Gametogenesis:
The process of formation
of gametes from primordial germ cells of gonads is called gametogenesis.
Types of Gametogenesis:
a. Spermatogenesis
b. Oogenesis
a. Spermatogenesis:
The process of formation
of haploid, male gametes or spermatozoa is called spermatogenesis.
It takes place in the
seminiferous tubules of testis. It is completed in about 74 days. It is a continuous
process.
It involves three phases
as
i. Multiplication
phase ii. Growth phase iii. Maturation phase
i. Multiplication phase: Primordial
germ cell (2n) which are present in the germinal epithelium of seminiferous
tubules of testis divides mitotically arid produce large number of
spermatogonia.
ii. Growth phase: Spermatogonia
increases in size due to accumulation of nutritive materials and chromatin
material present in the cytoplasm and develops into primary spermatocyte (2n).
iii. Maturation phase: Primary
spermatocyte undergoes first meiotic division and forms two haploid cells
called secondary spermatocyte (n). Secondary spermatocyte undergoes second
meiotic division and forms two haploid cells called spermatids.
Thus each diploid
spermatogium produces 4 haploid spermatids.
Each spermatid
metamorphoses into sperm.
The process of
transformation of spermatids into spermatozoa or sperm is called
spermiogenesis.
( refer diagram from textbook )
Spermiogenesis shows following changes:
(a) Increased length
(b) Differentiation of centrioles
and proximal and distal centrioles.
(c) Development of axial
filament from distal centriole.
(d) Coiling of mitochondria
(e) Development of Golgi
complex into acrosome
(f) Sperms are nourished by sertoli cells.
Structure of human sperm:-
1.
Human sperm is microscopic, flagellated structure.
2. It is motile, haploid and unicellular structure.
3. It shows presence of four parts.
A.V. Leeuwenhoek (1677) first reported the discovery of sperm of man.
Head:
It is flat, oval on
surface view and spoon shaped on side view.
Its major portion is occupied by haploid large nucleus (n=23) and a
cap like acrosome.
Nucleus contains 40% DNA
and is rich in arginine.
Head is covered by a cap
like structure called Acrosomes which is formed by Golgi apparatus.
Acrosomes contains
proteolytic and lysosomal enzymes popularly called sperm lysin, e.g.
hyaluronidase, corona penetrating
enzyme, acrosin and zona lysin.
Head is covered on the
outside by a double membrane head cap.
The surface of head
contain adhesion (decapacitation factors) which have to be removed before it
becomes capable of fertilizing an ovum.
A sheath present around
acrosome and part of nucleus is called Galea.
Neck:
It is a narrow
constricted region between head and middle piece.
It contains the two
centrioles and cytoplasm.
The proximal centriole
is free located toward the nucleus which plays an important role in first
cleavage of the zygote.
The distal centriole
forms basal body for axoneme of tail.
Middle piece:
The middle piece of
sperms is cylindrical.
It contains large number
of mitochondria arranged helically around a central axis formed by flagellum.
Mitochondrial spiral of sperm is called in German language Nebenkern. At the
end of the middle piece there is a ring centriole (annulus) with unknown
function.
Tail:
Tail is 10 times longer
than middle piece.
It is longest part of
sperm.
It contains axial
filament running throughout the length.
Axial filament is formed
by 11 (9+2) microtubules.
It consists of two major
portions or regions.
Major portion of tail is
covered by sheath called main piece and uncovered part is called end piece.
Lashing movement of the
tail helps in movement of sperm.
Fertilizability or
viability of human sperm in female genital tract is 12 to 24 hours. Entire
sperm is surrounded by plasma membrane.
Functions:
1. It fertilizes the
ovum.
2. It helps to restore
diploid condition in zygote.
3. It transmits the
parental characters in the offspring
( refer diagram from textbook )
1)
What is gametogenesis?
2)
What are the two types of
cells present in sexually reproducing organisms?
3)
What is the role of
somatic cells?
4)
Which type of divisions
takes place in gametic cells?
5)
What is spermatogenesis?
6)
What is oogenesis?
7)
When the term embryo is
applied to zygote?
8)
When the term foetus is
used for embryo?
9)
What is the site of
spermatogenesis?
10)
What is the lining of
seminiferous tubule?
11)
What are the terms
applied to germinal cells?
12)
Name the different phases
of spermatogenesis in sequence?
13)
Name the type of division
which takes place in primordial cell?
14)
Name the cells produced
by primordial cells with their ploidy level?
15)
What is primary spermatocyte?
16)
What changes occur in primary
spermatocyte?
17)
Which cells are produced
from primary spermatocyte?
18)
Which division takes
place in secondary spermatocyte?
19)
Which cells are produced
from secondary spermatocyte?
20)
Give the chromosomal
status of different cells produced in spermatogenesis starting form primordial
cell to spermatid?
21)
What is the difference
between spermatid and spermatozoa?
22)
What is spermiogenesis or
spermiation?
23)
What is spermatozoan?
24)
What are the different
changes take place during spermiogenesis?
25)
Which structure forms
acrosome?
26)
Which structure
develops/forms axial filament?
27)
Which structure is called
as paternal gamete?
28)
What is the length of
human sperm?
29)
What is the viability
period for sperm?
30)
When a sperm can show
better fertilization?
31)
What are the different
parts of human sperm?
32)
What are the different
components of head?
33)
What does acrosome
secrete?
34)
What is the role of hyaluronidase?
35)
What is the term applied
to anterior covering of nucleus and what is its nature?
36)
What is term applied to
posterior covering of nucleus?
37)
Which structure is
present in the neck of sperm?
38)
What is the role of
proximal centriole?
39)
What is the role of
distal centriole?
40)
What are the different
terms applied to middle piece of sperm?
41)
What is nebenkern?
42)
What is the role of
middle piece?
43)
Which component produces
sperm tail?
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