Ovulation Fertilization. Maturation of sperm Sperms –Incapable of fertilizing the oocyte...

Ovulation

Fertilization

Maturation of sperm

• Sperms– Incapable of fertilizing the oocyte immediately

after being released into the lumen of the seminefrous tubules

– Maturation• In epidydimus

• Epidydimus– Fluid reabsorption

• Increase concentrations of sperm by 100 folds– Estrogen dependent

– Quiescence of sperms• Slow activity• Movement depends on musculature

– Addition of chemicals• Glycoproteins

– Coating of sperm surface

• Maturation of sperm– Ability to move on its own– Nuclear condensation and loss of cytoplasm– Metabolism alteration

• External source

– Increased mobility– Changes in cell membrane

• Increased charge (glycosylation)• Changes in protein profile• Changes in lipid composition (fluidity)

• Maturation of sperm– Androgen dependent

• Very high concentrations (ABP)• Conversion to DHT

– Increased bioactivity

– Entire process will take several months to few days

Movement of sperms

• Entering vas deference from epidydimus– Muscular contraction

rather than fluid movement

– Dense mass– Serve as reservoir of

sperms• Lost in urine if no

ejaculation

Sperm in male reproductive tract

• Semen– Sperm plus seminal plasma

• Seminal plasma– Derived from accessory sex glands– Serves as fluid vesicle for transport

• Unnecessary for sperm function

• Seminal plasma– Derived from accessory sex glands– Serves as fluid vesicle for transport– Serves as buffer

• Basic in nature• Neutralizes acids

– Provides nutrients• Fluctose• sorbitol

• Semen composition– Immune cells– Infectious agents

Deposition of semen into the female reproductive tract

• Coitus– Physiological changes

• Genitalia• Other parts of body

– Phases• Described in humans by Johnson and Masters• Four phases

– Excitement (sexual arousal)– Plateau (intense arousal)– Orgasm (involuntary climax)– Resolution

• Absolute refractory phase in male– Immediately after orgasm

• Excitement– Vasocongestion

• Penile erection (engorgement of corpora cavanosa and corpus spongiosum)

• Vaginal lubrication (transudation)• Enlargement of labia minora and clitorus

– Vaginal expansion– “Tenting effect” on uterus

• Upward movement of uterus and cervix• Fibrilation of uterus (contraction)

– Sex flush– Myotonia

• Increased muscle tension

• Plateau– Increased testis size

• Elevated from the original position

– Secretion of fluid • Cowper’s gland

– Sex flushes– Engorgement of vaginal wall

• Orgasmic platform

– Formation of seminal pool

• Female orgasm– Rhythmic contraction

• Vaginal wall• Uterus

– Involuntary muscle spasm

• Male orgasm– Contraction of accessory sex gland

• Collection of semen into the urethral bulb• Contraction of bladder muscle• Emission stage of ejaculation

– Expulsion of semen/ejaculation• Muscle contraction• Reproductive tract contraction

• Resolution– Returning of body to nonaroused state

• Size of genitals

– Uptake of sperm by the cervix• Dipping into the seminal pool as the uterus returns

to the normal position• Opening of cervical canal

• Site of semen deposition– Varies among species

• Humans and cattle– Near external portion of cervix (cervical os)

• Uterus in other species

– Formation of “plug”• Enzymatic reaction

– Prevent backflow of semen– Buffers– Prevent sperm from other males to fertilize eggs?

Transport of sperm to the oviduct• Sperm must enter female reproductive tract and

reside there for some time– Maximum fertility

• Sperm – Viable inside of reproductive tract

• 24 to 48 hours in human • 30 to 48 hours in cows

• Oocyte– A finite life span

• 6 to 24 hours after ovulation in human

• Sperm must reach the oviduct at appropriate time in order to remain viable and fertile

Transport of sperm to the oviduct

• Semen – Deposited in the anterior part of the vagina near the cervix

• The majority of the sperm – Lost because of retrograde flow of the mucus out of the vagina– Killed because of the unfavorable environment for sperm survival

in the vagina.

Semen pool

Preovulatry follicle

Transport of sperm to the oviduct

• Some of the sperm – Transported through cervical canal into the uterus within minutes after sperm

deposition• Rapid transport• Rapid transport of sperm is caused by the contraction of myometrium (muscle layer of

the uterus) and not by active movement of sperm

– Sperm that enter the oviduct by rapid transport • Often non-viable and unlikely to participate in fertilization.

Transport of sperm to the oviduct

• Many sperm – Trapped in the folds

and crypts of the cervix

• Formation of sperm reservoirs within the cervix

– Critical • More sperm ultimately

reach the oviduct when more sperm are available in the cervical reservoir

Folds and crypts of the cervix

Transport of sperm to the oviduct

• Slow transport of sperm– After establishment of reservoir– Ensures the availability of sperm in the oviduct continuously

• Large number of sperms– Fails to reach the oviduct

• Many obstacles such as folding of the uterine endometrium and the narrow diameter of the uterotubal junction

• Presumably to prevent polyspermy (more than one sperm fertilizing the oocyte)

Transport of sperm to the oviduct

• Aggregation of sperms– The isthmus

– Sperm remain in the isthmus until ovulation

– Hyperactivation• Rapid movement of tail and erratic swimming motion

• Near ovulation

• Essential for their upward movement toward the ampulla.

Infundibulum

AmpullaIsthmus

Ampullary-isthmic Junction

Uterotubal Junction

Transport of sperm to the oviduct

• Capacitation – Changes in cell surface of the sperm– Required for proper attachment and penetration of the

zona pellucida when the sperm encounters the oocyte

Infundibulum

AmpullaIsthmus

Ampullary-isthmic Junction

Uterotubal Junction

• Capacitated sperms– Acrosome reaction when encountering the

oocyte• Ability to bind and penetrate zona pellucida

– Swelling of acrosome• Fusion of acrosomal membrane

– Exocytosis of acrosomal content• Ca and cAMP dependent

• Acrosome reaction– Triggered by the

protein in the zona pellucida

• ZP3• Sperms have receptors

for ZPs– Increased Ca uptake

– Release of hexosaminidase B (removal of ZP3)

• Acrosome reaction– Must take place near the oocyte

• Reduced sperm viability

Fertilization

• Sperm encountering the oocyte– Migration through the

remaining cumulus cells that surround the oocyte

– Sperm binds to the zona pellucida

• Mediated by the sperm binding sites present on zona pellucida

First polar bodyCumulus cells

Fertilization

• Within minutes of binding, the sperm begins to penetrate the zona pellucida, and eventually fuses with the oocyte– Equatorial and

postacrosomal region– Ca-dependent reaction– Immediately after the

sperm fuses with the oocyte, the oocyte extrudes the second polar body

– Oocyte becomes a haploid cell

Second polar body

Zona pellucida

Fertilization

• Once a sperm fuses with the oocyte, the zona pellucida becomes hardened– Release of Ca– Sperm binding sites

disappear from the zona pellucida

– Hardening of zona and loss of sperm binding sites are necessary in order to prevent polyspermy

Head of sperm

Fertilization

• Membrane of the sperm head – Degraded and

replaced by a new membrane

• Formation of a structure called the male pronucleus

– The maternal half of chromosomes

• The female pronucleus.Male pronucleus

Female pronucleus

Fertilization

• Male and female pronuclei move toward the center of the oocyte– Degradation of the

membrane that surrounds each pronucleus

– Mixing of maternal and paternal chromosomes

• Restoration of the total number of chromosomes

Zygote