Outline Questions
Understanding Gametogenesis
Origin or formation
Gamete: from Greek
gametē ‘wife,’ gametēs
‘husband,’ from gamos
‘marriage.’
•  What is cell differentiation?
–  What stages do cells go through on their way to a final role?
–  What are stem cells?
–  Why are they important? How does meiosis create cells capable of
fusing to form the next generation?
•  How is sperm produced in mammals?
•  How are oocytes produced in mammals?
•  What are the structures needed in mammalian systems to allow
gametogenesis?
•  How is timing regulated for gametogenic events (ie ovulation)?
Where are we in the Big Picture?
Page 336 in 4th edition
Back to how to make gametes.
Why create haploid cells?
“hap” sounds like “half”
MEIOSIS
Amount of hereditary material
is reduced by half
In animals, meiosis occurs prior
to production of eggs and sperm
Diploid adult (2n)
MITOSIS
Haploid
Egg
Sperm
gametes (1n)
Zygote (2n)
Cell divisions responsible for growth
(addition of somatic cells)
Normal amount of
hereditary material
is restored
Making gametes sets the stage for forming a zygote, which
creates all cell tyes
Conrad Hal Waddington 1905-1975
Landscape of development
Cellular Differentiation
Role determined
All fates possible
Mitosis, signaling, construction, etc
(4 loosely-defined stages:)
No structures built
Specific structures
potent
(all fates possible)
potent
(multiple fates
are possible)
(one fate possible,
not yet structured)
(recognizably structured
for a single cell fate)
Meiosis Review
MITOSIS
MEIOSIS
Diploid parent cell
Diploid parent cell
Chromosome replication
Chromosome replication
Prophase I
Prophase
Nuclear envelope breaks down;
chromosomes condense.
Tetrads form by the
synapsis of homologous
chromosomes.
Crossing over occurs.
Metaphase
Individual chromosomes
align at the metaphase plate.
Anaphase and Telophase
Metaphase I
Homologous pairs
align at the
metaphase plate.
Sister chromatids separate;
nuclear envelope re-forms.
Anaphase I and Telophase I
Homologous chromosomes
separate; two haploid cells result.
Meiosis II
Two diploid daughter cells of mitosis
Sister chromatids
separate.
Two (two identical
cells) and miTosis
both have Ts
Four haploid daughter cells of meiosis
Spermatogenesis
Spermatogonium (2n)
(May divide by mitosis to
form more spermatogonia)
Mitosis and
initial differentiation
Meiosis I
Front view
Primary
spermatocyte
(2n)
Secondary
spermatocyte
(1n)
Meiosis II
Urinary
bladder
Seminal
vesicle
Prostate gland
Spermatids
Bulbourethral
(1n)
gland
Vas deferens
Epididymis
Mature
sperm
cells (1n)
Testis
Oogenesis
Oogonium (2n)
(similar to
spermatogonium)
Mitosis and
initial differentiation
Meiosis I
Secondary
oocyte (1n)
Meiosis II
Primary
oocyte (2n)
1st
polar
body
2nd polar (1n)
body (1n)
Oviduct
Ootid (1n)
Uterus
Mature egg cell
(ovum) (1n)
Front
view
Ovary
Comparing Spermatogenesis and Oogenesis
Spermatogonium
(2n)
Primary
spermatocyte
(2n)
Secondary
spermatocyte
(1n)
Stem cells (2n)
Mitosis and
differentiation
Primary
oocyte
(2n)
Meiosis I
Meiosis II
Spermatids
(1n)
Mature
sperm
cells (1n)
Oogonium
(2n)
Secondary oocyte +
polar body (1n)
Ootid + polar body (1n)
Mature cells (1n)
Mature egg cell (ovum) (1n)
Female reproductive system:
Ovarian production of mature oocytes
Secondary oocyte
to oviduct
5. Degeneration of
corpus luteum
4. Ovulation
Oocytes
Follicle
cells
1. Formation of primary
oocytes within follicles
2. Follicle growth
3. Maturation of
follicle
Hormonal control of oogenesis/menstruation
Ovulation
Progesterone
Estradiol
Ovarian
hormone
cycle
Lutenizing Hormone
Pituitary
hormone
cycle
Menstruation
Follicle-Stimulating
Hormone
Menstrual
(uterine)
cycle
Days
0
7
14
21
28
Hormone Interactions
Ovulation
Progesterone
HIGH ESTRADIOL
Estradiol
LH
and
FSH
Lutenizing Hormone
Progesterone
low estradiol
Menstruation
Days
0
7
Follicle-Stimulating
Hormone
14
21
28
Key Concepts
•  Cells must differentiate as they divide and grow to
become all possible cell types in an organism.
•  Totipotent cells like gametes have the potential to become any
type of cell lineage
•  Pluripotent cells are stem cells that become one of a number of
possible final cell types (though not all options are available)
•  Determined cells have received the messages necessary to
decide their final cell fate but may not be completely
constructed for this role yet
•  Differentiated cells have built final specific necessary structures
•  Meiosis uses two rounds of division to create haploid cells
with new genetic combinations for the next generation
•  Spermatogenesis occurs in the testes and produces many
thousands of equal, small, viable sperm. These sperm are
maintained in protective seminal fluids.
•  Oogenesis is precisely controlled by hormonal signals.
These signals allow one human oocyte per month to mature
into a secondary oocyte which is released from the ovary
and implanted in the uterus.