Gametogenesis
The process of formation of gametes i.e. sperms and ova in
the primary sex organs i.e. Testes and Ovaries is called gametogenesis. It is
of two types :-
The process of formation
of sperms in the testes is called spermatogenesis.
The process of formation of ovum inside the ovary is called oogenesis.
Spermatogenesis
It is the process of formation of haploid male gametes
(spermatozoa) from diploid spermatogonia. In humans, it starts at puberty.
This
process is divided into two stages :-
- Formation of spermatids.
- Formation of spermatozoa from spermatids.
Formation of spermatids
The male germinal cells which produce sperms are called
primary germinal cells or primordial cells. They are present inside
seminiferous tubules. The process of spermatid formation occurs in 3 phases :-
- Multiplication phase
- Growth phase
- Maturation phase
The sperm mother cells or
spermatogonia are formed by repeated mitotic divisions of the primordial cells.
The spermatogonia are diploid. The spermatogonia may be of two types; type ‘A’ or stem cells and type ‘B’.
Growth phase
Type B-Spermatogonia stop dividing and grow
in size by accumulating cytoplasm. Now each grown spermatogonia is called
primary spermatocyte.
Maturation phase
Each primary spermatocyte undergoes first
meiotic division and produces two haploid secondary spermatocyte. Each
secondary spermatocyte undergoes 2nd meiotic division and produces two
spermatids. Hence from a single spermatogonia 4- haploid spermatids are formed.
These are non motile and rounded
Formation of spermatozoa
The transformation of a non – motile, rounded spermatid into
motile sperm is called spermiogenesis. It involves the movement of cell
organelle to specific positions, loss of water, cytoplasm and golgi apparatus
and changes in cell organelles. The fully formed tailed sperm becomes free in
the cavity of seminiferous tubules. The sertoli cells provide mechanical
support, nourishment, help in transportation to the sperms. After getting
nourishment from sertoli cells the spermetazoa become active and are released
from sertoli cells. This process is called spermiation.
Oogenesis
Oogenesis is the formation of functional haploid ovum from
the diploid germinal cells of the ovary.
Egg formation starts during embryonic developmental stage
but is completed only after fertilization.
Oogenesis is completed in three phases:-
- Multiplication phase
- Growth phase
- Maturation phase
Multiplication phase
During embryonic development, certain larger cells of the germinal epithelium of
ovary function as germ cells. These undergo repeated mitotic divisions to produce
diploid egg mother cells called oogonia. Oogonia multiply mitotically and
resembles like a cord and is called egg tube of pfluger, which later forms a
round mass called egg nest.
Growth phase
One cell (oogonium) in the egg net grows and becomes the
primary oocyte. The other oogonia form the nutritive follicular epithelium
around the primary oocyte. This str. is called the primary follicle. Growth phase is very long phase. In human it
takes about 12-13 yrs. A large number of primary follicles are degenerated
during the period from birth to puberty. Therefore at puberty only 60-80
thousands primary follicles are left in the ovary.
Maturation phase
After attainment of puberty the primary follicles and
primary oocyte in it resumes development. The primary follicle changes into
secondary and the secondary follicle into the tertiary follicle.
The primary oocyte grows further and completes meiosis I. It
is divided into two unequal parts, one larger secondary oocyte and one smaller
polar body. The tertiary follicle further changes into a Graafian follicle.
The
secondary oocyte forms a new membrane the Zona pellucida around it.
The Graafian follicle ruptures and the secondary oocyte is
released from the ovary and this process is called ovulation.
The second miotic division is completed only after the
secondary oocyte reaches the oviduct and is fertilized. Then it produces
another polar body and a larger ovum or ootid. The polar bodies take no part in
reproduction and may degenerate due to lack of cytoplasm.
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