lecture 14 - embryo development

Question 1

What is preformationism

Answer 1

Sperm or egg contains miniature preformed organism

Question 2

What is Epigenesis

Answer 2

Embryo develops progressively from undifferentiated egg

Question 3

What are the stages of embryo development

Answer 3

oocyte/sperm maturation
Ovulation/ejaculation
Fertilisation
Implantation
Embryo development
Foetal development
Parturition (birth)
Human gestation

Question 4

Describe early embryo development/cleavage

Answer 4

Week 1
- Fertilised oocyte, cleaved embryo, fragmented cell, blastocyst

Question 5

Describe how humans hatch too

Answer 5

Day 6-7
- Expanded blastocyst (trophectoderm) into zona pellucida

Question 6

Describe implantation

Answer 6

Week 2
- Blastocyst adhere to uterus wall
- bilaminar germinal disc (hypoblast, epiblast)

Question 7

Describe granulation

Answer 7

Week 3
- Embryo change from blastula to a gastrula (multiple layers in cells)
- Inner cell mass forms two germ layers - Hypoblast (primitive endoderm) - Epiblast (embryonic cells)
- Epiblast form three germ layers - ectoderm (outer), mesoderm (middle), endoderm (inner)
- Cells proliferate, differentiate and move

Question 8

Describe the germ layers

Answer 8

Endoderm - internal organs (internal)
Mesoderm - connective tissue (middle)
Ectoderm - skin, nerve cells (external)

Question 9

Describe gastrulation

Answer 9

Week 3
- Formation of primitive streak/groove in epiblast
- Pass through primitive streak and new layers beneath epiblast
- New cell lineages

Question 10

What forms in week 3

Answer 10

Neural groove -> neural tube (brain and spiral cord)
Somites (balls of mesoderm) -> connective tissue
Notochord - midline structure (not present in adults) - patterning signals by secreting sonic hedgehog

Question 11

What happens at week 4

Answer 11

Begin formation of
- Gut tube
- Liver
- Genital ridge
- Neural tube, brain vesicles
- Heart begins to beat

Question 12

Describe embryonic period

Answer 12

Week 5-8
- Heart descends into thorax
- Lung - buds form, descend into thorax
- Liver enlarges
- Ears - external form (cochlear otic vesicle)
- Eyes - retinal pigment present, eye & eyelids develop
- Limbs - buds form & elongate, hands/feet, fingers & toes lengthen
- Brain - cerebellum begins forming
- Skeleton - 33-34 cartilage vertebrae present, ossification of limb bones begins

Question 13

Describe foetal period

Answer 13

Weeks 9-38
- Organogenesis largely completed
- Extensive growth
- Ongoing differentiation and development of organ system

Question 14

What is the percentage of survival of a foetus

Answer 14

After 22 weeks - 15% survival
After 28 weeks - 90% survival - 1/3 significant morbidity

Question 15

What can go wrong in an oocyte

Answer 15

Cellular and mononuclear mechanisms required for fertilisation and early embryo development inherited to oocyte (metabolites, organelles and mRNA)
>50% of fertilised eggs lost early in development

Question 16

What happens to an oocyte during fertilisation

Answer 16

-cumulus expansion
- zona pellucida binding
- oolemma fusion
- oocyte activation
- Sperm processing
- Pronuclear formation

Question 17

What happens to sperm during fertilization

Answer 17

• Motility
• Morphology
• Acrosome
• Concentration

Question 18

What happens if things go wrong in the cleavage stage

Answer 18

First 2 weeks
- Exposure to teratogens usually cause complete loss of conceptus
- Abnormalities result in fertilisation or implantation failure
- No birth defects as no organs/structures developed yet
- Early embryo may compensate for damage
- Subtle effects on long term health (high BP, insulin resistance/diabetes)

Question 19

What happens if things go wrong in the embryonic period

Answer 19

-Gastrulation, embryo folding, formation of organ & systems
- Active period of development and differentiation
- Most vulnerable to major birth defects

Question 20

What happens if things go wrong in the foetal period

Answer 20

• mainly growth of organs & structures already developed
• Birth defects less sensitive
• Small size, mental retardation, defects in eyes, ears, teeth & external genitalia

Question 21

What are periods of susceptibility

Answer 21

• Different organs susceptible to teratogens at different stages
• Heart forms earliest so sensitive sooner
• Complex organs - brain are susceptible longer