events of fertilization

fertilization events

• intromission/semen deposition

  • sperm count and quality

  • location

  • timing

• cervix - complex sperm selection

• capacitation - preparing for the big event

• binding/penetration/fusion

• early embryonic events

sperm contribution

numbers count:

• up to 10 billion sperm cell - natural service

• usually about 10 million - AI

ejaculation location

site of deposition is related to how many sperm are selected out/expelled (vagina>cevix>uterus) 

• once in uterus, sperm transport is rapid 

• though sperm can be found in oviducts within minutes after deposit, most viable sperm are there for 8-12 hour after (require certain time in female tract to capacitate)

uterine sperm transport

super shuttle

aided by:

• cervical manipulation

• clitoral stimulation

• prostaglandins in seminal fluid

fertilization

time is of the essence:

• ovulation occurs 25-32 hours after onset of standing estrus (12-18 after end of estrus)

• oocyte “fertilizable”/viable for 8-12 hours after ovulation

• transport and capacitation of sperm may take 6-12 hours

• ideally, fertilization occurs 4-6 hours after ovulation (ideally sperm present at time of ovulation/oocyte entry)

optimal breeding time

• from middle of estrus 6-10 hours after end of esturs

• 6-24 hrs after ONSET of standing estrus

necessary sperm functions

• motility - get to oocyte

• hyper motility - penetrate the cumulus and zona pellucida or egg shell

• requires intact membrane

  • plasma membrane - keep things inside

  • acrosome - penetrate ZP

sperm essentials 

• be on time 

• be presentable: capacitate 

• takes some chemistry 

• show some affection: binding to oocyte (ZP and oocyte membrane) 

• worth the time and effort? Normal DNA (morphology, decondensation, maintain embryonic development) 

sperm to to list

• capacitation

• hyper activation

• penetration of cumulus

• acrosome reaction

• zona binding

• zona penetration

• sperm-oocyte fusion

• decondensation of sperm nucleus

• formation of pronuclei

human intervention

• damage to acrosome or plasma membrane is IRREVERSIBLE and can be cause by:

  • marked changes in pH

  • changes in osmotic balance

  • sudden cooling/heating

cow contributions - capacitation

cow contributions include involvement at the level of:

• uterine to oviductal pH changes

• oviductal fluid

• cumuls “matrix”

• zona pellucida

capacitation

freshly ejaculated sperm are unable or poorly able to fertilize 

• removal of adherent seminal plasma proteins (acrosome stabilizing factor)

• reorganization of plasma membrane lipids and proteins

• influx of extracellular calcium

• decrease in intracellular pH

acrosome reaction

• only occurs if sperm is capacitated (requires time in the oviduct)

• acrosome is specialized lysosome — filled with enzymes

• acrosome reaction:

  • release of hyaluronidase and acrosin

• induced when sperm binds to the zona pellucida — Ca2+ increase

release of hyaluronidase - breaks down hyaluronic acid

• acrosin is a proteolytic enzyme of the trypsin family - digest path through zona pellucida

zona pellucida

• sperm receptor

• induces acrosome reaction

• block to polyspermy

• physical protection

• keep cells together until compaction 

• protection from the immune system 

sperm-egg fusion

• sperm adds phospholipase C zeta

• increase Ca2+ (oscillations)

fertilization

series of biochemical changes in oocyte and sperm cells that leads to ion (Ca) concentration elevation → activation of egg and stimulation of cellular division

meiosis II completion

extrusion of 2nd polar body

post fertilization events 

sperm decondensation 

• nuclear envelope of the sperm disperses, and the chromatin rapidly loosens from its tightly packed state 

sperm mitochondria, are degraded rather than incorporated into the embryo 

assess maturation of oocytes

1. expansion of cumulus cells

• LH surge causes this

2. confirmation of polar body