Today: Group Quiz 6 Group Quiz 6 Intro to Development Intro to Development Thursday: Review Paper Draft Due for Peer Review!

Today:Today: Group Quiz 6Group Quiz 6 Intro to Intro to

DevelopmentDevelopment

Thursday: Review Paper Thursday: Review Paper Draft Due for Peer Review!Draft Due for Peer Review!

Backtracking: Advantages Backtracking: Advantages and Disadvantages of Sexual and Disadvantages of Sexual

Reproduction?Reproduction?

Generalizations about Sexual Generalizations about Sexual ReproductionReproduction

Offspring created by the Offspring created by the fusion of haploid fusion of haploid gametesgametes to form a diploid to form a diploid zygote.zygote.

The female gamete (The female gamete (ovumovum) is ) is generally large and generally large and

nonmotile.nonmotile.

The male gamete The male gamete ((spermatozoanspermatozoan) is ) is

generally smaller and generally smaller and motile.motile.

Some animals are Some animals are HermaphroditesHermaphrodites (single (single individual has both male and female individual has both male and female

reproductive systems)reproductive systems)

Common in sessile or Common in sessile or burrowing burrowing animals, and in parasitesanimals, and in parasites

Generalizations about Sexual Generalizations about Sexual ReproductionReproduction

Advantages? Does this generate genetic

diversity?

Fertilization OverviewFertilization Overview

Fertilization: the union of sperm and eggFertilization: the union of sperm and egg

Can be Can be EXTERNALEXTERNAL or or INTERNALINTERNAL

External FertilizationExternal Fertilization

Photo by Scott Egan,; University of Rhode Island Dept. of Natural Resources; A pair of wood frogs, Rana sylvatica, in amplexus

Internal fertilization requires Internal fertilization requires cooperative cooperative behaviorbehavior

andand

Sophisticated Sophisticated reproductive systemsreproductive systems including organs to deliver sperm and including organs to deliver sperm and

receptacles for its storage and transport to receptacles for its storage and transport to eggs.eggs.

Internal FertilizationInternal Fertilization

Mammalian Mammalian Gamete Gamete

Production: Production: FemalesFemales

Mammalian Mammalian Gamete Gamete

Production: Production: Males Males

Mammalian Gamete Production: Mammalian Gamete Production: MalesMales

Aside: FertilizationAside: Fertilization

Preformation vs EpigenesisPreformation vs Epigenesis

FertilizationFertilization Combines haploid Combines haploid

sets of sets of chromosomes to chromosomes to form single diploid form single diploid zygotezygote

Activation of the Activation of the eggegg

Model Organism = Model Organism = Sea UrchinSea Urchin

Why Urchins?

?

Studying Sea Urchin Studying Sea Urchin FertilizationFertilization

Source: NASA- Photo credit - Joseph Tash

External Fertilization in Sea UrchinsExternal Fertilization in Sea Urchins Sperm exposed to molecules from the jelly Sperm exposed to molecules from the jelly

coat surrounding an egg, the acrosome coat surrounding an egg, the acrosome discharges its contents by Exocytosisdischarges its contents by Exocytosis

= Acrosomal Reaction= Acrosomal Reaction The acrosomal process (elongating The acrosomal process (elongating

structure) then penetrates the jelly coat of structure) then penetrates the jelly coat of the egg and adheres to a specific receptor the egg and adheres to a specific receptor moleculemolecule

Fertilization: The Acrosomal Fertilization: The Acrosomal ReactionReaction

Fertilization: The Acrosomal Fertilization: The Acrosomal ReactionReaction

What needs to happen once one

sperm nucleus has entered the

egg?!

Fusion of the sperm and egg Fusion of the sperm and egg membrane causes membrane causes ion channelsion channels in in the egg’s membrane to openthe egg’s membrane to open

Sodium ions flow into the cell, Sodium ions flow into the cell, causing a causing a membrane membrane depolarizationdepolarization (change in (change in membrane potential)membrane potential)

Prevents more than 1 sperm from Prevents more than 1 sperm from entering (1-3 seconds!)entering (1-3 seconds!)

Fertilization: Fast Block to PolyspermyFertilization: Fast Block to Polyspermy

Membrane potential is restored within 2 minutes! (Uh oh…)

The Cortical The Cortical Reaction: Reaction:

Fusion of sperm and Fusion of sperm and egg triggers release egg triggers release of of calciumcalcium from the from the ER into the cytosolER into the cytosol

Calcium release Calcium release begins at site of begins at site of sperm entry and sperm entry and propagates across propagates across the fertilized eggthe fertilized egg

Fertilization: Slow Block to Fertilization: Slow Block to PolyspermyPolyspermy


The Cortical The Cortical Reaction:Reaction:

High calcium High calcium concentration causes concentration causes the the cortical cortical granulesgranules to fuse to fuse with plasma with plasma membranemembrane

Mouse oocyte stained to show cortical granules (small red dots). Genomic DNA in the metaphase plate of the secondary oocyte (top right) appears bluish-white.(From Biology of Reproduction 57:743-750, 1997, Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)


Enzymes from the Enzymes from the cortical granulescortical granules separate the separate the vitelline layervitelline layer from from the plasma membranethe plasma membrane

Water is drawn into the Water is drawn into the perivitelline perivitelline spacespace by osmosis, swelling it by osmosis, swelling it

The swelling pushes the vitelline The swelling pushes the vitelline layer away from plasma membrane layer away from plasma membrane where it is hardened by enzymes to where it is hardened by enzymes to form the form the Fertilization EnvelopeFertilization Envelope



Rise in Calcium also induces Rise in Calcium also induces metabolic changesmetabolic changes within the egg within the egg

Metabolism increases rapidly!Metabolism increases rapidly! Nucleus of sperm starts to swellNucleus of sperm starts to swell Sperm nucleus merges with egg Sperm nucleus merges with egg

nucleus (@ ~ 20 minutes)nucleus (@ ~ 20 minutes) First division occurs @ ~90 minutesFirst division occurs @ ~90 minutes

Fertilization: ActivationFertilization: Activation

Timing:Timing:

Sea Urchin Larva at 13

days

Fertilization: Sea Urchins Vs. Fertilization: Sea Urchins Vs. MammalsMammals

Internal Fertilization (terrestrial!)Internal Fertilization (terrestrial!)

Key Initial Differences:Key Initial Differences:

1. 1. CapacitationCapacitation: molecules in mammalian : molecules in mammalian female reproductive tract alter surface of female reproductive tract alter surface of sperm and increase the motility (~ 6 hrs)sperm and increase the motility (~ 6 hrs)

2. Mammalian egg 2. Mammalian egg cloaked by follicle cloaked by follicle cells-cells- capacitated sperm cell must capacitated sperm cell must migrate through this layer to reach the migrate through this layer to reach the zona pellucidazona pellucida


The Zona PellucidaThe Zona Pellucida = filamentous network = filamentous network of glycoproteins (extracellular matrix)of glycoproteins (extracellular matrix)

One of the glycoproteins, ZP3 functions One of the glycoproteins, ZP3 functions as a sperm receptoras a sperm receptor

Binding induces the acrosome to release Binding induces the acrosome to release its contents (as with sea urchins)its contents (as with sea urchins)

Enzymes from acrosome help sperm Enzymes from acrosome help sperm penetrate the zona pellucidapenetrate the zona pellucida


As with sea urchin, As with sea urchin, binding of egg binding of egg triggers triggers depolarizationdepolarization of egg of egg membrane (fast block to membrane (fast block to polyspermy)polyspermy)

A A Cortical ReactionCortical Reaction functions as the slow functions as the slow block (granules in cortex block (granules in cortex release contents; release contents; enzymes catalyze enzymes catalyze changes in the zona changes in the zona pellucida)pellucida)



Unfertilized Star Fish EggsUnfertilized Star Fish Eggs

Fertilization Reviewed

Fertilized egg (zygote) Fertilized egg (zygote) - the nucleus is no longer visible; the fertilization - the nucleus is no longer visible; the fertilization envelope is presentenvelope is present

Let’s Watch: Let’s Watch: http://www.youtube.com/watch_popup?v=jp-RgIRgcYE#t=000http://www.youtube.com/watch_popup?v=jp-RgIRgcYE#t=000

Stages of DevelopmentStages of Development

Fertilization is followed by 3 Fertilization is followed by 3 stages:stages:

1. Cleavage1. Cleavage

2. Gastrulation2. Gastrulation

3. Organogenesis3. Organogenesis

1.1. CLEAVAGECLEAVAGE

Succession of rapid cell divisionsSuccession of rapid cell divisions Partitions the cytoplasm into smaller Partitions the cytoplasm into smaller

cells, cells, BLASTOMERESBLASTOMERES, each with its own , each with its own nucleusnucleus

Each region of cytoplasm contains Each region of cytoplasm contains different cytoplasmic componentsdifferent cytoplasmic components

2-cell and 4-cell stage of Sea Urchin Cleavage (Blastomeres)

What happened to the size of each blastomere??

Polarity of Zygote and EggPolarity of Zygote and Egg

Most animals (except mammals) Most animals (except mammals) produce asymmetrical eggs and produce asymmetrical eggs and zygoteszygotes

Distribution of yolk, mRNA and Distribution of yolk, mRNA and proteins is proteins is not not uniformuniform

Sets the stage for subsequent

developmental events!

Yolk is Yolk is concentrated concentrated

at the at the vegetal vegetal

polepole..

The opposite The opposite pole is the pole is the

animal poleanimal pole

(Becomes the dorsal

side)

Polarity of Frog EggsPolarity of Frog Eggs

In the final stage, a In the final stage, a frog oocyte is frog oocyte is

pigmented dark pigmented dark brown in one brown in one hemisphere hemisphere

((animal poleanimal pole). ). The other hemisphere The other hemisphere

((vegetal polevegetal pole) ) shows the yellow shows the yellow color of the egg color of the egg

yolk.yolk.

CleavageCleavage

The first two cleavage divisions are The first two cleavage divisions are vertical producing four cells vertical producing four cells

extending from animal to vegetal extending from animal to vegetal pole.pole.

The third division is horizontal, The third division is horizontal, producing eight cells.producing eight cells.

Continued divisions (16-64 cells) Continued divisions (16-64 cells) produce a solid ball of cells, the produce a solid ball of cells, the

morula.morula.

A fluid-filled cavity, the blastocoel forms within the morula, creating a hollow ball of cells, the blastula.

Note the difference the presence of yolk makes!

Sea Urchin Sea Urchin MorulasMorulas - 8- - 8-16 cell stage 16 cell stage

Sea Urchin Sea Urchin BlastulaBlastula - 32-cell stage, - 32-cell stage, blastocoel blastocoel

Sea Urchin: Late Blastula - shows thickened Sea Urchin: Late Blastula - shows thickened layer of cells at one end, the vegetal pole layer of cells at one end, the vegetal pole

Morula (16-64 cells) Morula (16-64 cells) Blastula (>128 Blastula (>128 cells)cells)

A fluid-filled cavity, the blastocoel, forms A fluid-filled cavity, the blastocoel, forms within the morula creating a hollow ballwithin the morula creating a hollow ball

Source: courtesy of Dr. J. Hardin, Univ. of

Wisconsin

Blastula Blastula Gastrula Gastrula

Changes in cell motility, shape Changes in cell motility, shape and adhesion result in the spatial and adhesion result in the spatial rearrangement of an embryorearrangement of an embryo

Results in three familiar Results in three familiar embryonic tissue layers: embryonic tissue layers:

ectoderm, endoderm and ectoderm, endoderm and mesodermmesoderm

2. 2. GastrulationGastrulation

Source: raven.zoology.washington.edu/ embryos

Sea Urchin: Early Gastrula - blastocoel, Sea Urchin: Early Gastrula - blastocoel, blastopore, primitive gut (archenteron)blastopore, primitive gut (archenteron)

Sea Urchin: Late Gastrula - endoderm, Sea Urchin: Late Gastrula - endoderm, ectoderm, mesoderm ectoderm, mesoderm

Blastula Blastula

Gastrula Gastrula in a in a FrogFrog

Frog DevelopmentFrog Development

Great model organisms:Great model organisms:

HUGE eggs! (> 1mm)HUGE eggs! (> 1mm) External fertilization and developmentExternal fertilization and development Vertebrates!Vertebrates! Fertilization Fertilization Neuralation in 18 hours Neuralation in 18 hours

under good conditionsunder good conditions

anatomy.med.unsw.edu

Zooming in on the Frog Zooming in on the Frog GastrulaGastrula

How do we How do we know??know??

Stages of DevelopmentStages of Development

Fertilization is followed by 3 Fertilization is followed by 3 stages:stages:

1. Cleavage1. Cleavage

2. Gastrulation2. Gastrulation



Morphogenetic changes – folds, splits Morphogenetic changes – folds, splits and clusterings (condensation) begin and clusterings (condensation) begin

the process of organ buildingthe process of organ building

In chordates, the neural tube and In chordates, the neural tube and notocord form firstnotocord form first

Sea Urchin: the bipinnaria larva (bilateral Sea Urchin: the bipinnaria larva (bilateral symmetry) develops into a brachiolaria larva; this symmetry) develops into a brachiolaria larva; this larva undergoes metamorphosis to become the larva undergoes metamorphosis to become the

adult starfish, which has radial symmetry. adult starfish, which has radial symmetry.

Earliest Organogenesis in Frog Earliest Organogenesis in Frog (Chordate) Embryos(Chordate) Embryos

Let’s Watch –http://www.youtube.com/watch_popup?v=-KC1CA_Os1g#t=000

Formation of Formation of the Neural the Neural Tube in a Tube in a ChordateChordate

Species: Mouse Day Gestation: 8 Species: Mouse Day Gestation: 8 www.med.unc.eduwww.med.unc.edu

Documents

Today: Group Quiz 6 Group Quiz 6 Intro to Development Intro to Development Thursday: Review Paper Draft Due for Peer Review!