Gametogenisis

BY,
KRISHNAPRIYA P D
1ST MSc ZOOLOGY

 The origin and development of gametes is
called gametogenesis
 This may be divided into spermatogenesis and
oogenesis.
 Spermatogenesis deals with the development of
male sex-cells called sperms in the male gonad
or testis.
 Oogenesis is the development of female sex-
cells called ova or eggs in the female gonad or
ovary.

 In males, the spermatogonia enter meiosis and
produce sperm from puberty until death.
 The process of sperm production takes only a few
weeks. Each ejaculation has 100 to 500 million sperm.
 In females, this process is more complex. The first
meiotic division starts before birth but fails to
proceed.
 It is eventually completed about one month before
ovulation in humans. In humans, the second meiotic
division occurs just before the actual process of
fertilization occurs.
 Only I egg produced
 In addition, all meiosis is ended in females at
menopause.

Spermatogenesis:
The entire process of spermatogenesis can be divided
into following two phases:
(A) Formation of Spermatid:
The male gonad known as testis is the site of
spermatogenesis. In each vertebrate a pair of testes
remains attached to dorsal body wall by a connective
tissue called mesorchium .
Each testis is formed of thousands of minute elongated and
coiled tubules called seminiferous tubules.
The inner lining of seminiferous tubules is called as germinal
epithelium and is made of primordial germ cells (Primary
germ cells) as well as some supporting nutritive cells.

1. Multiplication Phase
The primary germ cells multiply by repeated mitotic division. The cells produced after the
final mitotic divisions are known as spermatogonia or sperm mother cells.
2. Growth Phase
The spermatogonia do not divide for sometime but increase in size by accumulating
nutritive materials from the supporting cells.
In mammals such supporting cells are called cells of Sertoli. The enlarged spermatogonia
are now called primary spermatocytes.
3. Maturation Phase:
During the phase of maturation, the primary spermatocytes divide by meiosis consisting of
two successive divisions.
The first division is reductional or disjunctional reducing the chromosome number from ‘2n’
to ‘n’. These cells are celled secondary spermatocytes.
Second division is equational resulting in formation of four daughter cells called
spermatids.

(B) Spermiogenesis (Spermatoleosis):
This is the second phase of spermatogenesis during which the spermatids produced
at the end of first phase are metamorphosed into sperm cells.
The spermatid is a typical cell containing a nucleus and cytoplasmic organelles such
as mitochondria, golgi bodies, centriole etc, but the nucleus only contains
haploid number of chromosomes.
During spermiogenesis or spermatoleosis the following
transformations occur in the spermatids:
1.The large spherical nucleus becomes smaller by losing water and usually changes
its shape into elongated structure.
2. The Golgi bodies condense into a cap called acrosome in front of the nucleus.
3. Nucleus and the acrosome combinedly form the head of the developing sperm
while the cytoplasm with mitochondria and centrioles move downwards and form
the cylindrical middle piece behind the head
4. The two centrioles of middle piece develop axial filaments which are bunched into
a single thread and extend behind in the form of a long vibratile tail. Thus,
spermatid is transformed into a motile sperm divisible into head, middle piece
and tail.

Sprem
 The mammalian sperm cell consists of a head, a midpiece
and a tail.
 The head contains the nucleus with densely coiled
chromatin fibres, surrounded anteriorly by an acrosome,
which contains enzymes used for penetrating the female
egg.
 The midpiece has a central filamentous core with many
mitochondria spiralled around it, used for ATP production for
the journey through the female cervix, uterus and uterine
tubes.
 The tail or "flagellum" executes the lashing movements that
propel the spermatocyte.

During fertilization, the sperm provides three essential parts to the oocyte:
(1) a signalling or activating factor, which causes
the metabolically dormant oocyte to activate;
(2) the haploid paternal genome
(3) the centrosome, which is responsible for maintaining the microtubule system.

Oogenesis:
It occurs in the ovary of female animals. It is comparable to
spermatogenesis so far as nuclear changes are
concerned.
But the cytoplasmic specialization in oogenesis is different
from spermatogenesis.
It is divisible into following three phases:
1.Multiplication Phase:
The primary germinal cells of the ovary with diploid number of
chromosomes (2n) divide several times mitotically so as to
form a large number of daughter cells known as oogonia

2. Growth Phase:
The oogonium does not divide but increases in size enormously to form
a primary oocyte.
The growth is associated with both nuclear and cytoplasmic growth. The
nuclear growth is due to accumulation of large amount of nuclear sap
and is termed as germinal vesicle.
The cytoplasmic growth is associated with increase in number of
mitochondria, endoplasmic reticulum and Golgi complex and
accumulation of reserve food material called yolk or vitellin.
3. Maturation phase:
The primary oocyte undergoes two successive divisions by meiosis. The
first division is meiosis-I and two unequal daughter cells are
produced.
The large cell is called secondary oocyte containing haploid (n) set of
chromosomes (due to reductional or disjunctional division) and entire
amount of cytoplasm.

The smaller cell is called first polar body or polocyte containing ‘n’
number of chromosomes and practically no cytoplasm.
The secondary oocyte and first polar body then undergo second
maturation division by meiosis-II which is an equational division.
As a result of this division one large ovum is formed containing entire
amount of cytoplasm and ‘n’ number of chromosomes and a second
polar body like the first polar body.
Simultaneously, the first polar body may divide into two polar bodies or
may not divide at all. Thus only one functional ovum is formed and the
two or three polar bodies soon degenerate.
In vertebrates the first polar body is formed after the primary oocyte is
released from ovary and has entered into the oviduct. The second
polar body is formed only when the sperm enters into ovum during
fertilization.

(C) Ripening of Egg:
Oogenesis is followed by the formation of protective coverings called
egg membranes.
Primary membrane is formed surrounding the plasma membrane of
ovum and is secreted by the ovum itself. It is called vitelline
membrane in frog and zona pellucida in rabbit.
The secondary membrane called chorion is formed from ovarian follicle
cells. The tertiary membranes are secreted in oviduct when the
ovum passes from ovary to outside.
The egg white (albumin), calcareous shell etc. come under this
category
The ripe ovum is spherical or oval and non-motile. Depending upon the
amount of yolk, it may be as small as 0.15 mm as in mammals
(microlecithal); it may be 2 mm as in frog (mesolecithal) or it may be
as large as 30 mm as in hen (megalecithal).

In a ripe ovum, the polarity is fixed. The top-most point is animal pole and the
bottom point is vegetal pole. The density of yolky cytoplasm increases from the
animal pole towards the vegetal pole.
In frog, the animal hemisphere is highly pigmented and appears black while the
vegetal hemisphere is highly pigmented and appears white.

Significance:
1.The process leads to formation of germ cells or gametes.
2. The normal body cells known as somatic cells are diploid
(2n) where as the germ cells are haploid (n).
3. During fertilization one halpoid sperm unites with one
haploid ovum to form a normal diploid somatic cell thus
keeping the chromosome number constant generation
after generation.
4. During first maturation division, the reshuffling of paternal
and maternal genes take place resulting in variation.

Zygote formation
 A zygote (from Greek "joined" or "yoked"), is a eukaryotic
cell formed by a fertilization event between two gametes.
 The zygote's genome is a combination of the DNA in each
gamete, and contains all of the genetic information
necessary to form a new individual.
 In multicellular organisms, the zygote is the earliest
developmental stage.
 In single-celled organisms, the zygote can divide asexually
by mitosis to produce identical offspring.
 Oscar Hertwig and Richard Hertwig made some of the first
discoveries on animal zygote formation.

In human fertilization, a release ovum (a haploid secondary oocyte
with replicate chromosome copies) and a haploidsperm cell
(male gamete)—combine to form a single 2n diploid cell called the
zygote.
Once the single sperm enters the oocyte, it completes the division of
the second meiosis forming a haploid daughter with only 23
chromosomes, almost all of the cytoplasm, and the sprem in its
own pronucleus.
The other product of meiosis II is the second polar body with only
chromosomes but no ability to replicate or survive.
In the fertilized dughter, DNA is then replicated in the two separate
pronuclei derived from the sperm and ovum, making the zygote's
chromosome number temporarily 4n diploid.

After approximately 30 hours from the time of fertilization,
fusion of the pronuclei and immediate mitotic division
produce two 2n diploid daughter cells
called blastomeres.
 It is not correct to call the conceptus an embryo, because it
will later differentiate into both intraembryonic and
extraembryonic tissues,and can even split to produce multiple
embryos (identical twins).
 After fertilization, the conceptus travels down
the oviduct towards the uterus while continuing to divide
mitotically without actually increasing in size, in a process
called cleavage.
 After four divisions, the conceptus consists of 16
blastomeres, and it is known as the morula.

Through the processes of compaction, cell division, and blastulation, the
conceptus takes the form of the blastocyst by the fifth day of
development, just as it approaches the site of implantation.]
When the blastocyst hatches from the zona pellucida, it can implant in
the endometrial lining of the uterus and begin the embryonic stage of
development.

Gametogenisis

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Gametogenisis