fertilization

mouse sperm and oocyte viability

sperm: 6-12 hrs oocyte: 6-15 hrs

pig sperm and oocyte viability

sperm: 25-50 hrs oocyte: 8-10 hours

cattle sperm and oocyte viability

sperm: 30-48 hrs oocyte: 8-12 hrs

human sperm and oocyte viability

sperm: 28-48 hrs oocyte: 6-24 hrs

important events of fertilization

capacitation, sperm penetration of cumulus cells, sperm attach to zona pellucida, sperm penetrate zona pellucida, fusion of sperm and oocyte membranes

sperm penetration between cumulus cells

embedded in an extracellular matrix which contains hyaluronic acid

sperm attachment sites/receptors

ZP1, ZP2, ZP3

ZP1

structural support

ZP2

structural; helps an acrosome reacted sperm move through the zona

ZP3

zona protein that binds to a receptor on the sperm plasma membrane and induces the acrosome reaction

sperm plasma membrane contains

ZBr that reacts with ZP3 for physical attachment and ARPR that binds to ZP3 and initiates the acrosome reaction

binding of the sperm to ZP

requires 10,000-50,000 ZP3 molecules

the acrosome reaction

must be completed before the sperm can fuse with the secondary oocyte

the acrosome reaction occurs when

sperm come into contact with the zona pellucida of the oocyte

during the acrosome reaction

perforations develop in the acrosome, point fusions of the sperm plasma membrane and the external acrosomal membrane oocur

the acrosome reaction is associated with

the release of acrosome enzymes that facilitate fertilization

purpose of acrosomal reaction

enable sperm to penetrate the zona pellucida, modifies equatorial segment so that it later fuse with the plasma membrane of the oocyte

during the acrosome reaction the sperm goes from

intact —> fusion —> vesiculation —> denuded; IAM & EqS intact

intact

all membranes of the head intact

fusion and vesiculation (during the reaction)

plasma membrane overlaying the acrosomal membrane fuses to the outer acrosomal membrane which leads to vesiculation creating pores for enzymes to pass through

after reaction

vesicles are sloughed , leaving the inner acrosomal membrane, the equatorial segment, and post nuclear cap intact

after the acrosomal reaction

fertilization occurs and polyspermy blocked

penetration of the zona pellucida around the oocyte

proacrosin converted into acrosin and zona remains predominately intact

when sperm reaches perivitelline space

it settles into a bed of microvilli

plasma membrane of oocyte

fuses with membrane of equatorial segment

supplementary sperm

extra sperm which pass through the ZP and into the PV space

supernumerary sperm

extra sperm which suceed in entering the vitellus, such ova are said tp show polyspermy

blocking polyspermy

when more than one sperm enters the ovum the fetus almost always aborts

onces sperm fuse with plasma membrane

the zona reaction occurs

zona reaction

makes the zona pellucida impermeable to other sperm and corticle granules undergo exocytosis (slow)

the zona reaction results in

a “zona” block-biochemical changes preventing other sperm from entering

vitelline block

reduction in the ability of the oocyte plasma membrane to fuse with more sperm (fast)

cows and pigs

use zona reaction

rabbits

use vitelline block

rats and mice

use both zona reaction and vitelline block

triploidy (in vitro)

there are 3 pronuclei within this one zygote, in the lab such embryos are discarded, in life such embryos almost always abort spontaneously

risk factors of polyspermy

age of oocyte, elevated temperatures, excessive sperm numbers

age of oocyte

time of insemination relative to ovulation, aged oocyte increases chances of polyspermy

elevated temperatures

increases metabolism and lifespan of egg, fever or environmental effects on body temperature

excessive sperm numbers

ath the site of fertilization sperm barriers reduce numbers at the AI-junction

at fertilzation

sperm is engulfed, 2nd division of oocyte is complete, sperm nucleus decondenses, and formation of male pronucleus

2nd meitoic division of oocyte

secondary oocyte was arrested in metaphase of 2nd meiotic division and now forms a mature ovum and another polar body

fusion of pronuclei

the male and female pronuclei are indistinguishable, and the second polar body can be seen, the plasma membranes of the two pronuclei dissolve and one diploid nucleus remains

syngamy

membranes of the pronuclei break down, chromosomes condense, and arrange themselves for mitotic division

on membrane dissolution

there is one diploid (2n) cell, the first cleavage follows shortly leaving two cells each with 2N chromosomes

full process of fertilization

hyperactive motility —> binding to zona pellucida —> acrosome reaction —> penetration of zona pellucida —> sperm-oocyte membrane fusion —> release of 2nd polar body —> sperm engulfed —> decondensation of sperm nucleus —> formation of male pronucleus

fertilization is completed

within 24 hours and approximately 400-600 million sperm are deposited at cervical opening by only 200 sperm make it to fertilization site

results of fertilization

stimulates the oocyte to complete meiosis, restores the normal diploid number of chromosomes, and results in variationof species as chromosome intermingle

embryo contains

only maternal mitochondria because sperm mitochondria are dispersed and discarded

the sex chromosome carried by the sperm

determines sex

monozygotic

a fertilized egg splits into two developing zygotes that are identical and same sex

dizygotic

results in fertilization of two eggs, not identical and increase with age of mother

how can fertilization go wrong?

too many sperm, infertility, bad timing