- Gametogenesis is the production of gametes (sex cells) via meiosis from germ cells. This involves the formation of haploid egg and sperm cells from diploid precursor cells. - Eggs undergo a process called oogenesis to form female gametes (ova/eggs). Sperm undergo spermatogenesis to form male gametes. Both involve mitosis, growth, and meiotic maturation. - Mature eggs contain stored nutrients, proteins, mRNA and other materials necessary to support early embryonic development before the embryo can feed itself. Eggs accumulate these materials during oogenesis.

1. Gametogenesis
Dr. Gauri Haval
A.G. College Pune

2. • Gametogenesis is the production
of gametes from haploid precursor cells. In
animals and higher plants, two morphologically
distinct types of gametes are produced (male and
female) via distinct differentiation programs.
Animals produce a tissue that is dedicated to
forming gametes, called the germ line.
Individual germline cells are called germ cells.
During the process of gametogenesis, a germ cell
undergoes meiosis to produce haploid cells that
directly develop into gametes. Hence, in animals,
meiosis is an integral part of gametogenesis.

3. Spermatogenesis
Means formation of sperm ; male gametes.
Oogenesis
Means formation of ovum ; female gametes.
Sperm and ovum are highly specialized sex cells.
Both types possess three main phases:
1. Period of multiplication: The primordial germ
cells multiply by mitotic cell division giving rise to
oogonia in case of females and spermatogonia in case
of males.
2. Period of growth: During these phase both oogonia
(female gamete) and spermatogonia (male gamete)
grow into primary oocyte or primary spermatocyte.

4. 3. Period of maturation: In case of female, the
primary oocyte undergoes two meiotic cell
division, the first gives rise to secondary oocyte
and primary polar body. The secondary
undergoes second meiotic division giving rise to
mature ova and secondary polar body (mature
ova leach oogonia).
In case of male, each primary spermatocyte
divides meiotically into secondary spermatocytes
and intern to spermatid (4 spermatid l each
primary spermatocyte).

11. Spermiation
• The mature spermatozoa released from the
protective Sertoli cells into the lumen of the
seminiferous tubule and a process called spermiation
then takes place, which removes the remaining
unnecessary cytoplasm and organelles.
• The resulting spermatozoa are now mature but lack
motility, rendering them sterile. The non-motile
spermatozoa are transported to the epididymis in
testicular fluid secreted by the Sertoli cells with the
aid of peristaltic contraction.
• In the epididymis they acquire motility and become
capable of fertilization. However, transport of the
mature spermatozoa through the remainder of the
male reproductive system is achieved via muscle
contraction rather than the spermatozoon's recently
acquired motility.

12. Oogenesis
• In some species, such as sea urchins and frogs, the female
routinely produces hundreds or thousands of eggs at a
time, whereas in other species, such as humans and most
mammals, only a few eggs are produced during the lifetime
of an individual. In those species that produce thousands of
ova, the oogonia are self-renewing stem cells that endure
for the lifetime of the organism. In those species that
produce fewer eggs, the oogonia divide to form a limited
number of egg precursor cells.
• In the human embryo, the thousand or so oogonia divide
rapidly from the second to the seventh month of gestation
to form roughly 7 million germ cells . After the seventh
month of embryonic development, however, the number of
germ cells drops precipitously. Most oogonia die during this
period, while the remaining oogonia enter the first meiotic
division

13. • These latter cells, called the primary oocytes,
progress through the first meiotic prophase
until the diplotene stage, at which point they
are maintained until puberty. With the onset
of adolescence, groups of oocytes periodically
resume meiosis. Thus, in the human female,
the first part of meiosis begins in the embryo,
and the signal to resume meiosis is not given
until roughly 12 years later. In fact, some
oocytes are maintained in meiotic prophase
for nearly 50 years.

14. • When the primary oocyte divides, its nucleus, called
the germinal vesicle, breaks down, and the metaphase
spindle migrates to the periphery of the cell. At
telophase, one of the two daughter cells contains
hardly any cytoplasm, whereas the other cell has
nearly the entire volume of cellular constituents. The
smaller cell is called the first polar body, and the larger
cell is referred to as the secondary oocyte. During the
second division of meiosis, a similar unequal
cytokinesis takes place. Most of the cytoplasm is
retained by the mature egg (ovum), and a second polar
body receives little more than a haploid nucleus. Thus,
oogenic meiosis conserves the volume of oocyte
cytoplasm in a single cell rather than splitting it equally
among four progeny.

15. Polar body formation

16. • The accumulated material in the oocyte
cytoplasm includes energy sources and energy-
producing organelles (the yolk and
mitochondria); the enzymes and precursors for
DNA, RNA, and protein syntheses; stored
messenger RNAs; structural proteins; and
morphogenetic regulatory factors that control
early embryogenesis. A partial catalogue of the
materials stored in the oocyte cytoplasm.
• Most of this accumulation takes place during
meiotic prophase I, and this stage is often
subdividedintotwophases, previtellogenesis (Gre
ek, “before yolk formation”) and vitellogenesis.

17. • The eggs of fishes and amphibians are derived
from an oogonial stem cell population that
can generate a new cohort of oocytes each
year. In the frog Rana pipiens, oogenesis takes
3 years. During the first 2 years, the oocyte
increases its size very gradually. During the
third year, however, the rapid accumulation of
yolk in the oocyte causes the egg to swell to
its characteristically large size. Eggs mature in
yearly batches, with the first cohort maturing
shortly after metamorphosis; the next group
matures a year later.

18. • Vitellogenesis occurs when the oocyte reaches the
diplotene stage of meiotic prophase. Yolk is not a single
substance, but a mixture of materials used for embryonic
nutrition. The major yolk component in frog eggs is a 470-
kDa protein called vitellogenin. It is not made in the frog
oocyte (as are the major yolk proteins of organisms such as
annelids and crayfishes), but is synthesized in the liver and
carried by the bloodstream to the ovary. This large protein
passes between the follicle cells of the ovary, and is
incorporated into the oocyte by micropinocytosis, the
pinching off of membrane-bounded vesicles at the bases of
microvilli. In the mature oocyte, vitellogenin is split into
two smaller proteins: the heavily phosphorylated
phosvitin and the lipoprotein lipovitellin. These two
proteins are packaged together into membrane-
bounded yolk platelets. Glycogen granules and
lipochondrial inclusions store the carbohydrate and lipid
components of the yolk, respectively.

19. • As the yolk is being deposited, the organelles also become
arranged asymmetrically. The cortical granules begin to
form from the Golgi apparatus; they are originally scattered
randomly through the oocyte cytoplasm, but later migrate
to the periphery of the cell. The mitochondria replicate at
this time, dividing to form millions of mitochondria that will
be apportioned to the different blastomeres during
cleavage. (In Xenopus, new mitochondria will not be
formed until after gastrulation is initiated.) As vitellogenesis
nears an end, the oocyte cytoplasm becomes stratified. The
cortical granules, mitochondria, and pigment granules are
found at the periphery of the cell, within the actin-rich
oocyte cortex. Within the inner cytoplasm, distinct
gradients emerge. While the yolk platelets become more
heavily concentrated at the vegetal pole of the oocyte, the
glycogen granules, ribosomes, lipid vesicles, and
endoplasmic reticulum are found toward the animal pole.
Even specific mRNAs stored in the cytoplasm become
localized to certain regions of the oocyte.

20. • Amphibian oocytes can remain for years in the
diplotene stage of meiotic prophase. This state
resembles the G2 phase of the cell division cycle (see
Chapter 8). Resumption of meiosis in the amphibian
primary oocyte requires progesterone. This hormone is
secreted by the follicle cells in response to
gonadotropic hormones secreted by the pituitary
gland. Within 6 hours of progesterone
stimulation, germinal vesicle breakdown (GVBD)
occurs, the microvilli retract, the nucleoli disintegrate,
and the chromosomes contract and migrate to the
animal pole to begin division. Soon afterward, the first
meiotic division occurs, and the mature ovum is
released from the ovary by a process called ovulation.
The ovulated egg is in second meiotic metaphase when
it is released

21. • Entry into the mitotic (M) phase of the cell cycle (in both
meiosis and mitosis) is regulated by mitosis-promoting
factor, or MPF (originally called “maturation-promoting
factor” after its meiotic function). MPF contains two
subunits, cyclin B and the p34cdc2 protein. The p34 protein
is a cyclin-dependent-kinase—its activity is dependent
upon the presence of cyclin. Since all the components of
MPF are present in the amphibian oocyte, it is generally
thought that progesterone somehow converts a pre-MPF
complex into active MPF.
• Reference:
• Gilbert SF. Developmental Biology. 6th edition. Sunderland
(MA): Sinauer Associates; 2000. Oogenesis. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK10008/

22. • All the material necessary for the beginning of
growth and development must be stored in
the mature egg (the ovum).
• The meiotic divisions that form the oocyte
conserve its cytoplasm (rather than giving half
of it away), and the oocyte either synthesizes
or absorbs proteins, such as yolk, that act as
food reservoirs for the developing embryo.
• Thus, birds' eggs are enormous single cells,
swollen with their accumulated yolk.

23. • Egg also has a remarkable
cytoplasmic storehouse that it
has accumulated during its
maturation.
• Proteins. It will be a long while
before the embryo is able to
feed itself or obtain food from
its mother. The early embryonic
cells need a supply of energy
and amino acids. In many
species, this is accomplished by
accumulating yolk proteins in
the egg. Many of the yolk
proteins are made in other
organs (liver, fat body) and travel
through the maternal blood to
the egg.

24. • Ribosomes and tRNA. The early embryo needs to make
many of its own proteins, and in some species, there is a
burst of protein synthesis soon after fertilization. Protein
synthesis is accomplished by ribosomes and tRNA, which
exist in the egg. The developing egg has special
mechanisms to synthesize ribosomes, and certain
amphibian oocytes produce as many as 1012 ribosomes
during their meiotic prophase.
• Messenger RNA. In most organisms, the instructions for
proteins made during early development are already
packaged in the oocyte. It is estimated that the eggs of sea
urchins contain 25,000 to 50,000 different types of mRNA.
This mRNA, however, remains dormant until after
fertilization
• Morphogenetic factors. Molecules that direct the
differentiation of cells into certain cell types are present in
the egg. They appear to be localized in different regions of
the egg and become segregated into different cells during
cleavage

25. • Protective chemicals. The embryo cannot run
away from predators or move to a safer
environment, so it must come equipped to
deal with threats. Many eggs contain
ultraviolet filters and DNA repair enzymes that
protect them from sunlight; some eggs
contain molecules that potential predators
find distasteful; and the yolk of bird eggs even
contains antibodies.

26. Egg Envelopes
• Egg envelope on the basis of origin are of three
types.
• (1) Primary egg envelops - These are secreted by
egg.e.g. Vitelline membrane-(Zona pellusida &
Zona Reticulate)
• (2) Secondary egg envelops - These are secreted
by ovary. e.g.- Chorion on insects.
• (3) Tertiary egg envelops - These are secreted by
oviduct. e.g.- Jelly coat of frog, Albumin, shell
membrane and shell of Hen

27. Egg Membranes
• Enclosing the cytoplasm is the egg plasma membrane. This
membrane must regulate the flow of certain ions during fertilization
and must be capable of fusing with the sperm plasma membrane.
• Primary Egg membrane:
• Outside the plasma membrane is the vitelline envelope, which
forms a fibrous mat around the egg. This envelope contains at least
eight different glycoproteins and is often involved in sperm-egg
recognition. The vitelline envelope is essential for the species-
specific binding of sperm. The envelope thus formed bears different
names in different animals – it is known as the vitaelline envelope
or vitelline membrane in insects, molluscs, amphibians, and birds;
in tunicates and fishes it is usually called the chorion. In mammals
the envelope of an exactly similar nature is called the zona
pellucida. The zona pellucida thus takes the place of the zona
radiata. The jelly coat surrounding the eggs of sea urchins belongs
in the same group.Vitelline membrane.
• In insects a second, thicker coat is secreted by the follicle cells on
top of the vitelline envelope. This second envelope is called the
chorion.

28. Secondary Envelope
• The second group of egg envelopes is those which are
secreted by oviducts and other accessory parts of the
genital organs while the egg is passing from the ovary to
the exterior.
• The eggs of amphibians are surrounded by a layer of jelly,
which protects the egg and sometimes serves to make the
eggs adhere to one another and to submerged objects such
as water plants. This jelly is secreted as the eggs pass
through the oviducts.
• When the amphibian egg is deposited in water, the jelly
absorbs water and swells. In the oviparous sharks and rays
the egg is surrounded in the oviducts (in the special parts
called the shell glands) by a hard shell of a complicated
shape. The shell is drawn out into long twisted horns which
serve to entangle the eggs among seaweed.

29. • The most complicated egg envelopes, however,
are found in the eggs of birds, where no less than
five envelopes can be distinguished, the
innermost being the vitelline envelope—a very
thin envelope covering the surface of the yellow
of the egg (which is the true egg cell). This
envelope actually has a double origin.
• An inner layer of the envelope is produced in the
ovary, in the space between the oocyte and the
follicle cells. This layer is composed of very rough
fibers. An outer, more finely fibrous layer is then
formed on top of the first layer when the egg
enters the upper portion of the fallopian tube.
• The next envelope is the white of the egg.

30. • Over most of the surface of the egg the shell envelopes are in
contact with one another, but at the blunt end of the egg they are
separated; the inner envelope adheres to the egg white, and the
outer envelope adheres to the shell, leaving a space in between
filled with air. The outermost envelope is the shell, which consists
chiefly of calcium carbonate (CaCO3), about 5 gm. in a hen’s egg.
• The shell is pierced by many fine pores which are filled by an
organic (protein) substance related to collagen. The envelopes of a
bird’s egg are secreted one after another as the egg proceeds down
the oviduct. The whole process takes slightly longer than 24 hours.
• After the egg has been released from the ovary, it quickly passes
into the oviduct and descends through it for about three hours,
during which most of the egg white is secreted and envelops the
egg cell. The lowest portion of the oviduct is widened and is termed
the uterus. Here the egg remains for 20 to 24 hours, while the
remainder of the egg white and eventually the shell envelopes and
the shell itself are secreted.
• Not only do the envelopes of a bird’s egg protect the egg cell, but
the egg white serves also as an additional source of nourishment
and is gradually used up in the course of the development of the
embryo.

31. • Secondary membranes:
• Many types of eggs also have an egg jelly
outside the vitelline envelope.
• This glycoprotein meshwork can have
numerous functions, but most commonly is
used either to attract or to activate sperm.