Human Development Chap 4

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  1. Structure of Gametes
    Image Upload 1First discovered by Leeuwenhoek and Hartsoeker (1678)

    Believed to be parasitic animals living with in the semen (spermatozoa)

    • Sperms are formed in testes by spermatogenesis
    •   - Maturation in Epididymis
    •   - Capacitation (able to fertilize) in female reproductive tract

    • Each sperm consists of:
    •   - Haploid nucleus
    •   - Propulsion system
    •   - Organelles (mitochondria and centrioles) 
    • - Acrosome

    • Three Region
    • - Head
    • - Mid Piece
    • - Tail

    • Centrioles: help to pull the sperm apart when the head hits the egg
    • Nucleus: haploid (23 chromosomes)
    • Mitochondria: helps the sperm to move.
  2. Propulsion of Sperm
    • Vary in different species
    • -In parasitic round worms, the sperm travels by amoeboid motion
    • -In most species the sperm travels by whipping its flagellum
    • -Fertilize external and development  external ex: Sea Urchin
    • - Fertilize internal and development intenal ex Humans

    • Force of sperm propulsion is provided by a protein ‘dynein’
    •   A Protein attached to microtubules
    •   Dynein hydrolyzes ATP to release energy
  3. Egg of Sea Urchin
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    • Consists of:
    •   Jelly layer - glycoprotein

      Vitelline envelope – fibrous mat of glycoproteins (sperm-egg recognition)

      Plasma membrane – regulates flow of ions

    •   Cortex – contains
    •   Actin filaments (forms   fertilization cone)

    • Cytoplasmic contents:
    • uProteins
    • uRibosome, tRNA, mRNA
    • uMorphogenetic factors
    • uChemical factors  
    •   Cortical granules   (prevents polyspermy)
  4. Fertilization
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    • Occurs in 4 stages:
    •   a. Contact and recognition between sperm and egg
    •   b. Regulation of sperm entry into egg
    •   c. Fusion of genetic material of sperm and egg 
    • d. Activation of egg metabolism to start development
  5. Contact and Recognition of Sperm and Egg
    “Chemo-attraction” - Sperm follows the chemical gradient secreted by egg. In sea urchin a 14 amino acid peptide called ‘resact’   Resact (sperm-activating peptide) à activates --> mitochondria of the sperm
  6. Acrosomal Reaction
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    • Divided into two steps:
    • 1.Fusion of Acrosomal Vesicle with sperm plasma membrane.
    • -Extension of acrosomal process
    • -Fucose-containing polysaccharide in egg jelly binds to sperm and initiates exocytosis
    • -Mediated by Ca++
    • 2.Exocytosis of sperm acrosomal vesicle releases proteolytic enzymes that can pave path in the jelly coat.
  7. Regulation of Sperm Entry into Egg
    • 1. Binding of sperm to extracellular envelope of the egg:
    • -Once the sperm penetrates the egg jelly, the acrosome contacts the surface of the egg
    • -This process is mediated by the protein ‘bindin’
    • -There are receptors for bindin on the surface of egg vitelline envelope or the plasma membrane

    • 2. Passing of the sperm through the extracellular envelope
    •   - digestion of the jelly by proteolytic enzymes
    •   - makes a hole for the passage of sperm.
  8. Acrosomal Reaction in Mammals
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    • In mammalian egg, instead of jelly coat, there are
    • -Cumulus layer – extracellular coat
    • -Zona pellucida – extracellular matrix

    • Sperm binds to Zona pellucida – initiated by zona binding protein, ZP3.
    •   - ZP3 binds to intact sperm head à triggers acrosomal reaction occurs

    In sea urchin egg the vitelline membrane is very thin and porous – once the sperm is bound, it is very close to egg plasma membrane

    In mammals the Zona pellucida is think, and the sperm is far from egg’s plasma membrane
  9. Fusion of genetic material of sperm and egg
    • Fertilization cone:
    • Sperm-egg binding --> extension of several microvilli --> fertilization cone
    • Plasma membranes of sperm and egg join together – mediated by fusogenic protein (bindin in sea urchin and CD9 protein in mammals)
    • Sperm nucleus and tail pass through this cytoplasmic bridge

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    • Entrance of multiple sperm into egg results in
    • oTriploid or polyploid nucleus
    • oEach sperm’s centriole divides to form two poles of mitotic apparatus resulting in bipolar mitotic spindle
    • oAbnormal development
  11. Prevention of Polyspermy
    • •Since polyspermy results in abnormal development, it needs to be prevented
    • •Blocking of polyspermy is done in two phases
    • •Fast block – initial quick reaction
    • •Slow block – later long-lasting reaction
  12. Fast block of Polyspermy
    •After the entry of first spermatozoa, the electric potential of plasma membrane of egg changes

    oSea water has higher Na+ compared to cytoplasm of egg

    oResting potential of plasma membrane of the egg is –70 mV

    • oWithin 1-3 seconds after the binding of first sperm, resting potential changes to +20 mV (due to small influx of Na+)
    • Sperm plasma membrane cannot fuse with egg plasma membrane with + 20 mV resting potential (possibly due to voltage sensor proteins

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    • A – Change in membrane potential
    • B – Normal embryonic divisions
    • C – Polyspermy embryonic division
    • D – Conc. of  NaCl and polyspermy
  13. Slow block of Polyspermy
    The gradient in membrane potential is transient (~ 1 min). More permanent blockade of polyspermy is accomplished by slow block (cortical reaction)

    In Sea Urchin – formation of Fertilization envelope

    oSince other sperms are already near the plasma membrane, their removal is accomplished by cortical reaction

    oCortical granules are beneath the plasma membrane of egg

    oUpon the entry of first sperm, the cortical granules fuse with egg plasma membrane and release their contents into the space between egg plasma membrane and vitelline envelope.

    In Mammals – the cortical reaction products modify receptors for ZP3 and ZP2 in the Zona pellucida so that they no longer bind to sperm
  14. Cortical Reaction
    • Cortical reaction products form fertilization envelope
    • oProteases – clip the binding receptors and any sperm attached to it
    • oMucopolysaccharides – produce osmotic gradient
    • oPeroxidase enzymes – hardens the fertilization cone
    • oHyaline – forms layer around the egg

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  15. Formation of Fertilization Envelope
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  16. Calcium initiates Cortical Reaction
    • Cortical Reaction is initiated by an increase Ca++ in egg
    • Ca++ promotes fusion of cortical granules to plasma membrane and release the contents by exocytosis
    • Calcium wave beings at the point of entry of the first sperm and removes excess sperm around the fertilized zygote as a wave.
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  17. Activation of pathways following fertilization
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  18. Probable mechanisms of Egg Activation
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  19. Activation of Egg Metabolism
    • •Mature egg is metabolically sluggish, that is activated by sperm.
    • •Categorized into two stages, Early and Late responses

    • •Early responses:
    • o Sperm-egg binding – 0 seconds
    • o Increase of membrane potential – 1 sec (fast block of polyspermy)
    • o Fusion of membranes of sperm and egg – 6 sec
    • o Increase in Calcium – 6 sec
    • o Cortical vesicle exocytosis - 60 sec (slow block of polyspermy)

    • •Late responses (sea urchin)
    • oActivation of kinases – Increase in NADH and NADPH – starts at 1 min
    • oSperm entry – 1-2 min
    • oIncrease in pH – 1-5 min
    • oMigration of sperm and egg nucleus to center – 2-12 min
    • oProtein synthesis - 5-10 min
    • oNucleic acid synthesis- 20-40 min
    • oMitosis – 60-80 min
    • oFirst cleavage – 85-95 min
  20. Fusion of Genetic material
    Sperm nucleus and centriole separate

    o Mitochondria and flagellum disintegrate

    o Egg nucleus – female pronucleus

    o Sperm nucleus decondenses (initiated by phosphorylation of histones)

    •Pronuclear membrane vesicules

    •Formation of male pronucleus

    oDecondensed male pronucleus begins transcription and replication

    oMicrotubules to form the aster

    oMicrotubules contact pronuclei and migrate towards each other

    Fusion forms diploid zygote nucleus
  21. Fusion of Genetic Material in Mammals
    • In mammals pronuclear migration is slow (about 12 hrs, compared to 1 hr in sea urchin)
    • oMale pronucleus enlarges while oocyte nucleus completes second meiotic division

    Note: Oocyte has not yet completed the meiotic division (ref: oogenesis

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    • Each pronucleus migrates towards each other – but do not fuse. Instead of fusing, chromosomes lineup randomly on mitotic spindle and move to each daughter cells
    • In mammals, true diploid nucleus is seen only in 2-cell stage.
  22. Cytoplasmic Changes – Gray crescent
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    • In Frogs, yolk is concentrated at one end of the egg – telolecithal
    • Ring of cortical cytoplasm surrounds inner cytoplasm

    • Following the entry of sperm, cortical cytoplasm shifts 300 towards the point of sperm entry
    • This shifting of dark cortical cytoplasm reveals area of lighter inner cytoplasm (gray crescent)
    • Gray crescent determines the anterio--posterior axis
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Human Development Chap 4
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