Tutorial for module BY1101: Embryology Joe Colgan ([email protected]) BY1101 Introduction to Molecular and Cellular Biology

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


EmbryologyWhat is embryology?

-The science of the development of an organism from the fertilisation of the ovum to the fetus


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)


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?


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


What are the major events in fertilisation?


1. Contact

2. Acrosomal reaction

3. Interaction of gametes

4. Fusion of nuclei

FertilisationBY1101 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?

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


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


Animal pole

Vegetal pole

What are the major events in cleavage formation in the frog?


Animal pole

Vegetal pole

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

• Holoblastic (Complete)

• Meroblastic (Partial)

What are the two types of cleavage?


Cleavage (Comparative)


Blastula Blastula Blastoderm

Holoblastic Holoblastic Meroblastic

Product of cleavage

Type of cleavage

Sea urchin Frog ChickModel organism


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


Triploblastic – Three layersDiploblastic – Two layers (Endo- and ectoderms only)

What is gastrulation?

What are the three embryonic germ layers?


What are the main events of gastrulation in the sea urchin?



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


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)


Blastopore Blastopore Primitive streakSite of involution during gastrulation

Sea urchin Frog ChickModel organism


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


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?


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


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


Organogenesis

Formation of organs from the 3 germ layers formed during gastrulation

Internal organs

Stage 5:

Event

Product

Stage 5: Organogenesis


Summary

Exam QuestionsBY1101 Introduction to Molecular and Cellular Biology

Notochord

Neural plate

Neural folds

Neural tube

Neural crest cells


Gametogenesis

Fertilisation

Cleavage

Gastrulation

Organogenesis


Amnion

Chorion

Yolk sac

Yolk sac

Allantois


2010/2011

2008/2009

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


Next week: Developmental genetics

Further reading: Campbell Chapter 47


Questions

Further reading: Campbell Chapter 47

Documents

Tutorial for module BY1101: Embryology Joe Colgan ([email protected]) BY1101 Introduction to Molecular and Cellular Biology