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Tutorial for module BY1101:
EmbryologyJoe Colgan ([email protected])
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
EmbryologyWhat 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 reproductionBY1101 Introduction to Molecular and Cellular Biology
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
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
How do the gametes form?
What differences are there in the gametes?
BY1101 Introduction to Molecular and Cellular Biology
SpermatogenesisProduct:SpermLocation: Testes (Seminiforous tubule)Specialised function:Location and penetration
OogenesisProduct:Ovum (Egg)Location:OvarySpecialised function:Nutrition, metabolism and development
Stage 1: Gametogenesis
Gametogenesis
Production of gametesFemale (Ovum) Male (sperm)
Haploid gametes
Stage 1:
Event
Product
BY1101 Introduction to Molecular and Cellular Biology
What are the major events in fertilisation?
BY1101 Introduction to Molecular and Cellular Biology
1. Contact
2. Acrosomal reaction
3. Interaction of gametes
4. Fusion of nuclei
What are the two defences to polyspermy?• Fast block against polyspermy• Acrosomal reaction -> Depolarisation
• Slow block against polyspermy• Cortical reaction -> Fertilization envelope
BY1101 Introduction to Molecular and Cellular Biology
Why is polyspermy a problem?
Stage 2: FertilisationBY1101 Introduction to Molecular and Cellular Biology
Fertilisation
Fusion of haploid gametes (ovum + sperm) to produce a diploid zygote
A diploid zygote
Stage 2:
Event
Product
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
BY1101 Introduction to Molecular and Cellular Biology
Morula Blastula Blastocoel
Fertilised zygote
Early division
Early cleavage
Latecleavage
Blastomeres
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
BY1101 Introduction to Molecular and Cellular Biology
Animal pole
Vegetal pole
What are the major events in cleavage formation in the frog?
BY1101 Introduction to Molecular and Cellular Biology
Animal pole
Vegetal pole
Is cleavage different in the chick?
BY1101 Introduction to Molecular and Cellular Biology
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
• Holoblastic (Complete)
• Meroblastic (Partial)
What are the two types of cleavage?
BY1101 Introduction to Molecular and Cellular Biology
Cleavage (Comparative)
BY1101 Introduction to Molecular and Cellular Biology
Blastula Blastula Blastoderm
Holoblastic Holoblastic Meroblastic
Product of cleavage
Type of cleavage
Sea urchin Frog ChickModel organism
BY1101 Introduction to Molecular and Cellular Biology
Cleavage
Rapid cell division without cell growth
Blastula (Frog)/Blastoderm (Chick)
Stage 3:
Event
Product
Stage 3: Cleavage
Next step: Gastrulation
• A series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a 3 layered embryo, the gastrula
BY1101 Introduction to Molecular and Cellular Biology
Triploblastic – Three layersDiploblastic – Two layers (Endo- and ectoderms only)
What is gastrulation?
What are the main events of gastrulation in the sea urchin?
BY1101 Introduction to Molecular and Cellular Biology
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
Two main cell types:
EpiblastsAll cells that will contribute to the formation of the embryo will originate as epiblasts
HypoblastsFunctional in the normal formation of the primitive streak
What differences occur in gastrulation of the chick?
Gastrulation (Comparative)
BY1101 Introduction to Molecular and Cellular Biology
Blastopore Blastopore Primitive streakSite of involution during gastrulation
Sea urchin Frog ChickModel organism
BY1101 Introduction to Molecular and Cellular Biology
Gastrulation
Formation of the embryonic germ layers
Gastrula (containing endoderm, mesoderm, ectoderm)
Stage 4:
Event
Product
Stage 4: Gastrulation
• The stage at which the embryonic germ layers localise into the basic structure of organs
BY1101 Introduction to Molecular and Cellular Biology
What is organogenesis?
How does it differ to gastrulation?
Case study: NeurulationBY1101 Introduction to Molecular and Cellular Biology
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
What are the steps involved in neurulation?
BY1101 Introduction to Molecular and Cellular Biology
SomitesLocation: Groups of cells located in strips of mesoderm lateral to the notochord form blocksFunction: Segmentation of the body structure, vertebrae
What are the two sets of specialised cells associated with neurulation?
Neural crest cellsLocation: Borders from where the notochord pinches off from the ectodermFunction: Formation of peripheral nerves, teeth and skull bones
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
Chorion-• Outer membrane• Gas exchange
Yolk sac-• Encloses the yolk• Blood vessels develop to bring nutrients
Allantois-• Waste disposal sac• Also gas exchange
Form during organogenesis- Endoderm- Mesoderm- Ectoderm
BY1101 Introduction to Molecular and Cellular Biology
Organogenesis
Formation of organs from the 3 germ layers formed during gastrulation
Internal organs
Stage 5:
Event
Product
Stage 5: Organogenesis
Exam QuestionsBY1101 Introduction to Molecular and Cellular Biology
Notochord
Neural plate
Neural folds
Neural tube
Neural crest cells
Exam QuestionsBY1101 Introduction to Molecular and Cellular Biology
Gametogenesis
Fertilisation
Cleavage
Gastrulation
Organogenesis
Exam QuestionsBY1101 Introduction to Molecular and Cellular Biology
Amnion
Chorion
Yolk sac
Yolk sac
Allantois
Essay practiceBY1101 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
BY1101 Introduction to Molecular and Cellular Biology
Next week: Developmental genetics
Further reading: Campbell Chapter 47