Embryology 2

Question 1

What is the primitive streak?

Answer 1

Formed in the midline of the epiblast, by the invagination of cells.
Once formed, the axis of the embryo forms.

Question 2

What is gastrulation?

Answer 2

The formation of three germ layers.
The ectoderm, mesoderm and endoderm.

Question 3

What causes gastrulation?

Answer 3

When epiblast cells migrate to lie between the epiblast and hypoblast layers.
Hypoblast cells are displaced.

Question 4

What does gastrulation form?

Answer 4

A trilaminar disc.
The cells have now become specialised.

Question 5

How are notochords formed?

Answer 5

A primitive streak forms in the ectoderm.
Cells sink to form a solid tube.
A notochord (a solid tube of cells) form below the mesoderm.

Question 6

What is neurulation?

Answer 6

A neural plate forms in the ectoderm.
It sinks down to form a neural tube.
The notochord induces ectodermal cells in the midline to form a neural tube.

Question 7

What does the mesoderm separate into?

Answer 7

The neural tube induces the mesoderm to thicken and separate into the paraxial mesoderm, intermediate plate mesoderm, and lateral plate mesoderm.

Question 8

What does the lateral plate mesoderm split into?

Answer 8

Somatic mesoderm.
Splanchnic mesoderm.
The intra-embryonic coelom is the space between.

Question 9

What are somites?

Answer 9

Formed by the segmentation of the paraxial mesoderm.
Each somite divides into the dermatome (dermis of the skin), myotome (muscles) and sclerotome (bones).

Question 10

What does the ectoderm develop into?

Answer 10

Neural tube - forebrain, midbrain, hindbrain, and spinal cord.
Epidermis of the skin.

Question 11

What do the layers of the mesoderm develop into?

Answer 11

Paraxial - 43 pairs of somites.
Intermediate - urogenital system.
Lateral plate - body cavities and pleura.

Question 12

What does the lateral plate mesoderm split into?

Answer 12

Somatic mesoderm.
Splanchnic mesoderm.
Intra-embryonic coelom (the space between).

Question 13

What does the endoderm form?

Answer 13

The gut.
The respiratory system.

Question 14

What are teratogens? Give examples.

Answer 14

Environmental factors that cause abnormal development.
Drugs, alcohol, tobacco, infectious agents that can transfer through the placenta, and radiation.

Question 15

Describe the levels of sensitivity to teratogens during embryonic growth.

Answer 15

Weeks 1-2 - high risk of death, low risk from teratogens.
Weeks 3-8 - period of greatest sensitivity to teratogens.
Weeks 9-38 - decreasing sensitivity.

Question 16

What does teratogen risk depend on?

Answer 16

Exposure during critical periods of development.
Drug dosage.
Genetic constitution of the embryo.

Question 17

What are the genetic factors that can cause abnormal development?

Answer 17

Chromosome number.
Structural changes.
Increased maternal age.
Damage from environmental factors.

Question 18

What prenatal tests can inform malformation diagnosis?

Answer 18

Blood - alpha-fetoprotein.
Ultrasound scan - 12 week anomaly scan.
Chorionic villus sampling.
Amniocentesis.

Question 19

What postnatal tests can inform malformation diagnosis?

Answer 19

Hip stability.
Testes (descent).
Fingers and toes.
Hearing.

Question 20

What else forms during the embryonic period?

Answer 20

Between weeks 4-8.
Limb buds form.
Pharyngeal arches form in the neck.