Gametogenesis
Gametogenesis
is a process by which diploid or haploid cells undergo cell division and
differentiation to form mature haploid gametes.
Gametogenesis:-
The process of gametes formation: It occurs in the gonads
(ovary or testis)
Gametogenesis
is divided in to four phases:-
1. Extra-gonadal origin of primordial germ cells
2. Proliferation of germ cells by mitosis
3. Meiosis
4. Structural and functional maturation of ova and
spermatozoa
Gonial
cells are the designation given to germ cells before they
enter meiosis
•Replicate mitotically.
•Genetically identical to the original zygote.
When they begin meiosis, they are called spermatocytes and oocytes.
Gametogenesis:-
·
Spermatogenesis
·
Oogenesis
Spermatogenesis:
Spermatogenesis is the process of formation of the male germ
cells (sperm formation).It occurs in the seminiferous tubules of the testis. It
is thread-like in shape.
Spermatogenesis occurs from puberty to old age, producing
immense numbers of spermatozoa at an average rate of 1.5 million spermatozoa
per minute. The sperm will mature in the epididymis, nourished by sertoli cells
for up to 10weeks. Humans aged 13-90 can make 1 billion sperm a day.
The sertoli cells are
supporting cells that have several functions. They form the blood-testes
barrier: nutrients, and circulating substances do not directly reach the germ
cells the sertoli cells determine which substances reach the germ cells the
spermatogonia are outside the blood-testis barrier. They also produce
antigen-binding proteins, which are necessary for spermiogenesis (morphological
development of spermatids to spermatozoa).
Spermatogenesis is the process by which male primary sperm cells
undergo meiosis, and produce a number of cells termed spermatogonia, from which
the primary spermatocytes are derived. Each primary spermatocyte divides into
two secondary spermatocytes, and each secondary spermatocyte into two
spermatids or young spermatozoa. These develop into mature spermatozoa, also
known as sperm cells. The primary spermatocyte gives rise to two cells, the secondary
spermatocytes, and the two secondary spermatocytes by their subdivision produce
four spermatozoa.
Spermatogonium:-
It is an intermediary male gametogonium(a kind of germ cell).Diploid
in number: 44 autosomes and 2 sex cells. It is very small and under the process
of development. Undergoing G1-S.
Primary
spermatocytes:-
·
Diploid in number: 44
autosomes and 2 sex cells
·
Increases in size
·
Undergoing G2.
Secondary
spermatocytes:-
·
Haploid in number: 22
autosomes (double stranded) and 1 sex cell each.
·
Can see the cleavage furrow
·
Undergoing Meiosis I
(Reduction Division)
·
Chromosomes are reduced into
half number.
Spermatid:
Haploid in number: 22 (single stranded) autosomes and1 sex
cell.
Undergoing Meiosis II (Equational Division)
Spermatozoa:-
It is a motile sperm cell, or moving form of the haploid cell
that is the male gamete. It joins an ovum to form a zygote. Matured Sperm Cell
are ready to fertilize the egg.
Oogenesis:-
Oogenesis is the process of formation of the female germ
cells (egg formation).It happens in the Ovary. It is Spherical in shape.
Oogenesis begins in
early foetal life. All oocytes formed in females are produced during foetal
life. Many of them degenerate with time and at birth the ovaries contain about
2 million oocytes. All the oocytes go into meiotic arrest when they reach the
first meiotic division during foetal life. The primary oocytes remain in the
prophase of the first meiotic division until the time of puberty, when they are
gradually released to complete meiosis at regular intervals known as the ovarian
cycle. On the average only one oocyte matures during each cycle, which occurs
at approximately monthly intervals, so that the total amount of oocytes to be
ovulated is about 500 oocytes in a lifetime.
Oogonium/oogonia:-
Diploid in number: 44 autosomes and 2 sex cells
It is very small and under the process of development.it
under goes GI-S phase.
Primary
oocytes:-
It is a cell whose function is to divide by the process of
mitoses. It is diploid in number: 44 autosomes and 2 sex cells. Increasing in
size. Undergoing G2 phase.
Secondary
oocytes:-
Undergoing meiosis I (reduction division). Chromosome are
reduce into half number. Begins during embryonic development but halts in
deplotene stage of prophase I until puberty. Primary oocytes that continue to
develop in each menstrual cycle, however, synapsis occur and tetrads form,
enabling chromosomal crossover to occur. Result of meiosis I, the primary oocyte
has now developed into secondary oocyte and first polar body.
Ootid
/ ovum:-
After meiosis I, the haploid secondary oocyte initiate meiosis
II (equational division). This process is halted at metaphase II stage until
fertilization. If such should occur. When meiosis II is completed, an otid and another
polar body have been created.
Polar
bodies:-
Both polar bodies are disintegrate at the end of meiosis,
leaving only the ootid, which then eventually undergoes maturation into a
mature ovum.
The function of
forming polar bodies is to discard the extra haploid sets of chromosome that
have resulted as a consequence of meiosis.
Gametogensis in human
Gametogensis in animals
Animals produce gametes directly through
meiosis in organs called gonads (testicles in males and ovaries in females).
Males and females of a species that reproduces sexually have different forms of
gametogenesis
·
spermatogenesis
(male)
·
oogenesis
(female)
Stages:-
However, before turning into gametogonia, the embryonic
development of gametes is the same in males and females.
Common
path:-
Gametogonia are usually seen as the initial stage of
gametogenesis. However, gametogonia are themselves successors of [[primoweranonymousment,
primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate
along the hindgut to the gonadal ridge. They multiply by mitosis, and, once
they have reached the gonadal ridge in the late embryonic stage, are referred
to as gametogonia. Once the germ cells have developed into gametogonia, they
are no longer the same between males and females.
Individual
path:-
From gametogonia, male and female gametes develop differently
- males by spermatogenesis and females by oogenesis. However, by convention,
the following pattern is common for both:
|
Cell type
|
chromosomes in humans
|
chromatids in human
|
Process
|
|
Gametogonium
|
diploid
(2N)/46
|
2C before
replication, 4C after / 46 before, 46 X 2 after
|
gametocytogenesis
(mitosis)
|
|
primary gametocyte
|
diploid
(2N)/46
|
2C before replication, 4C after /
46 before, 46 X 2 after
|
gametidogenesis
(meiosis I)
|
|
secondary
gametocyte
|
haploid
(N)/23
|
2C / 46
|
gametidogenesis (meiosis II)
|
|
Gametid
|
haploid
(N)/23
|
C / 23
|
|
|
Gamete
|
haploid
(N)/23
|
C / 23
|
|
In
vitro gametogenesis:-
In vitro gametogenesis (IVG) is the technique of developing
in vitro generated gametes, i.e., "the generation of eggs and sperm from
pluripotent stem cells in a culture dish” This technique is currently feasible
in mice and will likely have future success in humans and nonhuman primates.
In
gametangia:-
Fungi, algae, and primitive plants form specialized haploid
structures called gametangia, where gametes are produced through mitosis. In
some fungi, such as the Zygomycota, the gametangia are single cells, situated
on the ends of hyphae, which act as gametes by fusing into a zygote. More
typically, gametangia are multicellular structures that differentiate into male
and female organs:
·
Antheridium
(male)
·
Archegonium
(female)
In flowering plants:-
In angiosperms, the male gametes (always two) are produced
inside the pollen tube (in 70% of the species) or inside the pollen grain (in
30% of the species) through the division of a generative cell into two sperm
nuclei. Depending on the species, this can occur while the pollen forms in the
anther (pollen tricellular) or after pollination and growth of the pollen tube
(pollen bicellular in the anther and in the stigma). The female gamete is
produced inside the embryo sac of the ovule.
Gametogenesis in
flowering plant
