FIRST WEEK OF DEVELOPMENT

1-Fertilization

Dr Rania Gabr

OBJECTIVESOBJECTIVESBy the end of the lecture, the student should be able to:

Define the term ‘fertilization’.Describe the site, Explain phases of fertilization and its outcome.

DEFINITION

It is the process by which the sperm of the male & ovum of the female meet and fuse.

SITE OF FERTILIZATION

Normal Site: In the AMPULLA

of the uterine tube, for 2 reasons:It is close to

the ovary.It is the widest

part of the uterine tube.

It may occur in other parts of the tube but

NEVER occurs in the uterus.Chemical signals from the

oocyte attract the sperms.

Viability of GametesViability of Gametes

Human oocytes are usually fertilized within 12 hours after ovulation.

Oocyte cannot be fertilized after 24 hours as it degenerates shortly thereafter.

Human sperms do not survive for more than 48 hours.

SHAPE OF THE SPERM

It is small (with a length of about 50µ, and highly motile.

It consists of head, neck,body and a motile tail.

The head contains the nucleus that is covered by the acrosome (an organelle

containing enzymes).

There are two types of sperms according

to the chromosomal contents

i.e. the Y- bearing sperm

(having 22 autosomes + Y)

And the X bearing sperm

(having 22 autosomes +X).

MORPHOLOGY OF THE 2RY OOCYTE (OVUM)

It is large sized, immotile, covered by zona pellucida and corona radiata.

Its cytoplasm is abundant (containing yolk granules for nutrition of the embryo during the first week of development).

Its nucleus contains 22 chromosomes + one X chromosome.

How do the sperms reach

the site of fertilization?

1. By movement of their tail.

2. By the movement of uterine cilia.

3. By chemo attraction.

How does the oocyte reach

the site of fertilization?

1. By the movement fimbria of the uterine tube.

2. By peristaltic movement of the uterine tube.

BEFORE THE SPERM FERTILIZES THE OOCYTE IT MUST UNDERGO 2 PROCESSES:

1) Capacitation:

Capacitation occurs in the female reproductive tract.

Freshly ejaculated sperms are unable to fertilize.

The glycoproteincoat is removed from the

surface of the sperm's acrosome.

It takes about 7-8 hours. Only the capacitated

sperms can penetrate the corona radiata.

2) Acrosome reaction:

It occurs after capacitation where proteolytic enzymes as acrosin,

esterases, hyaluronidase, and

neuraminidase are liberated in order to penetrate the zona

pellucida.

PHASES OF FERTILIZATIONPHASES OF FERTILIZATION

1. Passage of sperm through the corona radiata.

2. Penetration of the zona pellucida.

3. Fusion of the oocyte and sperm cell membranes to form the zygote (the unicellular embryo).

STAGE 1: STAGE 1: PASSAGE OF SPERM THROUGH CORONA PASSAGE OF SPERM THROUGH CORONA RADIATARADIATA

It results from:

1- The action of an enzyme hyaluronidase released from the acrosome of the sperm which helps in dispersal of corona radiata cells.

2- Constant propulsive force of the sperm's tail.

STAGE 2: STAGE 2: PENETRATION OF ZONA PELLUCIDAPENETRATION OF ZONA PELLUCIDA

Constant propulsive force from the sperm’s flagellating tail &

acrosomal enzymes (esterases, acrosin & neuraminidase) allow one sperm to create a tract through the zona pellucida.

STAGE 3STAGE 3:: FUSION OF PLASMA MEMBRANES OF THE OOCYTE & THE SPERM & ENTRY OF SPERM CONTENTS INTO THE OOCYTE.

Once a sperm penetrates the zona pellucida, a zona reaction (a change in the properties of the zona pellucida) occurs that makes it impermeable to other sperms.

- This zona reaction is caused by the action of lysosomal enzymes released by cortical granules near the plasma membrane of the oocyte.

Stage 4Stage 4:: Completion of second meiotic division of the oocyte and formation of female pronucleus.

5. 5. Formation of male Formation of male pronucleus.pronucleus.

6. Fusion of pronuclei, formation of zygote 6. Fusion of pronuclei, formation of zygote and preparation of first mitotic divisionand preparation of first mitotic division

RESULTS OF FERTILIZATION

1. Stimulates the penetrated oocyte to complete its 2nd meiotic division.

2. Restoration of the diploid number of chromosomes (46 chromosomes).

3. Determination of the embryo's chromosomal sex (the female is 46,XX and the male is 46,XY).

4. Stimulation of cleavage (mitotic cell divisions of the zygote).

“The End ”

Thank you