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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
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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)
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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
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Seminal fluid
• Prostaglandins in seminal fluid contract smooth muscles in the vasa deferentia, thus aiding sperm passage during ejaculation
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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
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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
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Sperm size & structure (Fig 9-2)
- Size: 40- 250 μm long Structure:
- • Sperm head
- • Neck
- • Midpiece: contains mitochondriawhich produces energy for tailmovement
- • Tail
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Egg activation
- • Cortical reaction
- • Completion of 2nd stage of meiosis
- • Increase in egg metabolism
- • Synthesis of protein, RNA, and DNA
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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
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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
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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
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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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