BY1101 Introduction to Molecular and Cellular Biology
Tutorial for module BY1101:
Embryology
Joe Colgan (tcolgan@tcd.ie)
BY1101 Introduction to Molecular and Cellular Biology
Tutorial objectives
• Describe the main events in embryogenesis
• Compare and contrast the events between
different organisms
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
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)
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
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
BY1101 Introduction to Molecular and Cellular Biology
Stage 1: Gametogenesis
Stage 1:
Gametogenesis
Event
Production of gametes
Female (Ovum) Male (sperm)
Product
Haploid gametes
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
BY1101 Introduction to Molecular and Cellular Biology
Fertilisation
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?
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
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
Fertilised
zygote
Blastomeres
Morula
Early
division
Early
cleavage
Blastula
Blastocoel
Late
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
Animal pole
• Greater aggregation at
vegetal pole
Vegetal pole
BY1101 Introduction to Molecular and Cellular Biology
What are the major events in cleavage
formation in the frog?
Animal pole
Vegetal pole
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
BY1101 Introduction to Molecular and Cellular Biology
What are the two types of cleavage?
• Holoblastic (Complete)
• Meroblastic (Partial)
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
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)
BY1101 Introduction to Molecular and Cellular Biology
Next step: Gastrulation
What is gastrulation?
• A series of cell and tissue movements in which the blastulastage embryo folds inward, producing a 3 layered embryo, the
gastrula
Triploblastic – Three layers
Diploblastic – Two layers (Endo- and ectoderms only)
BY1101 Introduction to Molecular and Cellular Biology
What are the three embryonic germ layers?
BY1101 Introduction to Molecular and Cellular Biology
What are the main events of gastrulation in the sea
urchin?
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
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
BY1101 Introduction to Molecular and Cellular Biology
Gastrulation (Comparative)
Model organism
Sea urchin
Frog
Chick
Site of involution
during gastrulation
Blastopore
Blastopore
Primitive streak
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)
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?
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
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
BY1101 Introduction to Molecular and Cellular Biology
What are the four types of extra-embryonic
membranes seen in the chick?
Amnion•Fluid filled sac
•Bathes the embryo
Allantois• Waste disposal sac
• Also gas exchange
Form during
organogenesis
- Endoderm
- Mesoderm
- Ectoderm
Chorion• Outer membrane
• Gas exchange
Yolk sac• Encloses the yolk
• Blood vessels develop to
bring nutrients
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
BY1101 Introduction to Molecular and Cellular Biology
Summary
BY1101 Introduction to Molecular and Cellular Biology
Exam Questions
Notochord
Neural plate
Neural folds
Neural tube
Neural crest cells
BY1101 Introduction to Molecular and Cellular Biology
Exam Questions
Gametogenesis
Fertilisation
Cleavage
Gastrulation
Organogenesis
BY1101 Introduction to Molecular and Cellular Biology
Exam Questions
Amnion
Chorion
Yolk sac
Yolk sac
Allantois
BY1101 Introduction to Molecular and Cellular Biology
Exam Questions
2010/2011
2008/2009
BY1101 Introduction to Molecular and Cellular Biology
Essay practice
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
BY1101 Introduction to Molecular and Cellular Biology
Next week:
Developmental genetics
Further reading:
Campbell Chapter 47
BY1101 Introduction to Molecular and Cellular Biology
Questions
Further reading:
Campbell Chapter 47