Fertilization

Step prior to developing zygote =

fertilization

Fertilization:

1. Sperm must transverse the cervix

2. Be transported through the uterus to the oviduct

3. Undergo capacitation

4. Bind to the oocyte

5. Undergo the acrosome reaction

6. Penetrate the zona pellucida and fuse with the oocyte plasma membrane where the sperm enters the oocyte cytoplasm and its nucleus decondenses

When the male pronucleus is formed:

a successful fertilization has taken place

Immediate Transport:

• retrograde loss

• phagocytosis — cells eat other cells (white blood cells breaking down sperm)

• entrance into cervix/uterus

Cervix =

• “privileged” pathways

• removal of non-motile sperm

• removal of some abnormalities

Uterus =

• capacitation initiated

• phagocytosis

Oviduct =

• capacitation completed

• hyperactive motility

Fertilization =

• acrosome reaction

• spermatozoon penetrates oocyte

• male and female pronuclei form

Sperm actually become activated after:

capacitation

Semen deposition — Cow, sheep, rabbit, primates, dog, and cat

ejaculate into vagina

Semen deposition — Pigs

ejaculate into cervix

Semen deposition — Horses and camelids

ejaculate into cervical lumen where much of it goes directly into uterine body

Stallion ejaculates in bursts:

• First few bursts highly concentrated with sperm

• Last few bursts lower in concentration of sperm, mostly seminal plasma which serves to minimize retrograde sperm loss

Boars ejaculate a large volume:

• Most of the semen flows into uterus

• Ejaculate also differs over the ejaculation time

Ejaculate also differs over the ejaculation time:

• First fraction consisting of accessory fluids

• Then high sperm

• Followed by fraction that coagulates and reduces retrograde loss

Estradiol causes neutrophils to:

accumulate in the vagina and uterine mucosa

What do the Neutrophils do?

attack foreign materials, such as microorganisms introduced during copulation but also sperm!??!

6 to 12 hrs after introduction of sperm:

Large migration of neutrophils from uterine mucosa into uterine lumen

Sperm transport — Rapid transport phase:

sperm reach oviduct a few minutes after copulation

• These sperm are not fertile

• Reason not understood, due to muscle contractions?

Sperm transport — Sustained transport phase:

sperm transported to oviducts in a ‘trickle-like’ effect from reservoirs in the tract — Delivers sperm in a more uniform, sustained manner

Where are the reservoirs located?

in the cervix

____ assist with sperm transport?

Contractions of muscularis

____ stimulates contraction?

Estradiol

___in semen also increases contractions?

Prostaglandinsin

____assist with transport?

Seminal fluids (medium for them to swim through)

• Boar seminal plasma advances time of ovulation

In several species, sperm must navigate through:

the cervix

Bovine cervix produces two types of mucus:

• Sialomucin

• Sulfomucin

Sialomucin =

low viscosity, produced by cells in basal areas (divets) of crypts

Sulfomucin =

high viscosity, produced in apical (tip) portions of the epithelium covering the tips of the cervical folds (like molasses)

Straight shot =

sulfomucin — not privileged pathway

Sperm that swim into viscous sulfomucin:

Get stuck and end up getting flushed out

Sperm that swim into less viscous sialomucin:

Swim on through

The ‘privileged pathways’ are not most direct route:

• But require good sperm to make an effort

• Makes it easier to remove poor swimming/dead sperm as well as bacteria from tract

• capture mechanism for poor motility sperm

When sperm leave the male:

Not at maximum fertility

Sperm needs female reproductive tract to trigger:

capacitation

Capacitation =

• Biochemical changes occur in plasma membrane of the sperm, particularly the head

• Involves stripping of the various proteins that cover the sperm

What triggers capacitation?

chemical messengers in female mucus

Why do extra proteins on sperm head need to be removed?

underneath is stuff that will bing to the egg

As sperm move into oviduct, become:

hyperactive

Motility changes; switch from swimming in a straight line to a:

frenzied, circular motion

• Occurs in ampulla, and facilitates the sperm-oocyte interaction

Sperm contain:

zona binding proteins on the plasma membrane that are uncovered during capacitation

Primary zona binding region binds first and is:

responsible for adherence

Second binding site is:

acrosome reaction promoting ligand

• Causes a signal transduction which initiates acrosomal reaction

Acrosomal reaction =

An orderly fusion of spermatozoal plasma membrane and outer acrosomal membrane

Purpose of Acrosomal reaction?

1. Reaction enables sperm to penetrate the zona pellucida

2. Exposes equatorial segment so can later fuse with plasma membrane of oocyte

Acrosomal reaction begins when:

plasma membrane of sperm forms multiple fusion sites with outer acrosomal membrane of the sperm which forms vesicles (vesiculation)

After vesiculation:

acrosomal contents are dispersed and sperm nucleus is left with the inner acrosomal membrane surrounding it

If acrosome is damaged (heat, cold, pH):

reaction does not happen correctly

Acrosomal Reaction releases:

enzymes that allow sperm to digest its way through zona pellucida

• acrosin

• proacrosin

Acrosin =

Enzyme that hydrolyzes zona proteins and enhances sperm’s ability to bind

Proacrosin =

inactive form of acrosin, strong affinity for the zona

Penetration of zona only takes a few minutes:

Assisted by flagellar action

Enzymes produce:

a small hole in ZP, most of the zona stays intact

After sperm penetrates zona:

Plasma membrane of oocyte fuses with the membrane of the equatorial segment (assisted by fusion protein)

Cortical reaction:

Creates a zona block so that more sperm cannot penetrate

Polyspermy =

more than one sperm penetrate

Vitelline block =

Reduction in oocyte plasma membrane’s ability to fuse with additional sperm, also prevents polyspermy

Some species have:

both blocks, some have one or the other

When does cortical reaction occur?

when plasma membranes fuse

What are 2 methods of regulation of entry?

cortical reaction & vitelline block

After fertilization, sperm nucleus:

decondenses/unravels

Sperm nucleus becomes:

the male pronucleus

syngamy =

Female pronucleus and the male pronucleus fuse

What is present after syngamy occurs?

chromosomes can pair & have a single celled organism/zygote

Fertile life-span of sperm: Some Reptiles

retained 5 years

Fertile life-span of sperm: Bitch/female dog

9-11d

Fertile life-span of sperm: Camelids

4-5d

Fertile life-span of sperm: Cow

1.5-2d

Fertile life-span of sperm: Mare

4-5d

Fertile life-span of sperm: Woman

5-6d

Canine =

lengthy estrus and a lengthy life-span of sperm

• Possible for more than one sire in litter (superfecundation)

Humans =

• Lengthy fertile period of sperm

• Couple that with no defined period of sexual receptivity… results in a fairly high likelihood of pregnancy

On average, ____ chance of pregnancy when copulation occurs within 2 days of ovulation?

33%