1. Tutorial for module BY1101: Joe Colgan (tcolgan@tcd.ie)
BY1101 Introduction to Molecular and Cellular Biology
Tutorial for module BY1101:
Embryology
Joe Colgan

2. Tutorial objectives Describe the main events in embryogenesis
BY1101 Introduction to Molecular and Cellular Biology
Tutorial objectives
Describe the main events in embryogenesis
Compare and contrast the events between different organisms

3. Embryology What is embryology?
BY1101 Introduction to Molecular and Cellular Biology
Embryology
What is embryology?
-The science of the development of an organism from the fertilisation of the ovum to the fetus

4. Sexual reproduction What are gametes?
BY1101 Introduction to Molecular and Cellular Biology
Sexual reproduction
What are gametes?
Haploid germ cells (sperm or egg (ovum))
What is a zygote?
Single diploid cell (earliest stage of embryonic development)

5. How do the gametes form? Mitosis: Meiosis:
BY1101 Introduction to Molecular and Cellular Biology
How do the gametes form?
Mitosis:
Process of nuclear division in eukaryotic cells
Conserves chromosome number by allocating replicated chromosomes equally to each of the daughter nuclei
Meiosis:
Modified form of cell division in sexually reproducing organisms
Involves two round of cell division but only one round of DNA replication
Produces cells with half the numbers of chromosome sets as the original cell

6. What differences are there in the gametes?
BY1101 Introduction to Molecular and Cellular Biology
What differences are there in the gametes?
Spermatogenesis
Product:
Sperm
Location:
Testes (Seminiforous tubule)
Specialised function:
Location and penetration
Oogenesis
Product:
Ovum (Egg)
Location:
Ovary
Specialised function:
Nutrition, metabolism and development

7. Female (Ovum) Male (sperm)
BY1101 Introduction to Molecular and Cellular Biology
Stage 1: Gametogenesis
Stage 1:
Gametogenesis
Event
Production of gametes
Female (Ovum) Male (sperm)
Product
Haploid gametes

8. What are the major events in fertilisation?
BY1101 Introduction to Molecular and Cellular Biology
What are the major events in fertilisation?
1. Contact
2. Acrosomal reaction
3. Interaction of gametes
4. Fusion of nuclei
1. Acrosomal reaction
Release of hydrolytic enzymes from the acrosomal vesicle of a sperm cell when the sperm is near or in contact with an egg cell
2. Acrosomal process
Structure of actin filaments at the head of the sperm that binds to receptors of the jelly of the head preventing other sperm from initially binding

9. BY1101 Introduction to Molecular and Cellular Biology
Fertilisation

10. What are the two defences to polyspermy?
BY1101 Introduction to Molecular and Cellular Biology
What are the two defences to polyspermy?
Fast block against polyspermy
Acrosomal reaction -> Depolarisation
Slow block against polyspermy
Cortical reaction -> Fertilization envelope
Why is polyspermy a problem?

11. Fusion of haploid gametes (ovum + sperm) to produce a diploid zygote
BY1101 Introduction to Molecular and Cellular Biology
Stage 2: Fertilisation
Stage 2:
Fertilisation
Event
Fusion of haploid gametes (ovum + sperm) to produce a diploid zygote
Product
A diploid zygote

12. What are the major events in cleavage?
BY1101 Introduction to Molecular and Cellular Biology
What are the major events in cleavage?
Rapid cell division of zygote into blastomeres
5-7 divisions forms a ball of cells: Morula
Fluid filled cavity is the blastocoel
Surrounding hollow ball of cells are the blastula
Blastomeres
Morula
Blastula
Blastocoel
Fertilised zygote
Early
division
Early
cleavage
Late
cleavage

13. What is a major factor that influences cleavage?
BY1101 Introduction to Molecular and Cellular Biology
What is a major factor that influences cleavage?
Yolk proteins – importance constituent of developing embryos
Proportion varies across species
Yolk affects cleavage
Greater aggregation at
vegetal pole
Animal pole
Vegetal pole

14. What are the major events in cleavage formation in the frog?
BY1101 Introduction to Molecular and Cellular Biology
What are the major events in cleavage formation in the frog?
Animal pole
Vegetal pole

15. Is cleavage different in the chick?
BY1101 Introduction to Molecular and Cellular Biology
Is cleavage different in the chick?
All the cytoplasm of the cell is contained in a small disk
Cleavage planes cannot penetrate the yolk
Only cytoplasm at the animal pole is cleaved
Cleavage produces a small cap of cells – blastoderm
Blastoderm is the chick equivalent of blastula

16. What are the two types of cleavage?
BY1101 Introduction to Molecular and Cellular Biology
What are the two types of cleavage?
Holoblastic (Complete)
Meroblastic (Partial)

17. Cleavage (Comparative)
BY1101 Introduction to Molecular and Cellular Biology
Cleavage (Comparative)
Model organism
Sea urchin
Frog
Chick
Product of cleavage
Blastula
Blastula
Blastoderm
Type of cleavage
Holoblastic
Holoblastic
Meroblastic

18. BY1101 Introduction to Molecular and Cellular Biology
Stage 3: Cleavage
Stage 3:
Cleavage
Event
Rapid cell division without cell growth
Product
Blastula (Frog)/Blastoderm (Chick)

19. Next step: Gastrulation
BY1101 Introduction to Molecular and Cellular Biology
Next step: Gastrulation
What is gastrulation?
A series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a 3 layered embryo, the gastrula
Triploblastic – Three layers
Diploblastic – Two layers (Endo- and ectoderms only)

20. What are the three embryonic germ layers?
BY1101 Introduction to Molecular and Cellular Biology
What are the three embryonic germ layers?

21. What are the main events of gastrulation in the sea urchin?
BY1101 Introduction to Molecular and Cellular Biology
What are the main events of gastrulation in the sea urchin?

22. What differences occur in gastrulation of the frog?
BY1101 Introduction to Molecular and Cellular Biology
What differences occur in gastrulation of the frog?
1. Cells begin to invaginate at the dorsal lip of the blastopore
2. Involution – Cells move over the dorsal lip and begin to move into the blastopore
3. Blastocoel collapses and is replaced by the archenteron

23. What differences occur in gastrulation of the chick?
BY1101 Introduction to Molecular and Cellular Biology
What differences occur in gastrulation of the chick?
Two main cell types:
Epiblasts
All cells that will contribute to the formation of the embryo will originate as epiblasts
Hypoblasts
Functional in the normal formation of the primitive streak

24. Gastrulation (Comparative)
BY1101 Introduction to Molecular and Cellular Biology
Gastrulation (Comparative)
Model organism
Sea urchin
Frog
Chick
Site of involution during gastrulation
Blastopore
Blastopore
Primitive streak

25. BY1101 Introduction to Molecular and Cellular Biology
Stage 4: Gastrulation
Stage 4:
Gastrulation
Event
Formation of the embryonic germ layers
Product
Gastrula (containing endoderm, mesoderm, ectoderm)

26. How does it differ to gastrulation?
BY1101 Introduction to Molecular and Cellular Biology
What is organogenesis?
The stage at which the embryonic germ layers localise into the basic structure of organs
How does it differ to gastrulation?

27. Case study: Neurulation
BY1101 Introduction to Molecular and Cellular Biology
Case study: Neurulation
What are the steps involved in neurulation?
First steps in brain and spinal cord
Mesodermal cells form the notochord
Ectodermal cells above form the neural plate
Neural plate folds inwards to form the
neural tube
Neural tube will become the brain and spinal chord in the embryo

28. BY1101 Introduction to Molecular and Cellular Biology
What are the two sets of specialised cells associated with neurulation?
Neural crest cells
Location: Borders from where the notochord pinches off from the ectoderm
Function: Formation of peripheral nerves, teeth and skull bones
Somites
Location: Groups of cells located in strips of mesoderm lateral to the notochord form blocks
Function: Segmentation of the body structure, vertebrae

29. BY1101 Introduction to Molecular and Cellular Biology
What are the four types of extra-embryonic membranes seen in the chick?
Allantois-
Waste disposal sac
Also gas exchange
Amnion-
Fluid filled sac
Bathes the embryo
Form during organogenesis
- Endoderm
- Mesoderm
- Ectoderm
Chorion-
Outer membrane
Gas exchange
Yolk sac-
Encloses the yolk
Blood vessels develop to bring nutrients

30. Formation of organs from the 3 germ layers formed during gastrulation
BY1101 Introduction to Molecular and Cellular Biology
Stage 5: Organogenesis
Stage 5:
Organogenesis
Event
Formation of organs from the 3 germ layers formed during gastrulation
Product
Internal organs

31. BY1101 Introduction to Molecular and Cellular Biology
Summary

32. Exam Questions BY1101 Introduction to Molecular and Cellular Biology
Notochord
Neural plate
Neural folds
Neural tube
Neural crest cells

33. Exam Questions BY1101 Introduction to Molecular and Cellular Biology
Gametogenesis
Fertilisation
Cleavage
Gastrulation
Organogenesis

34. Exam Questions BY1101 Introduction to Molecular and Cellular Biology
Amnion
Chorion
Yolk sac
Yolk sac
Allantois

35. Exam Questions BY1101 Introduction to Molecular and Cellular Biology
2010/2011
2008/2009

36. Essay practice BY1101 Introduction to Molecular and Cellular Biology
Q. 5 What is organogenesis? Describe the importance of neurulation in establishing the body plan of the embryo.
Write an introductory paragraph to the essay above.
- Introduction should address the question being asked
- Should be concise (max. 1 paragraph)
Using bullet points, outline the main concepts you would have in your main text body to support your answer
- Including labelled drawings to support your answer are recommended
Write a concluding paragraph
- Should readdress the question
- Summarise the main points from your essay
- Be concise (max. 1 paragraph)
- Should not include any new information

37. Developmental genetics
BY1101 Introduction to Molecular and Cellular Biology
Next week:
Developmental genetics
Further reading:
Campbell Chapter 47

38. Questions Further reading: Campbell Chapter 47
BY1101 Introduction to Molecular and Cellular Biology
Questions
Further reading:
Campbell Chapter 47