Embryology S1

Question 1

What is the primitive streak and what is it important for?

Answer 1

A narrow groove that develops on the dorsal surface of the epiblast. It has an important role in the orientation of the embryo, determining the front and the back

Question 2

Explain briefly how the mesoderm is formed

Answer 2

• Cells on the epiblast migrate towards the primitive streak, invaginating into the epiblast, displacing the hypoblast, creating three layers
• As more cells migrate through the streak, they spread laterally and cephalic (towards the head)

Question 3

What is the function of the Zona pellucida

Answer 3

The glycoprotein shell that surrounds the oocyte that prevents polyspermy

Question 4

What is the bilaminar disc?

Answer 4

Made up of the epiblast and the hypoblast; derived from the embryoblast

Question 5

When is gastrulation complete?

Answer 5

Because development proceeds cephalocaudally (head to tail), gastrulation is not complete in the caudal region until week 4

Question 6

Name the three germ layers within the trilaminar disc and their positions within

Answer 6

Endoderm - The innermost layer
Mesoderm - The middle germ layer
Ectoderm - The outermost layer

Question 7

What is gastrulation

Answer 7

The reorganisation of the germ layers into the ectoderm, mesoderm and endoderm and hence, establishing the origin of all cells

Question 8

Where is the cytotrophoblast found?

Answer 8

It is the placental membrane around the yolk sac

Question 9

Name three derivatives of the mesodermal germ layer

Answer 9

Any from: Smooth, skeletal and cardiac muscle, connective tissue, bone, cartilage, blood and blood vessels and the urinary system

Question 10

What is a germ layer?

Answer 10

A layer from which other developed tissues originate from

Question 11

Where is the oocyte fertilised?

Answer 11

In the ampulla

Question 12

What is a fertilised oocyte called?

Answer 12

Zygote

Question 13

Name 3 important derivatives of the ectodermal germ layer

Answer 13

Any from: Skin, skin derivatives (including hair, nails, lens and cornea, mouth and anus lining)

Question 14

(Embryology) What are the three periods of human development?

Answer 14

1) Pre-embryonic: 1-2 weeks
2) Embryonic: 3-8 weeks
3) Fetal: 9-38 weeks

Question 15

(Embryology) What changes occur between fertilisation of the oocyte and compaction?

Answer 15

Zygote undergoes cleavage to form two blastomeres which make up the morula

Question 16

(Embryology) On which day does compaction occur?

Answer 16

Day 4

Question 17

(Embryology) What is the structure of the conceptus after compaction

Answer 17

Blastocyst:

• Inner cell mass - Embryoblast
• Outer cell mass - Trophoblast
• Blastocyst cavity
• inside zona pellucida

Question 18

(Embryology) How many cells will go on to form the embryo?

Answer 18

8

Question 19

(Embryology) What occurs on day 5?

Answer 19

Hatching - blastocyst moves out of the zona pellucida and is now no longer constrained and is free to enlarge and divide and interact with the uterine wall for implantation

Question 20

(Embryology) Describe the process of implantation

Answer 20

Occurs on days 6-7. Interstitial process whereby the blastocyst invades the uterine wall and implants in the storm

Question 21

(Embryology) What is the optimum site for implantation

Answer 21

Posterior uterine wall

Question 22

(Embryology) What problems can arise if the zygote implants at an alternative site to the posterior uterine wall?

Answer 22

Placenta praevia- conceptus implants in the lower uterine segment , covering the cervix opening, requires a caesarean section

Ectopic pregnancy- usually occurs when zygote implants in fallopian tube, can quickly become life threatening

Question 23

What changes occur on day 9?

Answer 23

Rapid development of the syncytiotrophoblast. Formation of the yolk sac

Question 24

What stimulates the process of gastrulation

Answer 24

Appearance of the primitive streak, node and pit on the dorsal surface of the epiblast

Question 25

What is the purpose of the notochord?

Answer 25

• Important signalling role
• Drives formation of nervous system
• Basis for axial skeleton
• Define the phylum Chordata

Question 26

Describe notochord formation and development

Answer 26

Notochord emerges from the cranial end of the primitive pit and forms a thick rod along the mesoderm, defining the midline

Question 27

What is the role of the notochord in the development of the neural tube in week 4?

Answer 27

Releases diffusion limited signalling molecules which stimulate thickening of the overlying ectoderm
This forms the neural plate
Edges of the plate are elevated above the plane of the disc and curl towards each other to form the neural tube

Question 28

What is the fate of the notochord?

Answer 28

Goes on to form the spinal chord

Question 29

What is the developmental and clinical significance of the myotome?

Answer 29

Developmental - Gives rise to muscle

Clinical - muscle or group of muscles supplied by a single spinal nerve

Question 30

What is the developmental and clinical significance of the dermatome?

Answer 30

Developmental - Gives rise to dermis

Clinical - Single strip of skin supplied by a single spinal never

Question 31

In which two directions does embryo folding occur?

Answer 31

Cephalocaudally and laterally