Gametogenesis
Developmental Biology
Gilbert 8th Ed. pp. 612- 617
9th Ed. Pp. 607-613
Today’s Objectives
• Identify the process by which meiotic
divisions lead to both male and female
mammalian games
• Identify the following important
components of the process of
fertilization: gametes, spermatogonia,
flagellum, tubulin, oocyte, pronuclei,
vitelline membrane, zona pellucida
Gametogenesis
• Process of creating gametes
• Germ cells are set aside early in
development
• Migrate to the gonad
• Undergo meiotic divisions to make
haploid germ cells
Meiosis - A review
• What is the ploidy of the somatic cell that will
undergo meiosis to form gametes?
• How many cell divisions take place?
• What happens to the genome before the first
division?
• What are the phases in each meiotic
division?
• How many daughter cells are made?
• What is the ploidy of those daughter cells?
Gametogenesis in Mammals
• Spermatogenesis
– Process of producing sperm
– Occurs in seminiferous tubules of the
testes
• Oogenesis
– Process of producing oocytes
– Occurs in ovaries
Spermatogenesis
1) Spermatogonia are the germ cells that will
eventually develop into the mature sperm; the first
step in this development is the duplication of
homologous chromosomes to get ready for meiosis
2) Primary spermatocyte :the first meiotic division
separates the homologous chromosomes from each
parent
3) Secondary spermatocyte :the second meiotic division
separates the 2 chromatids and creates 4 haploid
cells
4) Spermatids: Will produce 4 sperm cells by the
process of spermiogenesis.
Sperm cells differentiate into the shape we
commonly know(will talk more about structure next
time)
Simplified view
of
spermatogenesis
Spermatogenesis (more detail
- don’t memorize!)
When are sperm made in
mammals?
• In males, the spermatogonia enter meiosis and
produce sperm from puberty until death.
• The process of sperm production takes only a few
weeks.
• 100 to 500 million sperm can be released at once.
Oogenesis
1) Oogonia are the germ cells that will eventually
develop into the mature oocytes
2) Primary oocyte : Undergoes the first meiotic division,
separating homologous chromosomes
3) Secondary oocyte :the second meiotic division
separates the 2 chromatids and creates 4 haploid
cells
4) Egg:
• In females, it produces 1 egg and 3 polar bodies.
This allows the egg to retain more cytoplasm to
support early stages of development
• Simplified view
of Oogenesis
When does Oogenesis occur
in mammals?
• In females, this process is more complex than
in males
• The first meiotic division starts before birth
but fails to proceed.
• 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.
• Why not? What could happen?
• Only I egg produced per month (usually)
• What event provides an example of a human the
exception to this?
• In addition, all meiosis is ended in females at
menopause.
In older women, failure of the homologous chromosomes
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.
# of female germ cells over
time
• Next time, we’ll examine the unique
structure of each gamete