1. GAMETOGENESIS
BY:
MARTIN, Ezra D.
PLES, Janina Grace C.
AAPD2G

2. Meiosis is the first step in gametogenesis: separation of homologous
chromosomes into haploid daughter cells
Spermatogonia and oogonia are the germ cells that will eventually develop into the
mature sperm or egg
Primary spermatocyte or oocyte: the first step in this development is the duplication
of homologous chromosomes to get ready for meiosis
Secondary spermatocyte or oocyte: the
first meiotic division separates the
homologous chromosomes from each
parent
Spermatids or eggs: the second meiotic
division separates the 2 chromatids and
creates 4 haploid cells
In males, this eventually produces 4
sperm cells by the process of
spermiogenesis. In females, it produces
1 egg and 3 polar bodies. This allows
the egg to retain more cytoplasm to
support early stages of development

3. Meiosis generates tremendous genetic diversity. How many different types of
gametes can be generated by an individual (male or female) with 23 different
chromosomes?

4. More than 223 or 8,000,000 different gametes

5. The timing of meiosis differs in females and males
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.
Thus, in females, the
completion of meiosis
can be delayed for
over 50 years. This is
not always good.
Only I egg produced
In addition, all meiosis
is ended in females at
menopause.

6. Homologous chromosomes form the synaptonemal complex
which facilitates crossing over and genetic diversity
During meiosis, homologous chromosomes
join together in pairs to form the
synaptonemal complex.
Each pair of chromatids is connected by
axial proteins. The 2 homologous
chromosomes are held together closely by
central element proteins.
A recombination nodule forms that contains
enzymes for cutting and splicing DNA.
Chromosomes are cut and joined crosswise
at points called chiasmata,seen when they
separate.
The exchange of genetic material is evident
when the chromosomes separate
This process is dangerous as it leads to
deletions and duplications of genetic
material. However, it is also valuable
because it increases genetic diversity and
facilitates evolution.

7. In older women, failure of the synaptonemal complex
to separate properly can cause genetic disease
Down syndrome is trisomy 21. It
results in short stature, round face
and mild to severe mental
retardation.
This is the failure of the 2
chromatids to separate during
meiosis 2. It results in one oocyte
receiving 2 instead of 1 chromatid.
In older women, long term
association of chromatids (i.e., over
50 years) results in the axial
proteins failure to separate.
Down syndrome occurs with a
frequency of 0.2% in women under
30 but at 3% in those over 45 years
of age.

8. Spermatogenesis occurs in the seminiferous tubules
The mammalian testes are divided into many lobules, and each lobule contains many
tiny seminiferous tubules.Sperm develop in an ordered fashion in these tubules. Cells
start to mature on the outside and move inward (towards the lumen) as the become
mature sperm.
Spermatogonia are the most primative cells. They differentiate as primary
spermatocyte
Sertoli cells are supporting cells that stretch from the lumen to the edge of the
tubule. They nourish the developing sperm. They form a blood-testis barrier to
control spermatogenesis (similar to the blood-brain barrier). These cells also inhibit
spermatogenesis before puberty and stimulate the process after puberty.

9. Spermiogenesis is the maturation process into sperm
The golgi vesicles combine to
form an acrosomal vesicle that
lies over the nucleus. Its full of
enzymes
Centosomes start to organize
microtubules into long flagella
Mitochondria start to localize next to the
flagella to provide ready energy
The nucleus condenses in size and is stabilized by special
proteins called protamines
The excess cytoplasm is pinched off as a residual body
(no need for organelles and cytoplasmic proteins)
Sperm are tiny, but highly specialized missiles for delivering the male genome:
Microfilaments shoot the acrosome into the egg to ‘harpoon it’ and pull it in.
The acrosome has enzymes for breaking into the egg.
The midpiece has large numbers of mitochondria for horsepower.
The tail has a powerful flagellum for driving the sperm into the proximity of the egg (in
humans, through the uterus and up into the oviduct.

10. Spermatogonia and oogonia are stem cells
What is a stem cell?
Stem cells have 3 properties: 1. They are undifferentiated cells
2. They have potential for self renewal
3. They are able to undergo differentiation to form committed
progenitor cells (a fancy word for all types of differentiated
adult cells such as muscle, bone, skin, etc)

11. The goal of oogenesis is to produce one egg
with massive amounts of cytoplasm
In many organisms, such as frogs and birds, the egg must contain all the nutrients
to support the entire process of embryonic development
In humans, the egg does not need to grow so large because the fertilized egg only
needs to support growth until it implants in the uterus. The placenta then nourishes
development.
In some organisms, such as frogs,
oocytes grow to extremely large
size and they have very active
chromosomes that synthesize large
amounts of RNA. In contrast to
sperm which are tiny cells, oocytes
are among the largest cells in the
body.
Oocytes contain Lampbrush
chromosomes: look like brushes
that were used years ago to clean
lamps. Frog oocytes can contain
200,000 times as many ribosomes
as a normal cell.

12. Oocytes have a very small nucleus / cytoplasm ratio
Most normal cells have several times as much cytoplasm as nucleus. This allows the
nucleus to make enough mRNA and rRNA to keep up with the cytoplasm and cell
needs.
In some species, oocytes have a tremendously tiny nucleus to cytoplasm ratio. They
must have a large amount of cytoplasm and ribosomes to make all of the proteins
needed for embryonic development.
The nucleus is just not large enough to keep up and maintain enough transcription to
generate all of the needed components. However, oocytes have developed
specializations to deal with this problem.
1. Ribosomal RNA genes are often amplified in oocytes. This allows more templates
to transcribe more rRNA.

13. Specializations allow the egg to accumulate cytoplasm:
nurse cells allow oocytes of insects
to produce massive amounts of RNA
In Drosophila melanogaster, the oogonia are called
ctyoblasts, and they undergo an unusual specialization
They undergo multiple mitotic divisions, but fail to
undergo cytokinesis (cell division). Thus, they all remain
connected to the original cell as cytocytes
One of the lucky cytocysts becomes the oocyte
The other 15 become nurse cells. They make large
amounts of RNA and nutrients but they send it all to the
oocyte. This allows the oocyte to accumulate massive
amounts of cytoplasm to support development (15 nuclei
instead of 1).

14. What does a fly’s ovary look like?

15. Vitellogenesis is the process of producing the major yolk proteins
Yolk: animal eggs contain large amounts of protein, lipid, and glycogen to nourish
the embryo. These materials are collectively called yolk.
Yolk is minimal in animal eggs that sustain only the first portion of embryogenesis
(humans and many mammals that have a placenta need only support cleavage for
several days before implantation into the uterus).
However, yolk is stored in large amounts in the eggs of birds and reptiles because
their eggs have to support the entire process of development.
Yolk proteins are synthesized in the liver in vertebrates, or in the fat body of insects
(an analogous organ)
Animal – vegetal polarity: In eggs
that have a lot of yolk, the yolk is
concentrated in the vegetal pole.
The animal pole contains the
nucleus and relatively little yolk.
The yolk in the vegetal pole
interferes with cytokinesis during
the process of cleavage leading to
incomplete cleavage.

16. Maturation processes prepare the oocyte
for ovulation and fertilization
Most oocytes of different species are arrested in the first meiotic division.
Oocyte maturation begins officially when this block is removed and meiosis starts
once again.
1. The nuclear membrane breaks down and DNA starts to condense into
chromosomes
2. The permeability of the oocyte plasma membrane changes so it can function
outside of the ovary.
3. The plasma membrane develops receptors to interact with the sperm
Fertilization occurs at different stages of oocyte maturation:
How is oocyte maturation
initiated?

17. Control of oocyte maturation has been studied extensively in frogs
Oocyte maturation is controlled by hormone interactions between the pituitary and
follicle cells. Pituitary o gonadotropin hormone c
progesterone s triggers oocyte maturation by activating c-mos expression
C-mos activates maturation promoting factor, the same activity as M-phase promoting
factor, that is composed of cyclin B and cyclin dependent kinase 1The exact
mechanism isn’t understood.
If c-mos is inactivated by antisense
oligonucleotides, no oocyte maturation
occurs. On the other hand, if extra c-mos
is injected it triggers oocyte maturation
before it is ready.
MPF does many things, although the
exact pathways have yet to be found. It
causesbreakdown of the nuclear
envelopeby phosphorylating nuclear
lamins (proteins stabilizing the envelope),
it triggerschanges in the oocyte plasma
membrane, it stimulatesovulation, and it
causescondensation of chromosomes.

18. Development of mammalian oocytes occurs within the ovary
In the mammalian ovary, the oocytes are closely associated with somatic cells
called granulosa cells which aid oocyte maturation and ovulation.
The timing of oocyte maturation and ovulation varies in different mammals.
Ovulation can be stimulated by seasonal cues, the process of mating, or in
primates, by the monthly cycle regulated by hormones such as estradiol,
produced by the granulosa cells.

19. Eggs are protected by elaborate envelopes
Vitelline envelope: a glycoprotein layer covers the plasma membrane of all eggs.
This acts to protect the egg.
Eggs that are deposited in water have a jelly-like coating that surrounds the egg
(frogs eggs)
Eggs that are deposited on land have particularly elaborate envelopes. The eggs of
birds have a vitelline envelope, a fibrous layer, an outer layer of albumin (egg
white), and a shell composed of calcium carbonate. The outer envelopes are
synthesized in the oviduct after the egg has been fertilized.