Molecular basis of fertilization

COLLEGE OF HEALTH SCIENCEDEPARTMENT OF PHYSIOLOGY

ADVANCED REPRODUCTIVE PHYSIOLOGY

Molecular BASIS OF

FERTILIZATION

By TEKETEL ERISTU1Molecular Basis of Fertilization

Presentation Outline Introduction

Fertilization

Types of Fertilization

Cellular Events

Sperm Transport and Capacitation

Ovum Reaction to Sperm Penetration

References2Molecular Basis of Fertilization

ObjectivesAt the end of the presentation you will be able to:

Define Fertilization

Know the types of Fertilization

Describe Cellular Events

Explain Sperm Transport and Capacitation

Discuss ovum reaction to Sperm Penetration

3Molecular Basis of Fertilization

Fertilization

Father’s Sperm Mother’s Egg

Child’s Cell

INTRODUCTIO


23

46

46 23

23

23

23

23 46

sperm

mother

cell

ovum

mother

cell

sperms produced

by meiosis

fertilizationzygote

ova produced by meiosis

but only one develops to

maturity

46 46

46 4646 46 464646 46 46 46

4646

46 46

4646

Early EmbryoFertilization:Union of a Spermatozoal nucleus, of

paternal origin, with an egg nucleus, of maternal origin, to form the primary nucleus of an embryo.


6

CONT…

Molecular Basis of Fertilization

Types of Fertilization

1.External Fertilization:

is common in aquatic animals

Parents release their gametes into the environment


Internal Fertilization

occurs in nearly all terrestrial animals

Sperm are deposited in or close to the female

reproductive tract

Gametes unite within the tract following mating

CONT….


1.3 Cellular Eventsa) Ovum Maturation

Cell Type

Ploidy Process Process Completion

Oogonium

diploid Oocytogenesis (mitosis)

3rd trimester (forming Oocytes)

Primary Oocyte

diploid Ootidogenesis (Meiosis 1) (Folliculogenesis)

Halts in prophase 1

Secondary Ocyte

haploid Ootidogenesis(Meiosis 2)

Halted in metaphase 2 untill fertilization

Ovum haploid 9Molecular Basis of Fertilization

CONT…

2N (diploid) cell replicates its DNA, resulting in a 4N cell

(meiosis I)

2N cells

(meiosis II)

1N (haploid) cells


b) TransportThree Stages:• 1. Ovulation

Expulsion of the oocyte

Captured the Ciliated fimbriated end of the fallopian tube

The Ciliary action and Peristaltic waves of oviduct musculature bring the ovum into the ampulla of the fallopian tube

Fertilization Normally occurs in the ampulla ( ampulary-isthmic junction) of fallopian tube and Usually within 24 hrs of after ovulation.

Cont...


CONT…b.Transportation (utero-tubal Junction, junction b/n Uterus and

Oviduct) and Implantation of the Zygote

C.Utetro tubal junction uterine cavity


Cont….1. Follicle in Ovary is ready

to ovulate

2. New corpus Luteum

3. Ovulated Ovum in the second meiotic division

4. Formation of second polar body after fertilization

5. Fusion of egg and sperm Pronuclear; and

6. Beginning of first mitotic division of zygote.


1.4 Sperm Transport and Capacitation

a. Transport:• Velocity = 1-4mm/min (300 x106 200 sperms reach the oviduct).

• Flaggelar movement (to-and-fro) + ATP

The sperm are first deposited in the vagina:

they then pass up this cavity and through the cervix into the uterus ,

up the uterus, through the junction between the uterus and oviduct

(Uterotubal Junction), and up the isthmus of the oviduct to the usual

area of fertilization in the oviduct: the ampullary –isthmic junction.


Cont…1.Vaginal Sperm

After about one minute of deposition in the Vagina

the semen becomes thicker and less liquid

( Coagulation).

After about 20 minutes, the semen again liquefies,

Stimulates some sperm to swim more rapidly.


Cont…• The environment in the vagina is usually acidic

( about PH 4.2) and this level of acidity inhibit semen motility.

The presence of semen in the vagina, however,

increases the vaginal PH to a basic 7.2, which in turn

increase sperm motility.


Cont…2. Cervical Sperm

The cervical canal is lined by a complicated series of narrow folds and crypts and

is blocked by a sticky mass of cervical mucus and tiny cervical fibers.

Estrogen cause the mucus to become more liquid and makes gaps wider.


CONT…• The cervical fibers vibrate in rhythm with the tail

beat frequency of normal sperm,

this may allow the normal sperm to move through the cervix;

where as sperm with abnormal or absent tail beats are detained.

Other sperms enter cervical Crypts, where they may die or lost or they remain as a reservoir of sperm that may enter the uterus.


Cont…3. Uterine Sperm

Upon leaving the cervix, the sperm travel up the uterus to the uterotubal junction.

The uterus fluid is watery but sparse

The swimming rate of sperm (3mm/min) cannot account to climb up uterine lumen.


The muscle contraction and movement of cilia in the female reproductive tract that facilitate sperm transport.

Uterotubal junction allows the gradual entrance of sperm in the isthmus of the oviduct.

About half of the sperm enter the wrong oviduct and only a few hundred make it to the general proximity of the waiting egg.

CONT..


Cont…


b.Capacitation

• Sperms cannot fertilize oocytes when they are newly ejaculated. Thus Capacitation

• Is a process of sperm becoming fertile

• a process of essential changes in the spermatozoa that enables them to fertilize the egg.

• occurs during transport in female genital tract

• capacitated sperm have the ability to fertilize the egg

• The process of capacitation takes 5-7 hours.


Major Changes in sperm cell membrane during CapacitationChanges in surface glycoproteins, caused by secretion of FGT

Cholesterol is removed- increase Fluidity

Glycoproteins are lost- expose zona binding proteins

Proteins are phosphorylated

Fluctuations occur in the intracellular levels of calcium ions-essential for hyperactivation.

Cont.


Effects of Capacitation on Sperm

• Increased rate of metabolism

• Hyperactivation: flagellum beats more rapidly

• Changes in sperm glycoproteins allow sperm egg binding

• Pro-Acrosin (inactive) is converted to acrosin (active)

CONT….


1.5 Ovum Reaction to sperm penetration• Zona Binding • Zona proteins: ZP1, ZP2, ZP3

• Initiates acrosome reaction

a) Acrosomal Reaction

After binding to the corona radiata the sperm reaches the zona pellucida.

Sperm head binds to a ZP2 glycoprotein in the zona pellucida.

This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida. 25Molecular Basis of Fertilization

Cont…The acrosome reaction is associated with the release of

acrosome enzymes that facilitate fertilization

Acrosomal enzymes: esterases, acrosin,hyaluronidase ,and neuraminidase cause lysis of the zona pellucida


On binding sperm is induced to undergo the acrosomal reaction (ZP3)

Influx of Ca2+ into the sperm cytosol

Contents of the acrosome are released (+ exocytosis)

i. Proteinases/acrosin) + hyaluronidase penetration of ZP.

ii. Exposes other proteins on the sperm surface that bind to ZP2 help

the sperm maintain the binding to zona

iii.Exposes a protein in the sperm plasma membrane that mediates the

binding and fusion of this membrane with that of the egg.

• Penetration thru ZP 15-25min.



Cont…


Cont…



B. Cortical Reaction• Once sperm penetrates zona pellucida, the zona

reaction occurs:– This reaction makes the zona pellucida impermeable to

other sperms.

• Egg Activation

• Cortical Reaction– exocytosis of cortical granules

• Zona block

-biochemical changes eliminates

sperm binding

This prevents fertilization of an

egg by more than one sperm.32Molecular Basis of Fertilization

Mechanisms:Two mechanisms can operate to ensure that only one sperm

fertilizes the egg.

1.Primary block to polyspermy

Rapid depolarization of the egg plasma membrane

prevents further sperm from fusing and

thereby acts as a fast primary block to polyspermy.

But the membrane potential returns to normal soon after fertilization, so that a second mechanism is required to ensure a longer-term, secondary block to polyspermy

Cont…


ii. Secondary block to polyspermy

• A local increase in cytosolic Ca2+ (spreads in a wave).

• prolonged Ca2+ oscillations.

• Activate the egg to begin development,

• The cortical granules release their contents by exocytosis Proteolytic cleavage of ZP2 and the hydrolysis of sugar groups on ZP3

• The zona becomes “hardened,” so that sperm no longer bind to it.

CONT…



C. Nuclear Reaction

• Cortical reaction pulls the sperm nucleus.

• Loses the membrane that surround the head.

• Tail incorporated into the egg cytoplasm.

• Naked nucleus male pronucleus

• Approaches haploid nucleus of the ovum female pronucleus

• Male pronucleus + female pronucleus diploid zygote

• GENOME of a NEW ORGANISM… mitotic process….36Molecular Basis of Fertilization

2 References

1. Human Reproductive Biology Richard E.JONES, 3rd edition.

2. Advanced Reproductive Physiology Lecture notes, Dr. Getahun

3. Different internet Sources



Documents

Molecular basis of fertilization