Lecture 3: Early Embryology pt.1

Question 1

What is embryology?

Answer 1

Study of molecular, cellular & structural factors involved in progression from a single cell (pre-embryonic stage) to a baby (40 weeks)

Question 2

What happens during fertilisation?

Answer 2

Oocyte released from ovary and travels along fallopian tube.
Fertilisation by the sperm occurs in the ampulla and produces a zygote.
This zygote eventually divides into a blastocyst.
Implants itself into the upper posterior uterine wall.

Question 3

How long are sperm and oocytes viable for?

Answer 3

Sperm = 5 days
Oocyte = 1 day

Question 4

Why is ovulation typically described as 7 days?

Answer 4

Sperm is viable for 3 days whereas oocytes as only viable for 1 day. Thus, there are 3 days before & after the 1 day (the oocyte is viable) for a more likely chance of successful fertilisation.
3-1-3 = 7 days

Question 5

Order these in formation order… morula, blastocyst, blastomeres, zygote.

Answer 5

1. Zygote
2. Blastomeres
3. Morula
4. Blastocyst

Question 6

What is the purpose of the zona pellucida?

Answer 6

Sperm’s glycoprotein acts as a protective shell for the blastomere against other sperm. Prevents more than 2 sets of DNA.

Question 7

Define the following terms:

• pre-embryonic/germinal period
• embryonic period
• fetal period

Answer 7

Pre-embryonic/germinal period: fertilisation - end of week 2.

Embryonic period: 3rd week to end of 8th week.

Fetal period: 9th week till birth at 38 weeks (40 including conception)

Question 8

Describe what happens in the 1st week of human development.

• 30hrs after fertilisation
• day 2
• day 3

Answer 8

• 30hrs after fertilisation: mitotic cleavage into 2 blastomeres
• day 2: sperm’s zona pellucida protects blastomeres from other sperm
• day 3: blastomeres divide into morula

Question 9

Describe what happens in the 1st week of human development.

Answer 9

• day 4: compaction into blastocyst (1st cavity)
• day 5 hatching so blastocyst can enlarge and implant
• day 6/7: conceptus implantation begins

Question 10

Describe the 2nd week of human development.

• day 8
• day 9/10
• day 11

Answer 10

• Day 8: morula differentiates- embryoblast (epiblast + hypoblast) & trophoblast (cytotrophoblast + syncytiotrophoblast)
• Day 9/10: conceptus implants in uterine lining & establish maternal blood flow
• day 11: yolk sac moves from cytotrophoblast

Question 11

Describe the 2nd week of human development.

• day 12
• day 13
• day 14

Answer 11

• day 12: syncytio. makes contact with endometrium capillaries so bathed in maternal blood for O2, glucose.
• day 13: 2° yolk sac formed by pinching off from 1° yolk sac
• day 14: extraembryonic mesoderm spaces merge into a chorionic/embryonic cavity

Question 12

Describe the events of the 3rd week of human development. Try to include the following:

• primitive streak
• gastrulation
• Epiblast differentiation

Answer 12

• flat-disked epiblast develops primitive streak: signalling gastrulation
• cells invaginate in the primitive streak so the differentiate + displace hypoblast
• Epiblast differentiates ectoderm, mesoderm, endoderm

Question 13

Describe the 4th week of human development.

Answer 13

• Gastrulation created notochord from mesoderm’s middle clusters of cells
• Neuroectoderm forms a thickened neural plate which keeps thickening so curls into neural tube.
• Embryonic folding occurs where you have cephalocaudal folding (head then tail fold) then lateral folding

Question 14

What is important about gastrulation?

Answer 14

Establishes L+R side of the body.
If the ciliated cells become immobile then they will not beat the primitive node’s signals to the left. This is called side specific signalling.

This would cause situs invertus where the body’s internal structure is mirrored (eg: heart more towards RHS)

Question 15

Describe the notochord and its purpose.

Answer 15

Solid rod of mesoderm cells that define the midline.

They convert the ectoderm into neuroectoderm for neurulation which leads to the CNS formation.

Question 16

What are the 5 areas of the mesoderm?

Answer 16

Somatic mesoderm
paraxial mesoderm
Intermediate mesoderm
Splanchnic mesoderm
Intraembryonic coelom

Question 17

What does the ectoderm eventually develop into?

Answer 17

Organs, epidermis, nervous system

Question 18

What does the mesoderm eventually develop into?

Answer 18

CVS
Muscles
Bone
Supporting tissue
cartilage

Question 19

What does the endoderm eventually develop into?

Answer 19

Epithelial lining of GI, respiratory tract

Question 20

Consequences of embryonic folding.

Answer 20

• Ensures embryo fully within amniotic cavity
• Ectoderm on outside and endoderm on the inside
• Pulls connecting stalk around to the abdomen side

Question 21

Define the pre-embryonic period.

Answer 21

first 2 weeks after fertilisation

Question 22

Define the embryonic period.

Answer 22

3rd -8th week after fertilisation

Question 23

Define the fetal period.

Answer 23

9th-38t/40th week after fertilisation

Question 24

Discuss the changes taking place as a zygote travels down the fallopian tube.

Answer 24

After fertilisation on the ampulla, zygote moves via cilia.

As the zygote moves it cleaves into 2 blastomeres.

Blastomeres mitotically divide into a morula.

Compaction separates the inner mass into an embryo/trophoblast and makes a blastocoele.

Question 25

Define these terms:

• zygote
• cleavage
• zona pellucida

Answer 25

Zygote: fertilised ovum

Cleavage: the zygote after mitotically diving

Zona pellucida: sperm’s glycoprotein shell that protects the blastomere from other sperm

Question 26

Define these terms:

• morula
• ovary
• fallopian tube
• uterus

Answer 26

Morula: ball of cells formed after the blastomeres compact

Ovary: part of the female’s reproductive organs where the ova is released

Fallopian tube: tube that connects the ovaries to the upper part of the uterus. Egg usually fertilised in the ampulla.

Uterus: Reproductive organ in the lower part of the female pelvis where a conceptus can implant and develop into a fetus.

Question 27

Discuss the blastocyst formation and the initial stages of implantation.

Answer 27

Blastocyst = morula + blastocoele

Inner mass becomes embryoblast.

Outer mass becomes trophoblast.

Blastocyst hatches out of zona pellucida to expand and implant in the uterine epithelium.

Question 28

Define these terms:

• blastocyst
• trophoblast
• embryoblast

Answer 28

Blastocyst: structure after morula has developed a blastocoele

Trophoblast: outer cell mass flattened to form blastocyst wall

Embryoblast: inner mass of cells

Question 29

Define these terms:

• Implantation
• Cytotrophoblast
• Syncytiotrophoblast

Answer 29

Implantation: syncytiotrophoblast adheres + invades uterine epithelium

Cytotrophoblast: inner layer of cells connected to the yolk sac.

Syncytiotrophoblast: outer layer of cells that becomes support tissue.

Question 30

Describe the embryonic/bilaminar disc formation and the initial division within it.

Answer 30

Bilaminar disc made from the embryoblast differentiating into the epi/hypoblast. The epiblast cells form the amniotic cavity whereas the hypoblast forms the primitive yolk sac.

Question 31

Define these terms:

• bilaminar disc
• Germ layer
• Ectoderm
• Mesoderm
• Endoderm

Answer 31

Bilaminar disc: differentiation of the embryoblast and now consists of the epi/hypoblast

Germ layer: made of 3 layers and make up the trilaminar disc.

Ectoderm: apical layer of the trilaminar disc

Mesoderm: middle layer of the trilaminar disc

Endoderm: basal layer of the trilaminar disc

Question 32

Define these terms:

• primitive streak
• Gastrulation
• Trilaminar embryonic disc

Answer 32

Primitive streak: It is a divot in the epiblast disc formed at start of gastrulation on epiblast surface.

Gastrulation: beginning of week 3 where bilaminar disc becomes trilaminar disc due to the epiblast differentiating.

Trilaminar embryonic disc: epiblast differentiation into 3 layer disc (ectoderm, mesoderm, endoderm) that displaces the hypoblast.

Question 33

List the ectodermal derivatives

Answer 33

CNS
Skin
Organs

Question 34

List the mesodermal derivatives

Answer 34

CVS
Muscles
Bone
Cartilage

Question 35

List the endodermal derivatives

Answer 35

Internal structures
Epithelial linings
Gland parenchyma