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

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

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

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)

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

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

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

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

BY1101 Introduction to Molecular and Cellular Biology Fertilisation

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?

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

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

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

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

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

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)

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

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)

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)

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

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?

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

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

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

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)

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?

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

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? 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

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

BY1101 Introduction to Molecular and Cellular Biology Summary

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

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

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

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

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

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

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