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 is called
oogenesis.
• The primitive germ cells take their origin from the yolk sac at about
the end of 3rd week and their migration to the developing gonadal
ridge is completed round about the end of 4th week.
• In the female gonads, the germ cells undergo a number of rapid
mitotic divisions and differentiate into oogonia. The number of
oogonia reaches its maximum at 20th week, numbering about 7
million.
4.
5. • . At birth, there is no more mitotic division and all the oogonia are
replaced by primary oocytes which have finished the prophase of the
first meiotic division and remain in resting phase (dictyotene stage)
between prophase and metaphase. Total number of primary oocytes
at birth is estimated to be about 2 million.
• The primary oocytes do not finish the first meiotic division until
puberty is reached. At puberty, some 400,000 primary oocytes are
left behind, the rest being atretic. Out of these, some 400 are likely to
ovulate during the entire reproductive period.
6. • 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, so they contain double the amount of
normal protein content.
• 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
full maturation of 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. The two are of unequal size, the secondary oocyte
contains haploid number of chromosomes (23, X), but nearly all the cytoplasm
and the small polar body also contains half of the chromosomes (23, X) but with
scanty cytoplasm. Ovulation occurs soon after the formation of the secondary
oocyte
7. • The secondary oocyte completes the second meiotic division
(homotypical) 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 absence of fertilization,
the secondary oocyte does not complete the second meiotic division
and degenerates as such.
8. 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”. 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
(homotypical) 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
9.
10. PHYSIOLOGY OF OVULATION
• Ovulation is a process whereby a secondary oocyte is released from
the ovary following rupture of a mature Graafian follicle and becomes
available for conception.
• In relation to the menstrual period, the event occurs about 14 days
prior to the expected period. However, menstruation can occur
without ovulation and ovulation remains suspended during
pregnancy and lactation.
11. CAUSES
• Endocrinal
• • LH surge: Sustained peak level of estrogen for 24–36 hours in the late
follicular phase → LH surge occurs from the anterior pituitary. Ovulation
approximately occurs 16–24 hours after the LH surge. LH peak persists for
about 24 hours. The LH surge stimulates completion of reduction division
of the oocyte and initiates luteinization of the granulosa cells, synthesis of
progesterone and prostaglandins.
• • FSH rise: Preovulatory rise of progesterone facilitates the positive
feedback action of estrogen to induce FSH surge → increase in
plasminogen activator → plasminogen → plasmin → helps lysis of the wall
of the follicle. Thus, the combined LH/FSH midcycle surge is responsible for
the final stage of maturation, rupture of the follicle and expulsion of the
oocyte.
12. • Stretching factor: It is more a passive stretching than rise in intrafollicular
pressure which remains static at about 15 mm Hg
• Contraction of the micromuscles in the theca externa and ovarian stroma
due to increased prostaglandin secretion.
• EFFECT OF OVULATION: Following ovulation, the follicle is changed into
corpus luteum .The ovum is picked up into the Fallopian tube and
undergoes either degeneration or further maturation, if fertilization is to
occur.
• Menstruation is unrelated with ovulation and anovular menstruation is
quite common during adolescence, following childbirth and in women
approaching menopause.
13. FERTILIZATION
• Fertilization is the process of fusion of the spermatozoon with the
mature ovum. It begins with sperm egg collision and ends with
production of a mononucleated single cell called the zygote
• Its objectives are:
• (1) To initiate the embryonic development of the egg and
• (2) To restore the chromosome number of the species. Almost always,
fertilization occurs in the ampullary part of the uterine tube.
14.
15. • APPROXIMATION OF THE GAMETES: The ovum, immediately following
ovulation is picked up by the tubal fimbriae which partly envelope the
ovary, especially at the time of ovulation. The pick up action might be
muscular or by a kind of suction or by ciliary action or by a positive
chemotaxis exerted by the tubal secretion.
• The ovum is rapidly transported to the ampullary part. Fertilizable life
span of oocyte ranges from 12 to 24 hours whereas that of sperm is
48 to 72 hours.
16. • Out of hundreds of millions of sperms deposited in the vagina at single
ejaculation, only thousands capacitated spermatozoa enter the uterine tube
while only 300–500 reach the ovum. The tubal transport is facilitated by muscular
contraction and aspiration action of the uterine tube. It takes only few minutes
for the sperm to reach the Fallopian tube.
• : Complete dissolution of the cells of the corona radiata occurs by the chemical
action of the hyaluronidase liberated from the acrosomal cap of the hundreds of
sperm present at the site
• — Penetration of the zona pellucida is facilitated by the release of hyaluronidase
from the acrosomal cap. More than one sperm may penetrate the zona pellucida.
• — Out of the many sperms, one touches the oolemma. Soon after the sperm
fusion, penetration of other sperm is prevented by zona reaction (hardening) and
oolemma block. This is due to release of cortical granules by exocytosis from the
oocyte
17. • — Completion of the second meiotic division of the oocyte immediately follows,
each containing haploid number of chromosomes (23, X). The bigger one is called
the female pronucleus and the smaller one is called second polar body which is
pushed to the perivitelline space.
• — In the human, both the head and tail of the spermatozoon enter the cytoplasm
of the oocyte but the plasma membrane is left behind on the oocyte surface.
Head and the neck of the spermatozoon become male pronucleus containing
haploid number of chromosomes (23, X) or (23, Y).
• — The male and the female pronuclei unite at the center with restoration of the
diploid number of chromosomes (46) which is constant for the species. The
zygote, thus formed, contains both the paternal and maternal genetic materials.
• In some instances, an antigen called fertilizin present on the cortex and its coat of
the ovum, reacts with the antibody called antifertilizin liberated at the plasma
membrane of the sperm head. Thus the union between the two gametes may be
an immunological reaction (chemotaxis).
18. MORULA
• The two cell stage is reached approximately 30 hours after
fertilization. Each contains equal cytoplasmic volume and
chromosome numbers. The blastomeres continue to divide by binary
division through 4, 8, 16 cell stage until a cluster of cells is formed and
is called morula, resembling a mulberry.
• As the total volume of the cell mass is not increased and the zona
pellucida remains intact, the morula after spending about 3 days in
the uterine tube enters the uterine cavity through the narrow uterine
ostium (1 mm) on the 4th day in the 16-64 cell stage
19. BLASTOCYST
• While the morula remains free in the uterine cavity on the 4th and 5th day, it is
covered by a film of mucus. The fluid passes through the canaliculi of the zona
pellucida which separates the cells of the morula and is now termed blastocyst .
Zona hatching is the next step so that trophectoderm cells interact with
endometrial cells and implantation occurs.
• Due to blastocyst enlargement the zona pellucida becomes stretched, thinned
and gradually disappears. Lysis of zona and escape of embryo is called zona
hatching. The cells on the outer side of the morula (polar) become
trophectoderm and the inner cells (apolar) become inner cell mass by the
mediation of epithelial cadherin (E-cadherin) (protein).
• Trophectoderm differentiates into chorion (placenta) and the inner cell mass into
the embryo. Completely undifferentiated cells are called the pluripotent
embryonic stem (ES) cells. ES cells are able to produce mature somatic cells of
any germ layer (ectoderm, mesoderm and endoderm)
20.
21. IMPLANTATION
• Implantation occurs in the endometrium of the anterior or posterior
wall of the body near the fundus on the 6th day which corresponds to
the 20th day of a regular menstrual cycle. Implantation occurs
through four stages e.g. apposition, adhesion, penetration and
invasion
• ENDOMETRIUM AT THE IMPLANTATION SITE : (1) The endometrium is
in the secretory phase corresponding to 20–21 days of cycle. (2) The
microvilli on the surface of the trophectoderm interdigitate with the
decidual cells to form the junctional complexes. Endometrial
receptivity and molecular signaling during implantation is induced by
progesterone, LIF (leukemia inhibitory factor), prostaglandins and
COX-2.
22. • APPOSITION: Occurs through pinopod formation. Pinopods are long
finger like projections (microvilli) from the endometrial cell surface.
These pinopods absorb the endometrial fluid which is secreted by the
endometrial gland cells.
• This fluid, rich in glycogen and mucin provides nutrition to the
blastocyst initially. Unless this fluid is absorbed, adhesion phase
cannot occur. Adhesion of blastocyst to the endometrium occurs
through the adhesion molecules like integrin, selectin and cadherin
(glycoproteins).
23.
24. • PENETRATION:Actual penetration and invasion occur through the stromal
cells in between the glands and is facilitated by the histolytic action of the
blastocyst. With increasing lysis of the stromal cells, the blastocyst is
burrowed more and more inside the stratum compactum of the decidua
• The syncytium by penetrating the vessels, not only becomes continuous
with the endothelial lining but permits the maternal blood to enter into the
lacunar system. Ultimately erosion of few maternal arteries with formation
of blood space (lacunae) occurs.
• Nutrition is now obtained by aerobic metabolic pathway from the maternal
blood. Further penetration is stopped probably by the maternal
immunological factor and the original point of entry is sealed by fibrin clot
and later by epithelium. The process is completed by 10th or 11th day
which corresponds to D 24-25 from LMP
