2. • A gamete is either the sperm of the egg
• They are haploid meaning that the contain half the
usual number of chromosomes
• All the other cells are called somatic cells
• From somatic cells, some cells will become different and
undergo meiosis rather than mitosis. These cells form
the germ cell lineage
3. Chromosomal
Sex
Determination
• In most mammals, females have two X
chromosomes and males will have XY.
• This is not always the case in animals
• In birds it is reversed
• In some animals it makes no
difference
4. Mammalian Patterns of Sex
Determination
• The Y chromosome is critical.
• Meiosis in males produce sperm that is half X and half Y.
• In the rare case that there is XXY instead of XY, the
phenotype is male
• In the rare case where it is XO (just one X chromosome),
the phenotype is female, but their ovaries can’t be
maintained. The second X is needed.
5. What do the X
and Y
chromosome
do?
• Gonads in humans are bipotential
• in other words, they can develop into either testes or
ovaries
• XX gonadal cells activate a pathway called Wnt
• This produces a transcription factor called β-Catenin
which promotes the development of follicular cells in
the ovary
• XY gonadal cells activate a gene that encodes a Sry
transcription factor. This, in turn activates a Sox9 gene.
• The Sox9 gene ultimately causes the gonads to become
testes
• Leydig cells of the testes produce testosterone
• Sox9 also represses the Wnt pathway
6. Primary Sex
Determination
The determination of the gonad into
male or female is referred to as
Primary Sex Determination
Once the primary sex determination
is established in the gonads, the
hormones they make will cause
secondary sex determination.
7. Gonadal Sex Determination in
Mammals
Rudiments (immature parts of the organ) will become the
same mature organ.
• For example, rudiments of the kidney will become a full kidney
• The exception is the gonadal ruminant. It can become either testes or
ovaries
There are two gonadal ruminants found in weeks 4-7
• At this time they are not a particular sex
Germ Cells (precursors to sperm or eggs) migrate into gonads
in week 6. They are surrounded with mesodermal cells
8. If the fetus is
XY
Some of the mesoderm cells will become
Sertoli cells. There Sertoli cells will secrete
Anti-Mullerian Hormone (AMH).
AMH blocks development of female ducts
and they form the seminiferous tubules that
will make sperm
In week 8 the Sertoli cells start to organize
themselves into testes cords. These cords
will mature into Seminiferous tubules.
The other mesodermal cells (that didn’t
become Sertoli cells) start to become Leydig
cells which will produce testosterone
9. If the fetus is
XX
The germ cells start to make clusters
surrounded by Pre-Granulosa Cells.
The germ cells go through meiosis
and eventually become oocytes.
The cells surrounding them become
granulosa cells.
Other cells become Thecal Cells.
These cells secrete estrogens
10. Secondary Sex
Determination
in Mammals
• This is in response to hormones secreted by wither ovaries
or testes
• There are two times when this happens:
• Once during organogenesis and again in puberty
• In females: Mullerian ducts in the female and Wolffian
ducts in the male are both in the embryo.
• In females, the Mullerian ducts persist, and estrogen will
cause them to differentiate into the uterus, cervix, upper
vagina, and oviducts.
• The genital tubercle (a precursor to the external
genitalia) become the clitoris
11. In males:
• There are two hormones involved:
• Anti-Mullerian Hormone (produced by Sertoli cells)
cause degeneration of the Mullerian ducts.
• Leydig cells secrete testosterone which cause the
Wolffian ducts to differentiate into the vas deferens and
epididymis.
• Genital ducts become the penis
12. Gametogenesis in
Animals
• Gametogenesis is the differentiation of
germ cells into gametes (egg or sperm)
• Primordial Germ Cells are bipotential
precursors of egg and sperm
• Germ cells are set aside in the embryo.
They migrate from peripheral sites in the
embryo to the gonad.
13. Primordial
Germ Cells
(PGC) in
Animals
PCG’s enter the hindgut and
migrate to the bipotential gonads.
There are cells surrounding them
which secrete stem cell factor. SCF
is needed for motility
When the PCG’s are in the gonad,
proteins called BMPs make a niche
for them in the genital ridge.
14. Meiosis • A single cell divides twice to make four daughter cells
• The 4 daughter cells are haploid
• Meiosis produces gametes
• During interphase the DNA essentially doubles. We end up
with
• 46 pairs of chromosomes
• 23 pairs is the usual number, but it is doubled in
interphase.
• In meiosis I (during prophase I), homologous chromosomes
are going to exchange some of their DNA (crossing over)
• It then goes through another round of meiosis (meiosis II) and
they separate.
• This is important because it is the source for our genetic
diversity
15. Meiosis in
gonads
Meiosis is different in females and males in
gonads.
In females it starts in the embryonic gonads.
In males it does not start until puberty
In this case, meiosis is probably controlled by
stra8 transcription factor.
In developing ovaries, Stra8 is upregulated.
In developing testes it is downregulated.
16. Spermatogenesis
in Mammals
Occurs in puberty, not in
embryonic development
There are three phases:
• Proliferative Phase- sperm cells increase
by way of mitosis
• Meiotic Phase- creates the haploid state
through the two meiotic divisions
• Post Meiotic Phase- the spermatids eject
most of their cytoplasm to be more
streamlined
17. Oogenesis in
Mammals
• PCG’s reach the developing ovary and create about 7
million oogonia.
• Most of them will die, but some will become primary
oocytes.
• They stay suspended in prophase I in meiosis
• During puberty, One oocyte will undergo meiosis to make
the secondary oocyte.
• This secondary oocyte will stay stuck in metaphase II
• During fertilization, the secondary oocyte will finish meiosis
II to make the ovum.
