Oogenesis

1. Oogenesis
Dr. Ghizal fatima

2. • Oogenesis
• Oogenesis, ovogenesis, or oögenesis is the
differentiation of the ovum into a cell competent
to further develop when fertilized. It is developed
from the primary oocyte by maturation.
Oogenesis is initiated in the embryonic stage.
• Oogenesis, in the human female reproductive
system, is a growth process in which the primary
egg cell (or ovum) becomes a mature ovum,
These cells, known as the primary ova, number
around 400,000. The primary ova remain
dormant until just prior to ovulation, when an
egg is released from the ovary.

4. • the first part of oogenesis starts in the germinal
epithelium, which gives rise to the development of ovarian
follicles, the functional unit of the ovary.
• Oogenesis consists of several sub-processes:
oocytogenesis, ootidogenesis, and finally maturation to
form an ovum (oogenesis proper). Folliculogenesis is a
separate sub-process that accompanies and supports all
three oogenetic sub-processes.
• Oogonium —(Oocytogenesis)—> Primary Oocyte —
(Meiosis I)—> First Polar body (Discarded afterward) +
Secondary oocyte —(Meiosis II)—> Second Polar Body
(Discarded afterward) + Ovum
• Oocyte meiosis, important to all animal life cycles yet unlike
all other instances of animal cell division, occurs completely
without the aid of spindle-coordinating centrosomes.

5. • The creation of oogonia
• The creation of oogonia traditionally doesn't
belong to oogenesis proper, but, instead, to
the common process of gametogenesis, which, in
the female human, begins with the processes
of folliculogenesis, oocytogenesis,
and ootidogenesis. Oogonia enter meiosis during
embryonic development, becoming oocytes.
Meiosis begins with DNA replication and meiotic
crossing over. It then stops in early prophase

6. • Oogenesis
• Oogenesis starts with the process of developing
primary oocytes, which occurs via the
transformation of oogonia into primary oocytes,
a process called oocytogenesis.[13] Oocytogenesis
is complete either before or shortly after birth.
• There are three phases to oogenesis; namely,
multiplication phase, growth phase and
maturation phase.

7. • Multiplication Phase:
• During foetal development, it should be noticed
that certain cells present in the germinal
epithelium of the female ovary are bigger than
others.
• Hence, these cells split by mitosis, creating a
couple of million oogonia or mother egg cells in
each ovary present in the foetus. There are no
more oogonia which are formed or augmented
after birth.

8. • Growth Phase:
• This particular procedure of the primary oocyte tends
to be very long. In this, the oogonium nurtures into
bigger primary oocytes.
• After this, each primary oocyte gets surrounded by a
granulosa cells layer to create primary follicle. Later, a
large number of follicles get debased during the
duration from birth to puberty.
• Therefore, at puberty around 60,000 to 80,000
primary follicles can be found in each ovary.

9. • Maturation Phase:
• Similar to a primary spermatocyte, every primary
oocyte experiences two maturation divisions.
However, the outcomes of maturation divisions
under oogenesis are quite different to those
which occur in spermatogenesis.
• Considering the first meiotic division, the primary
oocyte segregates into two uneven haploid
daughter cells. These are known as the large
secondary oocyte and a small polocyte.

10. • Later, considering the second maturation
separation, the initial polar body might split to
create two, second polar bodies. Here, the
secondary oocyte once again divides to form
unfit daughter cells.

11. • Process of Oogenesis
• The process of oogenesis is completed in the
following three stages:
• Pre-natal Stage
• The primary oocyte grows while being arrested in
meiosis-I. The follicular cells proliferate and form
a stratified cuboidal epithelium. Such cells are
known as granulosa cells. These cells secrete
glycoproteins to form zona pellucida around the
primary oocyte.

12. • Antral Stage
• The fluid-filled area, present between
granulosa cells, combines to form a central
fluid-filled space called the antrum. These are
known as secondary follicles. In every month
cycle, these secondary follicles develop under
the influence of follicle-stimulating hormone
and luteinizing hormone.

13. • Pre-Ovulatory Stage
• This stage is induced by LH surge, and meiosis-I completes
here. Two haploid cells of unequal sizes are formed within
the follicle.
• One of the daughter cells that receive less cytoplasm forms
a polar body. This cell does not participate in ovum
formation.
• The other daughter cell is known as the secondary oocyte.
• The two daughter cells undergo meiosis-II. The polar body
replicates to form two polar bodies, while the secondary
oocyte arrests in the metaphase stage of meiosis-II.

14. • Ovulation
• Development of oocyte takes place in ovaries.
Every oocyte is neighboured by follicle cells to
form a follicle.
• As the menstrual cycle starts, primary oocytes
initiate to grow bigger, and follicle cells rise in
number, causing the follicle to grow larger too.
• Normally, some nurturing oocytes degenerate
and leave just one follicle to mature. Here,
fraternal twins may be born, which are distinct
genetically.

15. • When a follicle attains maturity, the primary
oocyte finishes its primary meiotic division
and becomes secondary oocyte. Soon after,
the follicle breaks and secondary oocyte is
liberated in the fallopian tube even when the
second meiotic division has not happened.
This release of a secondary oocyte from
ovaries is known as ovulation.