Today:

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Today

• Group Quiz 6
• Intro to Development

Thursday Review Paper Draft Due for Peer Review!
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Backtracking Advantages and Disadvantages of
Sexual Reproduction?
?
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Generalizations about Sexual Reproduction

• Offspring created by the fusion of haploid
  gametes to form a diploid zygote.
• The female gamete (ovum) is generally large and
  nonmotile.
• The male gamete (spermatozoan) is generally
  smaller and motile.

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Generalizations about Sexual Reproduction

• Some animals are Hermaphrodites (single
  individual has both male and female reproductive
  systems)
• Common in sessile or burrowing animals, and
  in parasites

Advantages? Does this generate genetic diversity?
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Fertilization Overview

• Fertilization the union of sperm and egg
• Can be EXTERNAL or INTERNAL

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

• Internal fertilization requires cooperative
  behavior
• and
• Sophisticated reproductive systems including
  organs to deliver sperm and receptacles for its
  storage and transport to eggs.

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Mammalian Gamete Production Females
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Mammalian Gamete Production Males
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Mammalian Gamete Production Males
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Aside Fertilization

• Preformation vs Epigenesis

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Fertilization

• Combines haploid sets of chromosomes to form
  single diploid zygote
• Activation of the egg
• Model Organism Sea Urchin

Why Urchins??
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Studying Sea Urchin Fertilization
Source NASA- Photo credit - Joseph Tash
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Fertilization The Acrosomal Reaction

• External Fertilization in Sea Urchins
• Sperm exposed to molecules from the jelly coat
  surrounding an egg, the acrosome discharges its
  contents by Exocytosis
• Acrosomal Reaction
• The acrosomal process (elongating structure) then
  penetrates the jelly coat of the egg and adheres
  to a specific receptor molecule

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Fertilization The Acrosomal Reaction
What needs to happen once one sperm nucleus has
entered the egg?!
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Fertilization Fast Block to Polyspermy

• Fusion of the sperm and egg membrane causes ion
  channels in the eggs membrane to open
• Sodium ions flow into the cell, causing a
  membrane depolarization (change in membrane
  potential)
• Prevents more than 1 sperm from entering (1-3
  seconds!)

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Membrane potential is restored within 2 minutes!
(Uh oh)
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Fertilization Slow Block to Polyspermy

• The Cortical Reaction
• Fusion of sperm and egg triggers release of
  calcium from the ER into the cytosol
• Calcium release begins at site of sperm entry and
  propagates across the fertilized egg

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Fertilization Slow Block to Polyspermy

• The Cortical Reaction
• High calcium concentration causes the cortical
  granules to fuse with plasma membrane

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 57743-750, 1997, Z. Xu, A. Abbott, G. Kopf, R. Schultz and T. Ducibella)
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Fertilization Slow Block to Polyspermy
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Fertilization Slow Block to Polyspermy

• Enzymes from the cortical granules separate the
  vitelline layer from the plasma membrane
• Water is drawn into the perivitelline space by
  osmosis, swelling it
• The swelling pushes the vitelline layer away from
  plasma membrane where it is hardened by enzymes
  to form the Fertilization Envelope

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Fertilization Slow Block to Polyspermy
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Fertilization Activation

• Rise in Calcium also induces metabolic changes
  within the egg
• Metabolism increases rapidly!
• Nucleus of sperm starts to swell
• Sperm nucleus merges with egg nucleus (_at_ 20
  minutes)
• First division occurs _at_ 90 minutes

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Timing
Sea Urchin Larva at 13 days
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Fertilization Sea Urchins Vs. Mammals

• Internal Fertilization (terrestrial!)
• Key Initial Differences
• 1. Capacitation molecules in mammalian female
  reproductive tract alter surface of sperm and
  increase the motility ( 6 hrs)
• 2. Mammalian egg cloaked by follicle cells-
  capacitated sperm cell must migrate through this
  layer to reach the zona pellucida

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Fertilization Sea Urchins Vs. Mammals
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Fertilization Sea Urchins Vs. Mammals

• The Zona Pellucida filamentous network of
  glycoproteins (extracellular matrix)
• One of the glycoproteins, ZP3 functions as a
  sperm receptor
• Binding induces the acrosome to release its
  contents (as with sea urchins)
• Enzymes from acrosome help sperm penetrate the
  zona pellucida

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Fertilization Sea Urchins Vs. Mammals

• As with sea urchin, binding of egg triggers
  depolarization of egg membrane (fast block to
  polyspermy)
• A Cortical Reaction functions as the slow block
  (granules in cortex release contents enzymes
  catalyze changes in the zona pellucida)

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Fertilization Sea Urchins Vs. Mammals
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Fertilization Reviewed

• Unfertilized Star Fish Eggs

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• Fertilized egg (zygote) - the nucleus is no
  longer visible the fertilization envelope is
  present
• Lets Watch http//www.youtube.com/watch_popup?v
  jp-RgIRgcYEt000

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Stages of Development

• Fertilization is followed by 3 stages
• 1. Cleavage
• 2. Gastrulation
• 3. Organogenesis

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CLEAVAGE

• Succession of rapid cell divisions
• Partitions the cytoplasm into smaller cells,
  BLASTOMERES, each with its own nucleus
• Each region of cytoplasm contains different
  cytoplasmic components

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What happened to the size of each blastomere??
2-cell and 4-cell stage of Sea Urchin Cleavage
(Blastomeres)
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Polarity of Zygote and Egg

• Most animals (except mammals) produce
  asymmetrical eggs and zygotes
• Distribution of yolk, mRNA and proteins is not
  uniform

Sets the stage for subsequent developmental
events!
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• Yolk is concentrated at the vegetal pole.
• The opposite pole is the animal pole

(Becomes the dorsal side)
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Polarity of Frog Eggs

• In the final stage, a frog oocyte is pigmented
  dark brown in one hemisphere (animal pole).
• The other hemisphere (vegetal pole) shows the
  yellow color of the egg yolk.

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Cleavage

• The first two cleavage divisions are vertical
  producing four cells extending from animal to
  vegetal pole.
• The third division is horizontal, producing eight
  cells.
• Continued divisions (16-64 cells) produce a solid
  ball of cells, the morula.

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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!
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• Sea Urchin Morulas - 8-16 cell stage

Sea Urchin Blastula - 32-cell stage, blastocoel
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• Sea Urchin Late Blastula - shows thickened layer
  of cells at one end, the vegetal pole

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Morula (16-64 cells) ? Blastula (gt128 cells)

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

Source courtesy of Dr. J. Hardin, Univ. of
Wisconsin
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Blastula ? Gastrula

• Changes in cell motility, shape and adhesion
  result in the spatial rearrangement of an embryo
• Results in three familiar embryonic tissue
  layers
• ectoderm, endoderm and mesoderm

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2. Gastrulation
Source raven.zoology.washington.edu/ embryos
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• Sea Urchin Early Gastrula - blastocoel,
  blastopore, primitive gut (archenteron)

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• Sea Urchin Late Gastrula - endoderm, ectoderm,
  mesoderm

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Blastula ? Gastrula in a Frog
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Frog Development

• Great model organisms
• HUGE eggs! (gt 1mm)
• External fertilization and development
• Vertebrates!
• Fertilization ? Neuralation in 18 hours under
  good conditions

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anatomy.med.unsw.edu
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Zooming in on the Frog Gastrula
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(No Transcript)
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How do we know??
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(No Transcript)
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Stages of Development

• Fertilization is followed by 3 stages
• 1. Cleavage
• 2. Gastrulation
• 3. Organogenesis

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3. Organogenesis

• Morphogenetic changes folds, splits and
  clusterings (condensation) begin the process of
  organ building
• In chordates, the neural tube and notocord form
  first

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• Sea Urchin the bipinnaria larva (bilateral
  symmetry) develops into a brachiolaria larva
  this larva undergoes metamorphosis to become the
  adult starfish, which has radial symmetry.

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Earliest Organogenesis in Frog (Chordate) Embryos
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Lets Watch http//www.youtube.com/watch_popup?v
-KC1CA_Os1gt000
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Formation of the Neural Tube in a Chordate

• Species Mouse Day Gestation 8 www.med.unc.edu