Gamete transport & fertilization 12

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jeannette_ruiz
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Gamete transport & fertilization 12
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2010-05-09 01:41:15
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Gamete transport fertilization
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Gamete transport & fertilization 12
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  1. FERTILIZATION=CONCEPTION
    • • Fusion of the nucleus of a haploid male gamete (sperm) and a haploid female gamete (ovum) to form a diploid individual
    • • Both sperm and ova travel in the female reproduction tract for a rendezvous in the oviduct
    • • While traveling, both the sperm and the ova go through important physical and biochemical changes´╗┐
  2. Semen release
    • • Sperm leave the epididymus and enter the vas deferentia where they are stored and transported
    • • 200 million sperm are produced each day
    • • Secretions of the male accessory glands(prostate gland and seminal vesicle)produce seminal plasma
    • • Sperm travels up vas deferentia and mixes with seminal plasma in the ejaculatory duct to form
    • semen (seminal fluid)
  3. Seminal plasma
    Contributes to maintenance, maturation, andtransport of sperm.
    Components of seminal plasma:
    • • Prostaglandin (contribute to sperm motility)
    • • Citric acid and ascorbic acid
    • • Bicarbonates
    • • Fructose
    • • Carnitine
    • • Glycerylphosphocholine
    • • Zinc
    • •ATP
  4. Seminal fluid
    • Prostaglandins in seminal fluid contract smooth muscles in the vasa deferentia, thus aiding sperm passage during ejaculation
  5. Sperm number
    • • 40 - 500 million (average 180 million) sperm in a single ejaculate
    • • 30% of ejaculated sperm are structurally or biochemically abnormal and are not capable of fertilization
    • • Some studies suggest that sperm count has decreased by 1% a year over the past 50 years
  6. Human semen characteristics
    • • Creamy texture with gray to yellow color
    • • Average volume: 2.5-3.5 ml after 3 days of abstinence (range 2-6 ml)
    • Fertility index = minimum qualifications formale fertility:
    • • ≥ 20 million sperm/ml
    • • ≥ 40% being able to swim
    • • ≥ 60% are normal shape and size
  7. Sperm size & structure (Fig 9-2)
    • Size: 40- 250 μm long Structure:
    • • Sperm head
    • • Neck
    • • Midpiece: contains mitochondriawhich produces energy for tailmovement
    • • Tail
  8. Sperm head structure (Fig 9-2)
    • • Contains an elongated haploid nucleus surrounded by nuclear membrane
    • •Acrosome: membrane bound vesicle external to nucleus that fits over the head of the sperm like a cap
    • • Acrosome is full of enzymes important in the penetration of the ovum
  9. Sperm flagellum structure (Fig 9-2)
    • • Flagellum is made of midpiece andtail configured in a “9+2”arrangement of microtubules
    • • This provides propulsive force,allowing locomotion of sperm celland penetration of ovum
  10. Sperm transport and maturation in female reproductive tract
    • • Vagina
    • • Cervix
    • • Uterus
    • • Uterotubal junction
    • • Oviduct isthmus
    • • Oviduct ampullary-isthmic junction which is most common site of fertilization
    • • 40 - 500 million (average of 180 million)sperm from single ejaculate is deliveredinto vagina
    • • < 1,000,000 make it to the uterus
    • • 100 - 1000 reach the oviduct
    • • 20 - 200 reach the egg
    • • Once sperm have made it to theoviduct, they can ‘smell’ the egg.
  11. Vaginal sperm
    • • Semen increases vaginal pH to a basic 7.2 which increases sperm motility
    • • Semen coagulation: about one minute after deposition into the vagina, the semen becomes thicker which may prevent sperm loss from vagina
    • • Semen liquefaction: about 20 minutes after deposition into vagina, semen again liquefies which enables sperm to swim faster toward cervix
    • • Female orgasm includes vaginal wall muscular contractions that produce pressure in vagina that is greater than in uterus which creates a pressure gradient that facilitates sperm movement through cervix
    • • Female orgasm can also cause the cervical os to dilate which allows the sperm to pass through more easily
  12. Cervical sperm
    • • Cervical canal lined by folds & crypts withtiny cervical fibers
    • •Mucus is thick and fibers are denselypacked during most of menstrual cycle
    • • Shortly before ovulation, the estrogen riseswhich causes the the mucus to becomemore liquid and the gaps between the fibersto become wider
    • • Cervical fibers may vibrate in rhythm withsperm tail beat frequency which aids spermmovement through the cervix
  13. Uterine sperm
    • • Uterine fluid is watery, but sparse
    • • Sperm “climb” up uterine wall by beating their tails
    • • However, uterine muscle contractions and cilia movement play a more important role in facilitating sperm transport up uterine wall
    • • Stimulation of cervix by penis duringcoitus stimulates release of oxytocinfrom woman’s posterior pituitary gland
    • • Oxytocin enhances force of rhythmic waves of uterine muscle contractions which help move sperm to the uterotubal junction
    • • About one half of sperm go into right oviduct and half go into left oviduct
  14. Sperm & ovum transport in oviduct
    • • On entrance of sperm into isthmus of oviduct,sperm tail beating is reduced while they “wait’for ovulation to occur
    • • In the oviduct the sperm have the ability to‘smell’ the egg.
    • • After ovulation, sperm move up to the ampullary-isthmic junction where it meets the ovum that has
    • traveled from the ovary through the oviductal infundibulum & ampulla
    • •Fertilization of ovum by sperm usually occurs at the ampullary-isthmic junction
  15. Sperm & ovum transport in oviduct
    • • Estrogens increase cilia number and progesterone increases cilia beating and eggtransport
    • • Oviductal cilia in the deep recesses beat toward the ovary to help move sperm
    • • Oviductal cilia in the ridges beat toward the uterus to help move egg & pre-embryo
    • • Sperm tend to travel in recesses, while ova & pre-embryo travel along ridges
  16. Sperm capacitation & activation
    • • Freshly ejaculated sperm is not capable of fertilization
    • • Sperm capacitation: during journey through female reproductive tract, the sperm matures and gains ability to fertilize eggs and increases tail movements of sperm
  17. Egg attraction of sperm
    • • Research suggests that human eggs produce chemicals that attract sperm and influence their swimming motion
    • • Human sperm have at least 20 chemical receptor molecules on itshead
  18. Fertilization
    • • Sperm passage through cumulus oophorus
    • • Sperm passage through zona pellucida
    • • Sperm attachment to egg plasmamembrane
    • • Cortical reaction
    • • Completion of second meiotic division of egg
    • • Formation/fusion of sperm and egg pronuclei
  19. Sperm passage throughcumulus oophorus (Fig 9-4)
    • • Cumulus oophrus is a layer of looselypacked follicle cells that surround the ovulated ovum
    • • As sperm enters the the cumulus oophorus,the enzyme hyaluronidase on the sperm head dissolves hyaluronic acid, acomponent of cementing material found between cells
    • • Hyaluronic acid breakdown enables spermto reach the zona pellucida
  20. Sperm passage tozona pellucida (Fig 9-4, 9-5)
    • • Zona pellucida is an extracellular matrix composed of three glycoproteins termed, ZP1, ZP2, and ZP3.
    • • Receptors on sperm plasma membrane attach to ZP3 (Fig 9-5A) which triggers sperm head to undergo the acrosomal reaction (Fig 9-5B &C)
    • • Acrosomal reaction: hydrolytic enzymes are released from acrosome which degrades the zona pellucida and creates a tunnel through which the sperm moves
  21. Sperm attachment to egg plasma membrane (Fig 9-5)
    • • Inner acrosomal membrane of sperm has has egg binding proteins that attach to egg cell membrane (Fig 9-5D) and enters in to the perivitelline space
    • • Sperm cell membrane fuses with the eggplasma membrane, allowing the spermnucleus and centriole to enter the eggcytoplasm (Fig 9-5E)
    • • Egg and sperm pronuclei migrate towardeach other within egg cytoplasm inpreparation for syngamy
  22. Sperm penetration of egg
    • • When sperm penetrates egg, calcium ions are released into the egg cytoplasm
    • • This sudden increase in calcium ions triggers egg activation with cortical granule release and completion of second stage of meiosis in egg before fertilization occurs
  23. The cortical reaction
    • • Once a sperm has penetrated an egg, a defense is mounted to prevent another sperm from penetrating the egg
    • • Cortical granules under the egg cell membrane release enzymes that debilitate ZP3 and ZP2 which prevents attachment of additional sperms which prevents polyspermy
    • • This cortical reaction is the first step in aseries of biochemical and physical changes known as egg activation
  24. Egg activation
    • • Cortical reaction
    • • Completion of 2nd stage of meiosis
    • • Increase in egg metabolism
    • • Synthesis of protein, RNA, and DNA
  25. Completion of second meiotic stageFig 9-6
    • • Ovulated egg is still arrested in second meiotic stage
    • • Penetration of egg by sperm initiates egg activation resulting in completion of second meiotic division
  26. Formation of sperm pronuclei
    • • Soon after sperm nucleus enters egg,its nuclear membrane breaks down
    • • Sperm DNA re-condenses as a result of exposure to egg cytoplasm
    • • A new membrane is formed to enclose the sperm pronucleus
  27. Formation and fusion of spermand egg pronuclei
    • • As sperm pronucleus and egg pronucleus approach each other, their cell membranes breakdown and syngamy occurs with the merging of the two haploid genomes to produce a diploid zygote
    • • Resultant diploid zygote cell divides mitotically to produce a two celled preembryo consisting of two identical daughter cells, called blastomeres
  28. Sperm & egg mitochondria
    • • Each sperm brings approximately 100 mitochondria into the egg.
    • • These mitochondria disappear soon after fertilization.
    • • All our mitochondria are inherited from our mothers.

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