Gametogenesis
The process of
formation of gametes is called as gametogenesis. It is basically of two types
viz
a) Spermatogenesis
b) Oogenesis
Spermatogenesis:
-
The process of formation of sperms i.e.
male gametes is called as spermatogenesis.
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It occurs in male gonads i.e. 1O
sex organ; testis.
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Development and maturation of sperms
takes place in seminiferous tubules inside testis.
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Seminiferous tubules are lined by
germinal epithelium which gives rise to germ cells i.e. gametes; sperms.
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Germinal epithelium cells divide and re-divide
to give rise to large number of cells called as Spermatogonia or Sperm Mother
Cell (SPM)
-
There are three phases in
spermatogenesis viz.
1. Phase
of multiplication
2. Phase
of growth
3. Phase
of maturation
1. Phase
of multiplication:
-
During this phase, germ cells undergo
repeated mitotic divisions to give rise to sperm mother cell (spermatogonia)
-
This process assures continuous
availability of spermatogonia.
-
It takes place at the basement membrane
of seminiferous tubule.
2. Phase
of growth:
-
Some of the spermatogonia cells move
towards the lumen of the seminiferous tubule.
-
The cells increase in size and become
larger.
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The increased size is due to
incorporation of nutrients to provide for succeeding stages of maturation.
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At this stage the cell is called as
Primary spermatocyte.
3. Phase
of maturation:
-
Each primary spermatocyte undergoes
maturation during which meiosis takes place.
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The first maturation is the completion
of first meiotic division i.e. reductional division.
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After reductional division secondary
spermatocyte is formed.
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Second maturation division i.e. second
meiosis is equational division which forms cells called as spermatids.
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Overall, four spermatids are formed from
single primary spermatocyte.
-
Spermatids develop into sperms after the
process of spermiogenesis.
-
Spermiogenesis:
§ It
takes place in the lumen of seminiferous tubules and the spermatid cells remain
attached to a nursing cell called as cell of Sertoli.
§ During
process of spermiogenesis, loss of cytoplasm takes place along with formation
of flagella.
§ When
the process of spermiogenesis is complete the mature sperm separates from cell of Sertoli and gains
entry to epididymis
Oogenesis:
-
The process of formation of female
gametes is called as oogenesis.
-
It takes place in primary sex organs of
females i.e. ovary.
-
has also three phases as that of
spermatogenesis. The phases are viz.
1. phase
of multiplication
2. phase
of growth
3. phase
of maturation
1. Phase
of Multiplication:
-
By primordial germ cells through
continuous cell division oogonia cell is formed.
-
Oogonia continue to multiply by mitosis
and after last division gives risae to primary oocyte.
2. Phase
of growth & Multiplication
-
This period is long in case of oogenesis
as compared to spermatogenesis.
-
In oogenesis, growth and maturation
takes place simultaneously.
-
During growth phase oocyte is completely
surrounded by special cells called as follicle cells.
-
In mammals these cells are derived from
germinal epithelium of ovaries.
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Initially young oocyte is surrounded by
single layer of follicle cells which are simple cuboidal epithelial cells.
-
Later on, number of follicle cells
increase greatly, the cells get arranged in several rows.
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When an egg approaches maturity, an
eccentric cavity appears in th mass of follicle cells.
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The cavity is filled by fluid secreted
by follicle cells
-
These follicle assist the growth of
oocyte by secreting substances which are taken up by oocyte.
-
Along with this the nuclei entres the
first meiotic division.
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Simultaneously the process of
vitellogenesis also initiate.
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In the process of vitellogenesis yolk is
incorporated in the developing ovum.
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Growth of ovum is much accelerated
during this period.
-
By this time first meiotic division is
complete and two cells are formed. One cell receives all the yolk and cytoplasm
and represents the secondary oocyte
while other cells does not receive any cytoplasm and is called as first polar
body.
-
Secondary oocyte again undergoes the
second meiotc division to give rise to haploid ovum.
-
This second meiotic division is also
unequal resulting in formation of mature ovum and second polar body. The second
meiotic division also takes place in first polar body and it gives rise to
third and fourth polar bodies.
-
Therefore, at end of oogenesis, a fully
mature ovum along with three polar bodies are formed.
Differences
in Spermatogenesis and Oogenesis:
|
Spermatogenesis
|
Oogenesis
|
|
1.
All spermatogonia are active and enter phase of growth.
|
1.
Few oogonia are active and very few or one enters
the phase of growth
|
|
2.
The first meiotic division is equal and two equal
secondary spermatocytes are formed.
|
2.
First meiotic division is unequal and one large
secondary oocyte and first polar body is formed.
|
|
3.
Second meiotic division is equal and equal
spermatid cells are formed.
|
3. Second meiotic division is unequal and
ovum and total three polar bodies are
formed.
|
|
4.
Spermatids undergo process of differentiation
before they become spermatozoa.
|
4.
Ovum does not undergo any maturation
division.
|
|
5.
Sperms become capable of fertilizing the egg some
time after completion of reduction divisions and differentiations.
|
5. Ovum becomes capable of being
fertilized even before it has completed
its
second meiotic division
|
|
6.
One spermatogonium form four sperms
|
6.
One oogonium cell forms one ovum
|
|
7.
Motile gametes are formed
|
7. Immotile gamete is formed
|
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