Gamete transport & fertilization 12

• • 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

• • 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)

• • Prostaglandin (contribute to sperm motility)
• • Citric acid and ascorbic acid
• • Bicarbonates
• • Fructose
• • Carnitine
• • Glycerylphosphocholine
• • Zinc
• •ATP

• Prostaglandins in seminal fluid contract smooth muscles in the vasa deferentia, thus aiding sperm passage during ejaculation

• • 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

• • 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

• Size: 40- 250 μm long Structure:
• • Sperm head
• • Neck
• • Midpiece: contains mitochondriawhich produces energy for tailmovement
• • Tail

• • 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

• • 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

• • 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.

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • 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

• • Cortical reaction
• • Completion of 2nd stage of meiosis
• • Increase in egg metabolism
• • Synthesis of protein, RNA, and DNA

• • Ovulated egg is still arrested in second meiotic stage
• • Penetration of egg by sperm initiates egg activation resulting in completion of second meiotic division

• • 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

• • 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

• • Each sperm brings approximately 100 mitochondria into the egg.
• • These mitochondria disappear soon after fertilization.
• • All our mitochondria are inherited from our mothers.