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
Meiosis generates tremendous genetic diversity. How many different types
of gametes can be generated by an individual (male or female) with 23
different chromosomes?
More than 223 or 8,000,000 different gametes
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.
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.
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.
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  secondary  spermatid  sperm are released into lumen.
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.
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.
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)
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.
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.
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).
What does a fly’s ovary look like?
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.
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?
Control of oocyte maturation has been studied extensively in frogs
Oocyte maturation is controlled by hormone interactions between the pituitary
and follicle cells. Pituitary  gonadotropin hormone  stimulates follicle cells 
progesterone  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 1
The exact mechanism isn’t understood.
If c-mos is inactivated by antisense
oligonucleotides, no oocyte maturation
occurs. On the other hand, if extra cmos is injected it triggers oocyte
maturation before it is ready.
MPF does many things, although the
exact pathways have yet to be found. It
causes breakdown of the nuclear
envelope by phosphorylating nuclear
lamins (proteins stabilizing the
envelope), it triggers changes in the
oocyte plasma membrane, it stimulates
ovulation, and it causes condensation
of chromosomes.
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.
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.