DR (MRS) V B BUCKTOWAR
LECTURER, OBG DEPT.
DR. D Y PATIL MEDICAL COLLEGE
Knowing the necessary steps which lead to spermatozoa being
Describing how the enabling of the spermatozoa takes place
Describing how the spermatozoon penetrates into the oocyte
Knowing the process whereby a zygote is formed
We have learned that while the ovarian follicle is growing, the
oogonium within it undergoes maturation
Oogonium enlarges to form a primary oocyte.
The primary oocyte undergoes the first meiotic division to shed off
the 1st polar body and becomes secondary oocyte.
At ovulation, the second meiotic division is in progress and a spindle
has formed for separation of the second polar body.
At this stage the ovum enters the infundibulum of the uterine tube
and passes into the ampulla.
Fertilization - the action or process of fertilizing an
egg, involving the fusion of male and female
gametes to form a zygote.
Complex sequence of cordinated molecular events that begins with contact between a sperm and
an oocyte and ends with the intermingling of maternal and paternal chromosomes .
For successful fertilization, a mature spermatozoon (sperm cell) must penetrate into the mature oocyte.
At the same time this is also an encounter between one of the smallest human cells with one of the
The head of the sperm has a diameter of only 4mm, whereas the egg cell has one of 20mm
Fertilization of the ovum occurs in the ampulla of the fallopian tube.
One spermatozoon pierces the zona pellucida and enters.
The fertilization process takes approximately 24 hours.
One can speak of a successful fertilization when the genetic
material of the male spermatozoon has united with that of the
female oocyte thereby creating a new, unique individual.
An Oocyte(2) following ovulation,
surrounded by a cloud of cumulus cells(1),
the corona radiata, which form a cloak
Spermatozoa in a native ejaculate. These must
still mature in several steps during their way
through the female genital tract before they are
able to fertilize the oocyte.
When the spermatozoon leaves the testis it is not yet capable of fertilizing an oocyte.
What changes must it undergo to make it capable of uniting with the oocyte
How does the oocyte get from the ovary into the fallopian tube?
What makes it possible for the spermatozoon to penetrate into the oocyte?
How is it made sure that a spermatozoon from another species is unable to deposit its
When the fertilization is completed, the genes of the two parents have united. How
does this union of the two genomes take place?
The female genital tract
4 Fallopian or uterine
5 Ampullary part of the
6 Uterine musculature
7 Uterine mucosa
11 Ligamentum ovarii
12 Suspensory ligament
of the ovary
13 Ovary cut open
(follicles in various stages)
Approximately one-and-a-half days before the midpoint of the cycle, the
concentration of the luteinizing hormone (LH) rises steeply.
The LH peak is responsible for a whole series of processes: triggering ovulation.
With the LH peak the following maturation steps are now triggered in and around the
oocyte - up to ovulation:
In the oocyte:
Termination of the first meiosis with ejection of the first polar body
Begin of the second meiosis with arrest in the metaphase
Maturation of the oocyte cytoplasma by preparing molecules and structures that will
be needed at the time of fertilization.
The results of these processes are:
The correct placement of the uterine tube infundibulum upon the ovarian surface
The rupture of the follicle wall and the flow of the follicle fluid with the oocyte into the
The inhibition of the maturation of further follicles
The acquisition of the oocyte by
the fallopian tube
The fimbriae of the tube place themselves around the ovarian
stigma and seal this location off.
Via a rotation about the axis of the suspensory ligament of the ovary
and the ovarian ligament the ovary can turn the follicle that is
about to rupture towards the fallopian tube.
When the surface ruptures the mass of cumulus cells, which are
saturated with hyaluronic acid and which shelter the oocyte, reach
the fallopian tube together with serous yellow follicle contents.
2 Follicle that is about to rupture
5 Fallopian tube
6 Ligamentum ovarium proprium
7 Suspensory ligament of the ovary
OOCYTE ACQUISITION MECHANISM
1 Fallopian tube cut open with the tube
mucosa that lies in folds
2 Closely apposed fimbriae
3 Follicle fluid that has flowed out
4 Secondary oocyte with corona radiata
5 Ovary with follicles in various stages of
development and atresia
6 Pellucid zone
7 First polar body
8 Secondary oocyte
9 Cells of the corona radiata
10 Arrested spindle apparatus
The oocyte in the cloud of cumulus
cells following ovulation
In the fallopian tube, the secondary oocyte
is surrounded by the corona radiata and
scattered parts of cumulus cells (so-called
cumulus cell cloud).
The fluid that lies in between is sticky and
stringy (effect of the hyaluronic acid) with a
high concentration of progesterone (to
attract the spermatozoa).
Secondary oocyte (in arrested
metaphase of the 2nd meiosis)
Scattered groups of cumulus cells
The oocyte now "waits" in the fallopian tube on fertilization by the
The matrix of hyaluronic acid holds it "captive" there, so to speak.
After a number of hours the matrix liquefies more and more
and the oocyte is gradually transported towards the uterus by the
ciliary beats of the tube's epithelium cells.
Spermatozoa maturation steps
The spermatozoa go through several temporal maturation steps in a series of
different locations in order to be capable of penetrating into the oocyte.
Oocyte's maturation steps involve the storing of yolk and the process of
FUNCTIONAL MATURATION STEPS are required with the spermatozoa, which
involve their motile abilities & ability to penetrate through the egg covering.
The spermatozoa experience an initial maturation step during the time they are
"stored" in the epididymis.
When the ejaculation occurs, a second step follows that leads to a sudden
activation of their motility.
Spermatozoa maturation stepssummary
The third step takes place during their stay in the female genital tract, especially during the
ascension towards the ovary through the uterus and fallopian tube.
The spermatozoa experience thereby the so-called capacitation.
Finally, the last activation step follows: the acrosome reaction in the immediate vicinity of the
The maturation and activation of the spermatozoa occur in the following four steps:
Storage in the epididymis -Maturation
Ascension to the ovary -Capacitation
Near the oocyte -Acrosome reaction
Only a spermatozoon that has undergone an acrosome reaction is capable of binding to the
pellucid zone of the oocyte.
Maturation steps of the
spermatozoa in the epididymis:
Through the deposition of new proteins in the nucleus, the DNA becomes more
The sperm head becomes smaller, thereby, and more compact.
The cytoplasma is further reduced, making the sperm cells more slender.
The ability for motility is achieved.
The structure of the plasma membrane is altered. This has effects on the motility, the
capacitation ability and the ability for the acrosome reaction.
Only spermatozoa that have passed through the epididymis are mature enough to
be capable of motility.
While the spermatozoa are pushed through the deferent duct and
the urethra, a large volume of secretions of various glands are
This fluid part of the ejaculate is known as the seminal plasma.
The path of the sperm cells to the
oocyte - capacitation
Following the ejaculation a large number of sperm cells are deposited in vagina, near
the uterine cervix (portio vaginalis uteri).
The path that the sperm cells must travel from the portio to their meeting with the
oocyte in the ampullary part of the fallopian tube is 13-15 cm long.
Along this stretch, sperm cells go through a further maturation process, the so-called
Of the roughly 200 million ejaculated sperm cells only a few hundred are able to
traverse the long way through the cervix, the uterus, and past the fallopian tube isthmus
to the tube's ampullary region to there meet oocyte.
The cervical canal
At the time of ovulation the
properties of the cervix mucus
changes from a "sperm-hostile
"environment to a very "spermfriendly" one.
Before the ovulation (Fig. 26) the
cervical canal is narrow and the
cervix mucus is strongly meshed (it
forms the so-called cervical barrier)
that hinders the passage of sperm
At the time of ovulation (Fig. 27) the
cervix wall becomes looser and the
1 Sperm cells
2 Mucus fibers
3 (strongly meshed)
Crypt of a cervix
Capacitation is what one calls the changes that lead to
hyperactivity of the spermatozoon and which later allow the
spermatozoon to go through the acrosome reaction.
Much of it in uterine tube
Only capacitated sperm can pass through corona cells and
undergo acrosome reaction
Penetrating the cumulus cells
Enzymes are released by the acrosome reaction; the hyaluronidase dissolves the
intercellular matrix between the cumulus cells, other enzymes dissolve the pellucid zone
that lies around the oocyte.
Normally, the acrosome reaction of the spermatozoa takes place first when they
encounter the pellucid zone.
Upon arriving at the pellucid zone, these sperm cells themselves undergo an acrosome
reaction and a further amount of hyaluronidase and other enzymes are released.
In this way, the cumulus cells is further loosened up and more and more sperm cells
obtain the possibility of undergoing the acrosome reaction themselves at the pellucid
The hyperactivity of the spermatozoa caused by the capacitation is a decisive factor to
go through the mass of cumulus cells.
The acrosome reaction
During the acrosome reaction the contents of
the acrosome are released outwardly.
The cell membrane of the spermatozoon fuses
with the outer membrane of the acrosome.
The contents of the acrosome flow out
through the resulting pores.
After binding to zona pellucida
Induced by zona proteins
Culminates release of enzymes needed to
penetrate the zona pellucida including acrosin
and trypsin-like substance
RESULT: The penetration of the pellucid
of corona radiata
of zona pellucida
Phase-3: fusion of
oocyte and sperm
Phase-1: penetration of corona radiata
deposited in female genital
reach the site of
pass through corona
binding and induces
•Acrosin allows sperm
to penetrate zona
penetration of zona
Phase-3: fusion of oocyte and sperm cell membrane
•Fusion between plasma
membrane of oocyte and
•Head & tail of spermatozoon
enter cytoplasm of oocyte
•Plasma membrane left on
Phases of fertilization
Completion of the second meiotic division of the secondary oocyte and
formation of female pronucleus.
Formation of the male
The condensed DNA must be
unpacked as a first step, i.e.,
A nucleic membrane encloses the
The formation of a (pro)nucleus is
necessary for the subsequent
synthesis phase in which the DNA is
Oocyte containg two haploid
pronuclei is called as Ootid
The formation of the maternal pronucleus
At the same time that the paternal pronucleus forms, the maternal pronucleus is also
The penetration of the spermatozoon into the secondary oocyte causes the
resumption and termination of its second meiosis.
After the 2nd polar body is created the maternal chromosomes that remain in the
oocyte are enclosed by a nucleic membrane and decondensed.
Synthesis phase of the DNA in the pronuclei
The DNA must be duplicated before each commencing cell division so it can be
distributed among the daughter cells.
In an impregnated oocyte this happens for every chromosome set – the one from
the father and the one from the mother – each in a separate pronucleus.
The time needed for the duplication of the DNA amounts to roughly 12-18 hours.
During this time span, the two pronuclei also get closer to each other spatially.
Phases of fertilization- Approach of
The paternal and the maternal
pronuclei move towards each other
with the help of microtubules, which
begin to be formed immediately
after penetration of the
They grow in a star-like pattern out of
the paternal centrosome directly
beside the forming paternal
pronucleus (= formation of an aster
made of dozens of microtubules).
The microtubular proteins themselves
arise from the cytoplasma of the
Maternal astral microtubule
Phases of fertilization- Approach of
Simultaneously , the synthesis of
the DNA is taking place in the
the microtubules of the aster pull
the pronuclei together in the
center of the oocyte.
This duplication takes roughly 12
hours. The pronuclei grow in size in
Duplicated paternal centrosome
The formation of the zygote
After the two pronuclei have come close together the nucleic membranes of both pronuclei
dissolve and the chromosomes of both align themselves on the spindle apparatus at the equator.
The zygote, the first cell of a new organism with an individual genome (2n4C) is created by the
alignment of the maternal chromosomes together with the paternal ones on a common spindle
Nucleic membranes of the
pronuclei, as they are dissolving
Microtubules of the mitotic spindle
The formation of the zygote-contd.
The mitotic spindle divides the
chromosomes that have just been
brought together into the two first
cells of the embryo.
This proceeding towards the twocell stage occurs on average
between 22 and 26 hours after
The goal of the fertilization cascade
is thus achieved:
The fabrication of a diploid set of chromosomes
The determination of the chromosomal gender of the new individual
The induction of normal "cleavage division" for embryogenesis.
The zygote - by definition the first cell of the embryo
Results of fertilization
Restoration of diploid chromosomal number
Primary sex determination
Initiation of the cleavage of the zygote
Two celled zygote
Cells are called blastomeres
Clevage normally occurs as the zygote passes along the uterine tube
towards the uterus
cell mass – embryo proper
cell mass – Trophoblast - Placenta
The cleavage divisions up to the
Approximately 24 hours after fertilization the impregnated oocyte begins with
the first cleavage division.
The morula, a collection of around 30 cells (blastomere), is created at about
Because these cells arise only through the cleavage of the zygote and all are
found inside the pellucid zone, which cannot expand, no growth is seen.
Every new cell is thus only half as large as the cell from which it derives.
The name of this stage comes from its resemblance to a mulberry, since it
really looks like a collection of spherical cells.
How a blastocyst is engendered
On the 4th day after insemination the outermost cells of the morula are still enclosed
within the pellucid zone.
These begin to join up with each other (so-called compaction). An epithelial cellular
layer forms, thicker towards the outside, and its cells flatten out and become smaller.
A cavity forms in the interior of the blastocyst into which fluid flows (the so-called
The two to four innermost cells of the morula develop into the so-called inner cell mass
of the blastocyst.
The actual embryo will develop solely from these cells (embryoblast).
These cells are concentrated at one pole, the embryonic pole of the blastocyst.
Thus,an outer cell mass (the trophoblast), consisting of many flat cells, and the
embryoblast, formed from just a few rounded cells.
From the trophoblast the infantile part of the placenta and the fetal membranes will