Transcript Primordial Germ Cells

• Mitosis produces two identical daughter cells,
but meiosis produces 4 very different cells.
Fig. 13.8
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• Meiosis reduces
chromosome number by
copying the
chromosomes once, but
dividing twice.
• The first division,
meiosis I, separates
homologous
chromosomes.
• The second, meiosis II,
separates sister
chromatids.
Fig. 13.6
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Division in meiosis I occurs in four phases:
prophase, metaphase, anaphase, and telophase.
• During the preceding interphase the
chromosomes are replicated to form sister
chromatids.
– These are genetically identical
and joined at the centromere.
• Also, the single centrosome
is replicated.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In prophase I, the chromosomes condense and
homologous chromosomes pair up to form
tetrads.
– In a process called synapsis, special proteins attach
homologous chromosomes tightly together.
– At several sites the chromatids of
homologous chromosomes are
crossed (chiasmata) and segments
of the chromosomes are traded.
– A spindle forms from each
centrosome and spindle fibers
attached to kinetochores on
the chromosomes begin to
Fig. 13.7
move
the
tetrads
around.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• At metaphase I, the tetrads are all arranged at
the metaphase plate.
– Microtubules from one pole are attached to the
kinetochore of one chromosome of each tetrad,
while those from the other pole are attached to the
other.
• In anaphase I,
the homologous
chromosomes
separate and
are pulled toward
opposite poles.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In telophase I, movement of homologous
chromosomes continues until there is a haploid
set at each pole.
– Each chromosome consists of linked sister
chromatids.
• Cytokinesis by the same
mechanisms as mitosis
usually occurs simultaneously.
• In some species, nuclei
may reform, but there is
no further replication
of chromosomes.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Meiosis II is very similar to mitosis.
– During prophase II a spindle apparatus forms,
attaches to kinetochores of each sister chromatids,
and moves them around.
• Spindle fibers from one pole
attach to the kinetochore of
one sister chromatid and
those of the other pole to
the other sister chromatid.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• At metaphase II, the sister chromatids are
arranged at the metaphase plate.
– The kinetochores of sister chromatids face opposite
poles.
• At anaphase II, the
centomeres of sister
chromatids separate
and the now separate
sisters travel toward
opposite poles.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• In telophase II, separated sister chromatids
arrive at opposite poles.
– Nuclei form around
the chromatids.
• Cytokinesis separates
the cytoplasm.
• At the end of meiosis,
there are four haploid
daughter cells.
Fig. 13.7
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Primordial Germ Cells (PGCs) retain the ability to undergo meiosis
Frogs & Drosophila - Germ plasm/polar granules
determination of germ line and inhibition of somatic gene expression
m-RNA/protein in vegetal/posterior cytoplasm from egg
Mammals-future posterior region of embryo/extraembryonic region
gene expression and interaction produces PGCs which then migrate to
the gonads
Primordial
germ cell
PGC
Fig. 46.11
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
 Sperm structure:
 Haploid nucleus.
 Tipped with an acrosome.
 Contains enzymes that help the
sperm penetrate to the egg.
 A large number
of mitochondria
provide ATP to
power the
flagellum.
Fig. 46.12
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 46.13
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
March 11, 2004 Nature
For nearly a century, scientists have firmly believed that whereas men can produce sperm throughout their lives, women are born with all
the eggs they will ever have. But new research suggests that this basic tenet of reproductive biology is wrong, a discovery that could have
enormous repercussions for fertility treatment.
Egg cells, or oocytes, that are not fertilized are known to undergo a natural process of cell death. While investigating the effects of
chemotherapy drugs on fertility in mice, Jonathan L. Tilly of Massachusetts General Hospital and his colleagues began to count the normal
rate at which oocytes die. "We certainly didn’t set out two years ago to overturn dogma," says Tilly, lead author of the report detailing the
findings, published today in Nature. But to their surprise, the scientists found that the number of oocytes dying over a period of several
days was far greater than would be sustainable over the long term if the egg supply was not being replenished. In fact, at that rate, mice
would be fertile for just two weeks following birth, as opposed to more than a year.
The team subsequently conducted a series of experiments to verify the observation. Careful examination of the lining of adult mouse
ovaries revealed cells that closely resemble germline stem cells, those continuously dividing cells that give birth to oocytes. The
researchers determined that these ovarian cells express a protein that is associated with meiosis, the process by which sex cells divide.
When they transplanted the putative germline stem cells into a strain of transgenic mice whose cells all express a green fluorescent
marker, they found that the transplanted cells had divided and produced oocyte follicles in the host tissue. Tilly and his collaborators also
analyzed the effects of a chemotherapy agent called Busulfan on the mouse ovaries. The drug, which has been used to study sperm
proliferation, destroys the ability of male germline stem cells to divide into new sperm, but does not harm existing sperm. Several weeks
after injecting the ovaries with Busulfan, the researchers found that the number of oocytes had decreased dramatically. There was no sign
of the cell death that marks oocyte degeneration, however. The drug apparently targeted the female stem cells, preventing their ability to
produce oocytes.
Early in the 20th century, some scholars had suggested that eggs could in fact be replenished in adult mammals. But a 1951 study
definitively argued that egg numbers are determined at birth, shutting the door to further work for the next half century. "People were
viewing ovaries very differently than we do now," Tilly reflects. "The technology was just based on histological analysis." Although
biological markers for germline stem cells have since been developed, conclusions about mammalian ovaries were never reexamined
because they were believed to be "as sound as telling people that the sun sets in the west."
The team is now working to demonstrate the existence of germline stem cells in human ovaries, and it is confident that the finding will
carry over. The next step is to figure out which genes instruct a germ stem cell to differentiate into an egg. Ultimately, Tilly says,
transplanting these cells into the ovaries of menopausal women or women whose ovaries have been prematurely damaged by cancer
treatment could restore their fertility. --Alla Katsnelson
 Usually one follicle matures and releases its egg during each
menstrual cycle.
 After ovulation the remaining follicular tissue develops
into the corpus luteum.
 Secretes estrogens and progesterone.
 Maintain the uterine
lining during pregnancy.
 If pregnancy does not
occur the corpus luteum
disintegrates.
Growth of Primary Oocyte
Lampbrush Chromosomes 5% of genome
Increased number of Nucleoli
gene amplification
Increased number of Organelles
Yolk Production
Egg Types
Isolecithal
Mesolecithal
Telolecithal
Isolecithal
Centrolecithal
Gene for Twinning?
Fraternal (non-identical) twinning occurs when two eggs are ovulated simultaneously. Thus, the twins derive from two separate fertilization
events. Multiple ovulations are uncommon in humans, cattle, and many breeds of sheep. (Obviously multiple births are normal in mice,
which have eight to sixteen live births at a time, depending on the strain of mouse). It has long been thought that pituitary gonadotropin
levels can regulate the number of eggs that mature, but it has also been thought that the ovary must also be able to regulate egg maturation.
The clue to the ovarian regulator came in 1968, when Derek Weir, who farmed in the South Island of New Zealand, noticed he had a
prolific breeder on his hands. This particular ewe produced 33 lambs in 11 years. Hearing about a research project on good breeders, Weir
offered up his ewe. That sheep was used to start a colony, and the gene for multiple births was found to reside on the X chromosome of this
"Inverdale" strain. In 1998, the team heard of the work of Finnish researchers who had mapped the mouse and human versions of a GDF9B
to the X chromosome. The function of the GDF9B gene was not known, but it was known to be expressed in primary oocytes.
The New Zealanders thought the sheep equivalent of GDF9B (also known as BMP 15) might be causing the Inverdale effect. Using human
primers from the Finnish group, the New Zealand group used PCR to clone the homologous gene in the sheep. They demonstrated that the
BMP15 gene is expressed in the sheep oocytes and that there was a one base pair difference between the BMP15 genes of wild-type and
Inverdale sheep (Galloway et al. 2000; Griggs 2000). It appears that one dose of the mutant BMP15 gene causes twin and triplet births,
while homozygosity at that locus produces ovarian failure and sterility.
The BMP15 gene may possibly explain X-linked ovarian dysgenesis syndromes. In XO mice and humans (Turner’s syndrome), premature
ovarian failure is caused by the absence of an X chromosome. Thus, in addition to the pituitary factors regulating ovulation, the oocytes,
themselves, help regulate the number of eggs that will be released at ovulation. The basis for this cooperation has yet to be found.
Egg Envelopes
I. Produced within the ovary
Vitelline Membrane sea urchin, frog, bird
Zona pellucida mammal
II. Produced outside of the ovary
jelly
albumin & shell membranes