2. GAMETOGENESIS
• The process involved in the maturation of the two highly
specialized cells, spermatozoon in male and ovum in female
before they unite to form zygote, is called gametogenesis.
3. OOGENESIS
The process involved in the development of a mature ovum
The primitive germ cells take their origin from the yolk sac at end
of 3rd week
their migration to the developing gonadal ridge is completed at
end of 4th week.
The germ cells undergo rapid mitotic divisions and differentiate
into oogonia.
The number of oogonia reaches its maximum at 20th week,
numbering about 7 million.
4. • At birth, no more mitotic division and all the oogonia are
replaced by primary oocytes finished the prophase of meiosis I
and remain in resting phase between prophase and metaphase.
• Total number of primary oocytes at birth - 2 million.
• At puberty, some 400,000 primary oocytes are left behind, the
rest being atretic.
• The primary oocytes do not finish the first meiotic division
until puberty is reached,
• Out of these, some 400 are likely to ovulate during the entire
reproductive period.
5. Maturation of the oocytes:
• The essence of maturation is reduction of the number of
chromosomes to half.
• Before the onset of first meiotic division, the primary oocytes
double its DNA by replication
• There are 22 pairs of autosomes which determine the body
characteristics and 1 pair of sex chromosomes, named 'XX’.
• The first stage of maturation occurs with the ovarian follicle
just prior to ovulation but the final maturation occurs only after
fertilization.
• The primary oocyte undergoes first meiotic division giving rise
to secondary oocyte and one polar body.
6. •They two are of unequal size,
• secondary oocyte - haploid number of chromosomes (23, X) with all
cytoplasm
• small polar body - half of the chromosomes (23, X) but with scanty
cytoplasm.
•Ovulation occurs soon after the formation of the secondary oocyte
•The secondary oocyte completes the second meiotic division only after
fertilization by the sperm in the Fallopian tube and results in the
formation of two unequal daughter cells, each possessing 23
chromosomes (23, X), the larger one is called the mature ovum and the
smaller one is the second polar body containing the same number of
chromosomes.
•The first polar body may also undergo the second meiotic division. In the
7. Structure of a mature ovum:
A fully mature ovum is the largest cell in the body and is about 130
microns in diameter.
It consists of cytoplasm and a nucleus with its nucleolus which is
eccentric in position and contains 23 chromosomes (23, X).
During fertilization, the nucleus is converted into a female pronucleus.
The ovum is surrounded by a cell membrane called vitelline membrane.
There is an outer transparent mucoprotein envelope, the zona pellucida.
The zona pellucida is penetrated by tiny channels which are important
for the transport of the materials from the granulosa cells to the oocyte.
In between the vitelline membrane and the zona pellucida, there is a
narrow space called perivitelline space which accommodates the polar
bodies.
9. SPERMATOGENESIS
The process involved in the development of spermatids from the
primordial male germ cells and their differentiation into spermatozoa is
called spermatogenesis.
Shortly before puberty, the primordial germ cells develop into
spermatogonia and remain in the wall of seminiferous tubules.
The spermatogonia, in turn, differentiate into primary spermatocytes
which remain in the stage of prophase of the first meiotic division for a
long time (about 16 days).
Each spermatocyte contains 22 pairs of autosomes and 1 pair of sex
chromosomes, named 'XY.
10. With the completion of the first meiotic division, two secondary
spermatocytes are formed having equal share of cytoplasm and haploid
number of chromosomes either 23, X or 23, Y.
Then immediately follows the second meiotic division with the
formation of four spermatids, each containing haploid number of
chromosomes, two with 23, X and two with 23, Y.
Immediately after their formation, extensive morphological
differentiation of the spermatids occurs without further cell division to
convert them into spermatozoa. The process is called spermiogenesis.
In man, the time required for a spermatogonium to develop into a
mature spermatozoon is about 61 days.
11. Structure of a mature spermatozoon:
• It has got two parts, a head and a
tail.
• The head consists principally of
the condensed nucleus and
acrosomal cap. Acrosome is rich in
enzymes.
• The tail is divided into four zones-
the neck, the middle piece, the
principal piece and the end piece
