3. HUMAN REPRODUCTION
2. Oogenesis
- It
is the process of formation and maturation of ovum.
- It
takes place in Ovarian follicles.
-
Oogenesis is
initiated in embryonic stage when 2 million of egg mother cells (oogonia)
are formed within each ovary.
-
No more oogonia are formed and added after
birth.
-
Oogonia multiply to form
primary oocytes. They enter prophase-I of the meiosis and
get temporarily arrested at that stage.
-
Each primary oocyte gets surrounded by a
layer of granulosa cells to form primary follicle.
-
Many primary follicles degenerate during the
phase from birth to puberty. Therefore, at puberty, only 60,000-80,000
primary follicles are left in each ovary.
-
Primary follicles get
surrounded by more layers of granulosa cells and a new theca to form secondary
follicles.
- The
secondary follicles transform into a tertiary follicle. It has a fluid
filled cavity (antrum). The theca layer forms an inner theca interna
and an outer theca externa.
- The
primary oocyte in tertiary follicle grows and undergoes first unequal meiotic
division to form a large secondary oocyte (n) & a tiny first
polar body (n). So, secondary oocyte retains nutrient rich cytoplasm of
primary oocyte.
- It
is unknown that whether the first polar body divides further or degenerates.
-
The tertiary follicle further changes into
the mature follicle (Graafian follicle).
- Secondary oocyte forms a new membrane (zona pellucida).
- Graafian
follicle now ruptures to release the secondary oocyte (ovum) from the
ovary. This is called ovulation.
Schematic representation of oogenesis
Structure of ovum (egg)
- Spherical
and non-motile. About 0.2 mm in diameter.
- Ovum
has 3 membranes:
a. Plasma membrane:
Innermost layer.
b. Zona pellucida: Outer to the plasma membrane.
c. Corona radiata: Outer
layer formed of follicle cells.
Spermatogenesis & Oogenesis- A comparison
Spermatogenesis
Oogenesis
Occurs in testis.
Occurs in ovary.
Limited growth phase.
Elaborated growth phase
Each primary spermatocyte gives 4 sperms.
Each primary oocyte gives one ovum.
No polar body formation.
Polar bodies are formed.
Begins at puberty and extends up to senility.
Begins at embryonic stage but suspends up to puberty. It ceases around the age of fifty.