Download PHYSIOLOGY OF CONCEPTION

GAMETOGENESIS AND
FERTILIZATION
By DR.ARCHANAVIKRAM
YMC
GAMETOGENESIS :The process involved in the maturation of the two
highly specialised cells, spermatozoon in male and ovum
in female, before they unite to form zygote, is called
gametogenesis.
OOGENESIS :•Definition – The process involved in the development of
a mature ovum is called oogenesis.
•The primitive germ cells take their origin from the yolk
sac at about the end of 3rd week .
•In the female gonads, the germ cells undergo a number of
rapid mitotic division and differentiate into oogonia.
•The number of oogonia reaches it’s maximum at 20th
week, numbering about 7 million.
•While the majority of the oogonia continue to divide,
some enter into the prophase of the first meiotic division
and are called primary oocytes.
•These are surrounded by flat cells and are called
primordial follicles and are present in the cortex of the
ovary.
•At birth, all the oogonia are replaced by primary oocytes
which have finished the prophase of the first meiotic
division and remain in the resting phase (dictyotene stage)
between the prophase and the metaphase.
•Out of these, some 400 are likely to ovulate during the
entire reproductive period.
Maturation of the oocytes – The essence of maturation
is reduction of the number of chromosomes to half.
-Before the onset of first meiotic division, the primary
oocytes double its DNA by replication, so they contain
double the amount of normal protein content.
-There are twenty two pairs of autosomes which
determine the body characteristics and one pair of sex
chromosomes, named “XX”.
-The first stage of maturation occurs with full maturation
of the ovarian follicle just prior to ovulation but the final
maturation occurs only after fertilization.
•The secondary oocyte completes the second meiotic
division only after fertilization by the sperm in the
fallopian tube and results in the formation of two
unequal daughter cells, each possessing 23
chromosomes (23x), the larger one is called the mature
ovum and the smaller one is called the second polar
body containing the same number of chromosomes.
•The first polar body may also undergo the second meiotic
division.
•In the absence of fertilization, the secondary oocyte does
not complete the second meiotic division and degenerates
as such.
Structure of a mature ovum:•A fully mature ovum is the largest cell in the body and is
about 130 microns in diameter. It consists of cytoplasm
and a nucleus with it’s nucleolus which is eccentric in
position.
•It contains 23 chromosomes (23x).
•During fertilization, the nucleus is converted into a female
pronucleus.
SPERMATOGENESIS:•Definition – The process involved in the development of
spermatids, from the primordial male germ cells and their
differentiation into spermatozoa is called spermatogenesis.
•Shortly before puberty, the primordial germ cells develop
into spermatogonia and remain in the wall of seminiferous
tubules.
•The
spermatogonia
differentiate
into
primary
spermatocytes which remain in the stage of prophase of
the first meiotic division for a long time (about 16 days)
•Each spermatocyte contains 22 pairs of autosomes and 1
pair of sex chromosomes named “XY”.
•With the completion of the first meiotic division, 2
secondary spermatocytes are formed having equal share of
cytoplasm and haploid number of chromosomes either
23X or 23Y.
•Immediately follows the second meiotic division with the
formation of four spermatids, each containing haploid
number of chromosomes, two with 23X and two with
23Y.
•Extensive
morphological
differentiation
of
the
spermatids occurs without further cell division to
convert them into spermatozoa. This process is called
spermiogenesis, after which maturation & capacitation
of spermatozoa occurs.
•The time required for a spermatogonium to develop
into a mature spermatozoon is about 61 days.
Structure of a mature spermatozoon:
• The spermatozoon has a head, a neck, a middle piece
and a principal piece or tail.
• An axial filament passes through the middle piece and
extends into the tail.
1. The head :
- The head of the human spermatozoon is derived from
the nucleus . It consists of chromatin (mostly DNA).
- The head is covered by a cap like structure called the
acrosome (also called the acrosomic cap or galea
capitis).
2. The neck :
-The neck is narrow
-It contains a funnel shaped basal body and a spherical
centriole.
-The basal body is also called the connecting piece
because it helps to establish an intimate union between the
head and the remainder of the spermatozoon.
3. The axial filament :
• The axial filament begins just behind the centriole.
• It passes through the middle piece and most of the tail.
• At the point where the middle piece joins the tail, the
axial filament passes through a ring like structure called
the annulus.
• The part of the axial filament which lies in the middle
piece, is surrounded by a spiral sheath made up of
mitochondria.
-The axial filament is composed of several fibrils.
-There is a pair of central fibrils, surrounded by 9 pairs
(doublets) arranged in a circle around the central pair.
-Outside the fibrils, there is a fibrous sheath.
-In the region of the middle piece, the fibrous sheath is
surrounded by spirally arranged mitochondria.
-Finally, the entire sperm is enclosed in a plasma
membrane.
OVULATION :
•Definition –. it is a process whereby a secondary
oocyte is released from the ovary following rupture of a
mature graffian follicle and becomes available for
conception
•If gonadatrophin stimulation is adequate, one of the
several follicle units propelled to varying degrees of
maturity will advance to ovulation.
FERTILIZATION
FERTILIZATION :
•Definition – It is the process of fusion of the
spermatozoon with the mature ovum.
• it occurs in the ampullary part of the uterine tube.
•The fertilizable life of the human oocyte is between 12
and 24 hrs.
•Out of hundreds of millions of sperms deposited in the
vagina, only thousands of sperms enter the uterine tube
while only 300 to 500 reach the ovum.
•The tubal transport is facilitiated by muscular contraction
& aspiration action of the uterine tube.
Morula:- After the zygote formation, typical mitotic
division of the segmentation nucleus occurs producing 2
blastomeres.
•The 2 cell stage is reached approximately 30hrs after
feritilization.
•The blastomeres continue to divide by binary division
through 4,8,16 cells stage until a cluster of cells is formed
called morula.
•The morula after spending 3 days in the uterine tube
enters the uterine cavity through the narrow uterine ostium
on the 4th day.
•The central cell of the morula is known as inner cell mass
-the embryo proper and the peripheral cells are called outer
cell mass which forms the protective & the nutritive
membranes of the embryo.
Blastocyst:- While the morula remains free in the
uterine cavity on the 4th & 5th day, it is covered by a film
of mucus.
•The fluid passes through the canaliculi of the zona
pellucida which seperates the cells of the morula & is now
called the blastocyst.
•Lysis of the zona and escape of embryo is called as zona
hatching.
•The cells on the outer side of the morula become the
trophectoderm and the inner cells become the inner cell
mass.
•Trophectoderm differentiates into the placenta and inner
cell mass into the embryo.
IMPLANTATION :
Implantation is defined as the process by which an
embryo attaches to the uterine wall and penetrates first
the epithelium and then the circulatory system of the
mother to form the placenta.
Implantation begins 2 – 3 days after the fertilized egg
enters the uterus; entry is on day 18 or 19 of the cycle.
Thus, implantation occurs 6 – 7 days after fertilization.
Implantation consists of 3 stages:
1. Apposition
2. Adhesion
3. Invasion
(also called migration to denote its benign nature).
INVASION AND PLACENTATION:
In the second week after ovulation, the placenta is formed.
By this time, the trophoblasts at the implantation site have
formed masses of cytotrophoblasts and syncytiotrophoblasts,
and invasion of maternal blood vessels has begun.