Embryology

Question 1

Define the pre-embryonic, embryonic and foetal periods of development.

Answer 1

Pre-Embryonic = First 2 weeks

Embryonic = week 2-8

Foetal = week 9-38

Question 2

Describe the changes of the fertilised zygote travelling to uterus

Answer 2

FERTILISATION

• Capacitation of sperm > acrosome reaction
• Ampulla of fallopian tube
• Zygote

CLEAVAGE

• 30 hours
• 2 blastomeres equal size with zona pellucida
• MORULA (ball of totipotent cells)

COMPACTION - first differentiation

• Day 4
• Embryoblast and Trophoblast
• Blastocyst cavity
• Pluripotent cells

HATCHING

• Day 5
• Blastocyst hatches from zona pellucida
• Enlarges
• Implants in uterine surface

IMPLANTATION

• Day 6-7
• Superior posterior uterine wall
• Trophoblast invades into endometrium

Question 3

Define zygote

Answer 3

• Single cell
• Product of fertilisation of ovum by sperm
• Produced by fusion of male and female pronuclei

Question 4

Define morula

Answer 4

Ball of undifferentiated, totipotent stem cells

Question 5

Define cleavage

Answer 5

A series of mitosis divisions into a clump of identical cells

Question 6

Define zona pellucida

Answer 6

A glycoprotein shell surrounding the changing zygote

Question 7

Define blastocyst

Answer 7

Differentiation of morula into inner and outer cell masses + internal cavity

• Inner = Embryoblast
• Outer = Trophoblast

Question 8

Define the term bilaminar disc

Answer 8

• 2 layered embryonic disc

- Consists of epiblast and hypoblast from inner cell mass

Question 9

Define the term germ layer

Answer 9

3 primary tissue layers of the trilaminar disc that arise through gastrulation:

• Endoderm
• Mesoderm
• Ectoderm

Question 10

Define the term mesoderm

Answer 10

Middle layer of the 3 embryonic germ layers of the trilaminar disc formed during gastrulation

Question 11

Define the term ectoderm

Answer 11

Outermost layer of the 3 embryonic germ layers of the trilaminar formed during gastrulation

Question 12

Define the term primitive streak

Answer 12

• Begins as a thickened region of epiblast at tail end of bilaminar disc.
• Site from which gastrulation proceeds

Question 13

Define the term gastrulation

Answer 13

The process during week 3 of development resulting in formation of 3 germ layers of the trilaminar disc

Question 14

Define the term trilaminar disc

Answer 14

3 layered germ disc produced by gastrulation:

• Endoderm
• Mesoderm
• Ectoderm

Question 15

Describe what happens in week 2 of development

Answer 15

Week of 2’s

Trophoblast >

• Syncytiotrophoblast
• Cytotrophoblast

Embryoblast > bilaminar disc

• Epiblast
• Hypoblast

Hypoblast >

• Yolk sac
• Chorionic cavity

Question 16

Explain the use of the terms ‘dermatome’ and ‘myotome’ in describing the innervation of skin and muscles in an adult.

Answer 16

Dermatome / myotome are used clinically to define an area of skin / muscle supplied from a single spinal nerve root.

Each spinal cord segment gives rise to a nerve root