary tract
• umbilical cord
• amnion.

Question 17

How do monozygotic twins arise?

Answer 17

Splitting of the zygote at various stages of development.

• usually occurs at the blastocyst stage so the 2 embryos have a common placenta and chorionic cavity but separate amniotic cavities.
• can also occur at the 2 cell stage and this results in 2 placentas, 2 chorionic cavities and 2 amniotic cavities.

Rarely it can occur at the bilaminar germ disc stage and this results in only 1 placenta, 1 chorionic cavity and 1 amniotic cavity.

Question 18

What is the placenta?

Answer 18

Tissue/organ that forms during pregnancy which connects the fetus (via the umbilical cord) to the uterine wall.

Question 19

What is the role of the placenta?

Answer 19

Organ that facilitates nutrient and gas exchange between maternal and fetal parts.
Produces hormones.

Question 20

What are chorionic villi?

Answer 20

Tissue which extends from the chorion

Fetal portion of the placenta

Finger like projections that contain capillaries for blood flow - they are the border of fetal v maternal blood

Question 21

How are dizygotic twins created?

Answer 21

2 eggs and 2 sperm. They usually implant separately and have own placenta. Occasionally placentas fuse.

Question 22

What is a vanishing twin?

Answer 22

Pregnancy starts out as twin pregnancy but at scan only one baby. Second twin has been miscarried and then either:

• mummified (papyraceous)
• resorption of twin

Question 23

What causes triploidy? What are the mechanisms called?

Answer 23

Diandry/diandric - fertilisation of one egg by 2 sperm. Less likely to be diploid sperm.
Digyny/digenic - diploid egg fertilised by sperm is most likely. Rare cause - fusion of 2 eggs (dieggy).

Question 24

What is a partial mole?

Answer 24

An additional paternal haploid chromosome set.
Usually by fertilisation of egg by 2 sperm.
69,XXX or 69,XXY. 69,XYY v rare due to earlier lethality.
Rarely - fertilisation by a diploid sperm.

Question 25

What do we need to bare in mind with a triploidy tissue detected by QF-PCR?

Answer 25

If there isn’t a maternal blood sample to compare then we do not know if the extra set of chromosomes are paternal (diandric) - resulting in a partial mole.
This has consequences due to gestational trophoblastic disease.
Partial mole has a lower risk than complete mole but still increases risk of neoplasia.

Question 26

What is a complete mole?

Answer 26

Tissue with a entirely paternal chromosome set - usually from doubling up of a 23,X sperm.
Rare cause is 2 sperms.
Regardless, there is no maternal contribution.

Question 27

What is omphalocele?

Answer 27

Portions of gut tube herniate through umbilical cord (belly button).
Chromosome abnormalities in 15% of cases.

Question 28

What is diaphragmatic hernia?

Answer 28

Common malformation.
Failure of one or both membranes to close so that stomach, spleen liver, intestines can enter the thoracic cavity.
Squashes heart and lungs.

Question 29

What cells does the heart originate from?

Answer 29

Mesoderm and neural crest cells.

Question 30

What is the origin of DiGeorge Syndrome/22q11.2 Deletion Syndrome?
How does this origin link to the typical phenotype?

Answer 30

Abnormal neural crest development.
Neural crest cells play a role in the development of both the face and heart. Hence this syndrome is associated with heart and facial defects.

Question 31

What germ cell layer does a blood sample represent?

Answer 31

Mesoderm layer.

Question 32

What germ cell layer does a buccal scrape represent?

Answer 32

The ectoderm layer.