2. G A M E T O G E N E S I S
F E R T I L I Z AT I O N
C O N T E N T S
3. GAMETOGENESIS
• Gametogenesis is the process whereby a haploid cell (n) is formed from a diploid
cell (2n) through meiosis and cell differentiation.
• Gametogenesis in the male is known as spermatogenesis and produces spermatozoa.
• Gametogenesis in the female is known as oogenesis and result in the formation of
ova.
Fig 1 – SpermatogenesisFig 2 – Oogenesis
4. Spermatogenesis
• Males start producing sperm when they reach puberty, which is usually from 10-16 years old. Sperm are produced in
large quantities (~200 million a day) to maximise the likelihood of sperm reaching the egg. Sperm are continually
produced as males need to be ready to utilise the small window of fertility of the female.
• Sperm production occurs in the testes of the male, specifically in the seminiferous tubules. The tubules are kept separate
from the systemic circulation by the blood-testis barrier.
• The blood-testis barrier is formed by Sertoli cells and is important in preventing hormones and constituents of the
systemic circulation from affecting the developing sperm, and also in preventing the immune system of the male from
recognising the sperm as foreign – as the sperm are genetically different from the male and will express different surface
antigens. Sertoli cells also have a role in supporting the developing spermatozoa.
• Spermatogonia are the initial pool of diploid cell that divide by mitosis to give two identical cells. One of these cells will
be used to replenish the pool of spermatogonia – these cells are A1 spermatogonia. This replenishment of spermatogonia
means that males are fertile throughout their adult life. The other cell – type B spermatogonium – will eventually form
mature sperm.
• Type B spermatogonia replicate by mitosis several times to form identical diploid cells linked by cytoplasm bridges,
these cells are now known as primary spermatocytes. Primary spermatocytes then undergo meiosis:-
1. Meiosis I produces two haploid cells known as secondary spermatocytes
2. Meiosis II produces four haploid cells known as Spermatids
• The cytoplasmic bridges break down and the spermatids are released into the lumen of the seminiferous tubule – a
process called spermiation. The spermatids undergo spermiogenesis (remodelling and differentiation into mature
spermatozoa) as they travel along the seminiferous tubules until they reach the epididymis.
5. • From the seminiferous tubule they travel to the rete testis,
which acts to “concentrate” the sperm by removing excess
fluid, before moving to the epididymis where the sperm is
stored and undergoes the final stages of maturation.
• Spermatogenesis takes approximately 70 days, therefore in
order for sperm production to be continuous and not
intermittent, multiple spermatogenic processes are occurring
simultaneously within the same seminiferous tubule, with
new groups of spermatogonia arising every 16 days
(spermatogenic cycle). Each of these populations of
spermatogenic cells will be at different stages of
spermatogenesis.
• Note that once sperm leave the male body and enter the
female reproductive tract, the conditions there cause the
sperm to undergo capacitation, which is the removal of
cholesterol and glycoproteins from the head of the sperm
cell to allow it to bind to the zona pellucida of the egg cell
6. • Oogenesis differs from spermatogenesis in that it begins in the foetus prior to birth.
Primordial germ cells (which originate in the yolk sac of the embryo) move to
colonise the cortex of the primordial gonad and replicate by mitosis to peak at
approximately 7 million by mid-gestation (~20 weeks). Cell death occurs after this
peak to leave 2 million cells which begin meiosis I before birth and are known as
primary oocytes. Therefore, a human female is born with approximately 2 million
primary oocytes arrested in meiosis and these make up a finite supply of potential
ova.
• The primary oocytes are arranged in the gonads in clusters surrounded by
flattened epithelial cells called follicular cells and these form primordial follicles.
The primary oocytes are arrested in prophase stage of meiosis I.
• During childhood, further atresia (cell death) occurs, leaving ~40,000 eggs at
puberty.
• Once puberty begins, a number of primary oocytes (15-20) begin to mature each
month, although only one of these reaches full maturation to become an oocyte.
• The primary oocytes undergo 3 stages:
1. Pre-antral
2. Antral
3. Preovulatory
Oogenesis
7. 1.Pre-antral Stage
The primary oocyte grows dramatically whilst still being arrested in meiosis I. The
follicular cells grow and proliferate to form a stratified cuboidal epithelium. These cells
are now known as granulosa cells and secrete glycoproteins to form the zona
pellucida around the primary oocyte. Surrounding connective tissue cells also
differentiates to become the theca folliculi, a specialised layer of surrounding cells
that is responsive to LH and can secrete androgens under its influence.
2.Antral Stage
Fluid filled spaces form between granulosa cells, these eventually combine together
to form a central fluid filled space called the antrum. The follicles are now called
secondary follicles. In each monthly cycle one of these secondary follicles becomes
dominant and develops further under the influence of FSH, LH and oestrogen. (See
article on the menstrual cycle).
3.Pre-Ovulatory Stage
The LH surge induces this stage and meiosis I is now complete. Two haploid cells are
formed within the follicle, but they are of unequal size. One of the daughter cells
receives far less cytoplasm than the other and forms the first polar body, which will
not go on to form an ovum. The other haploid cell is known as the secondary oocyte.
Both daughter cells then undergo meiosis II, the first polar body will replicated to give
two polar bodies but the secondary oocyte arrests in metaphase of meiosis II, 3 hours
prior to ovulation.
8. • Ovulation
The follicle has grown in size and is now mature – it is called a Graafian follicle.
The LH surge increases collagenase activity so that the follicular wall is
weakened, this combined with muscular contractions of the ovarian wall result in
the ovum being released from the ovary and being taken up into the fallopian
tube via the fimbriae (finger-like projections of the fallopian tube).
• Fertilisation
The secondary oocyte will only complete meiosis II on fertilisation, giving off a third
polar body once meiosis II is completed and a fertilised egg. If fertilisation never
occurs, the oocyte degenerates 24 hours after ovulation, remaining arrested in
meiosis II.
If the egg is fertilised however, the peristaltic movements of the fallopian tube move
the egg to the uterus where it can implant into the posterior uterine wall.
9. Stages of human development
1. Zygotic stage: The zygote is formed when the male gamete (sperm)
and female gamete (egg) fuse.
2. Blastocyst stage: The single-celled zygote begins to divide into a
solid ball of cells. Then, it becomes a hollow ball of cells called a
blastocyst, attaching to the lining of the mother's uterus.
3. Embryonic stage: The major internal organs and external features
begin to emerge, forming an embryo. In this stage, the heart, brain,
and spinal cord become visible. Arms and legs start to develop.
4. Fetal stage: Once the formed features of the embryo begin to grow
and develop, the organism is considered a fetus. Differentiation and
specialization of structures happens during this time.
10. Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse.
Because each of these reproductive cells is a haploid cell containing half of the
genetic material needed to form a human being, their combination forms a diploid
cell. This new single cell, called a zygote, contains all of the genetic material needed
to form a human—half from the mother and half from the father.
FERTILIZATION
Figure 3. Before fertilization, hundreds of capacitated sperm must break
through the surrounding corona radiata and zona pellucida so that one can
contact and fuse with the oocyte plasma membrane.
11. Transit of Sperm
• During ejaculation, hundreds of millions of sperm (spermatozoa) are released into
the vagina. Almost immediately, millions of these sperm are overcome by the acidity
of the vagina (approximately pH 3.8), and millions more may be blocked from
entering the uterus by thick cervical mucus.
• Of those that do enter, thousands are destroyed by phagocytic uterine leukocytes.
Thus, the race into the uterine tubes, which is the most typical site for sperm to
encounter the oocyte, is reduced to a few thousand contenders.
• Their journey—thought to be facilitated by uterine contractions—usually takes from
30 minutes to 2 hours. If the sperm do not encounter an oocyte immediately, they
can survive in the uterine tubes for another 3–5 days. Thus, fertilization can still
occur if intercourse takes place a few days before ovulation.
• During the journey, fluids in the female reproductive tract prepare the sperm for
fertilization through a process called capacitation, or priming. The fluids improve the
motility of the spermatozoa. They also deplete cholesterol molecules embedded in
the membrane of the head of the sperm, thinning the membrane in such a way that
will help facilitate the release of the lysosomal (digestive) enzymes needed for the
sperm to penetrate the oocyte’s exterior once contact is made.
• Sperm must undergo the process of capacitation in order to have the “capacity” to
fertilize an oocyte. If they reach the oocyte before capacitation is complete, they will
be unable to penetrate the oocyte’s thick outer layer of cells.
12. Contact Between Sperm and Oocyte
• Fertilization must occur in the distal uterine tube because an unfertilized oocyte
cannot survive the 72-hour journey to the uterus.
• Oocyte (specifically a secondary oocyte) is surrounded by two protective layers:
1. The corona radiata is an outer layer of follicular (granulosa) cells that form
around a developing oocyte in the ovary and remain with it upon ovulation.
2. The underlying zona pellucida (pellucid = “transparent”) is a transparent, but
thick, glycoprotein membrane that surrounds the cell’s plasma membrane.
• To reach the oocyte itself, the sperm must penetrate the two protective layers.
The sperm first burrow through the cells of the corona radiata. Then, upon
contact with the zona pellucida, the sperm bind to receptors in the zona
pellucida. This initiates a process called the acrosomal reaction in which the
enzyme-filled “cap” of the sperm, called the acrosome, releases its stored
digestive enzymes. These enzymes clear a path through the zona pellucida that
allows sperm to reach the oocyte.
• Finally, a single sperm makes contact with sperm-binding receptors on the
oocyte’s plasma membrane. The plasma membrane of that sperm then fuses
with the oocyte’s plasma membrane, and the head and mid-piece of the
“winning” sperm enter the oocyte interior.
13. • When the first sperm fuses with the oocyte, the oocyte deploys two mechanisms to
prevent polyspermy, which is penetration by more than one sperm.
• This is critical because if more than one sperm were to fertilize the oocyte, the resulting
zygote would be a triploid organism with three sets of chromosomes. This is
incompatible with life.
• The first mechanism is the fast block, which involves a near instantaneous change in
sodium ion permeability upon binding of the first sperm, depolarizing the oocyte plasma
membrane and preventing the fusion of additional sperm cells.
• The fast block sets in almost immediately and lasts for about a minute, during which
time an influx of calcium ions following sperm penetration triggers the second
mechanism, the slow block.
• In this process, referred to as the cortical reaction, cortical granules sitting immediately
below the oocyte plasma membrane fuse with the membrane and release zonal
inhibiting proteins and mucopolysaccharides into the space between the plasma
membrane and the zona pellucida.
• Zonal inhibiting proteins cause the release of any other attached sperm and destroy the
oocyte’s sperm receptors, thus preventing any more sperm from binding.
• The mucopolysaccharides then coat the nascent zygote in an impenetrable barrier that,
together with hardened zona pellucida, is called a fertilization membrane.
14. The Zygote
• At the point of fertilization, the oocyte has not yet completed meiosis; all secondary
oocytes remain arrested in metaphase of meiosis II until fertilization.
• Only upon fertilization does the oocyte complete meiosis.
• At this moment, the oocyte has become an ovum, the female haploid gamete.
• The two haploid nuclei derived from the sperm and oocyte and contained within the
egg are referred to as pronuclei.
• They decondense, expand, and replicate their DNA in preparation for mitosis.
• The pronuclei then migrate toward each other, their nuclear envelopes disintegrate,
and the male- and female-derived genetic material intermingles.
• This step completes the process of fertilization and results in a single-celled diploid
zygote with all the genetic instructions it needs to develop into a human.
• Most of the time, a woman releases a single egg during an ovulation cycle. However,
in approximately 1 percent of ovulation cycles, two eggs are released and both are
fertilized. Two zygotes form, implant, and develop, resulting in the birth of dizygotic
(or fraternal) twins. Because dizygotic twins develop from two eggs fertilized by two
sperm, they are no more identical than siblings born at different times.
• Much less commonly, a zygote can divide into two separate offspring during early
development. This results in the birth of monozygotic (or identical) twins. Although
the zygote can split as early as the two-cell stage, splitting occurs most commonly
during the early blastocyst stage, with roughly 70–100 cells present
15. IN VITRO FERTILIZATION
• IVF, which stands for in vitro fertilization, is an assisted reproductive technology.
• In vitro, which in Latin translates to “in glass,” refers to a procedure that takes place
outside of the body.
• PROCEDURE
1. A typical IVF procedure begins with egg collection. A normal ovulation cycle produces
only one oocyte, but the number can be boosted significantly (to 10 –20 oocytes) by
administering a short course of gonadotropins. The course begins with follicle -
stimulating hormone (FSH) analogs, which support the development of multiple
follicles, and ends with a luteinizing hormone (LH) analog that triggers ovulation. Right
before the ova would be released from the ovary, they are harvested using ultrasound -
guided oocyte retrieval. In this procedure, ultrasound allows a physician to visualize
mature follicles. The ova are aspirated (sucked out) using a syringe.
2. In parallel, sperm are obtained from the male partner or from a sperm bank. The sperm
are prepared by washing to remove seminal fluid because seminal fluid contains a
peptide, FPP (or, fertilization promoting peptide), that —in high concentrations—
prevents capacitation of the sperm. The sperm sample is also concentrated, to
increase the sperm count per milliliter.
3. Next, the eggs and sperm are mixed in a petri dish. The ideal ratio is 75,000 sperm to
one egg. If there are severe problems with the sperm —for example, the count is
exceedingly low, or the sperm are completely nonmotile, or incapable of binding to or
penetrating the zona pellucida—a sperm can be injected into an egg. This is called
intracytoplasmic sperm injection (ICSI).
The embryos are then incubated until they either reach the eight -cell stage or the
blastocyst stage. In the United States, fertilized eggs are typically cultured to the
blastocyst stage because this results in a higher pregnancy rate. Finally, the embryos
are transferred to a woman’s uterus using a plastic catheter (tube).
16. Figure 4:In vitro fertilization involves egg collection from the ovaries, fertilization in a petri dish, and
the transfer of embryos into the uterus.
17. • Sexual reproduction starts with the combination of a sperm and an egg in a process
called fertilization. This can occur either inside (internal fertilization) or outside
(external fertilization) the body of the female.
1. External Fertilization
• External fertilization usually occurs in aquatic environments where both eggs and
sperm are released into the water. After the sperm reaches the egg, fertilization
takes place.
• Most external fertilization happens during the process of spawning where one or
several females release their eggs and the male(s) release sperm in the same
area, at the same time.
• Pairs of fish that are not broadcast spawners may exhibit courtship behavior. This
allows the female to select a particular male. The trigger for egg and sperm release
(spawning) causes the egg and sperm to be placed in a small area, enhancing the
possibility of fertilization.
• External fertilization in an aquatic environment protects the eggs from drying out.
• EXAMPLE:Salmon reproduce through spawning.
18. 2. Internal Fertilization
• Internal fertilization occurs most often in land-based animals, although some aquatic
animals also use this method. There are three ways that offspring are produced following
internal fertilization.
• In oviparity, fertilized eggs are laid outside the female’s body and develop there, receiving
nourishment from the yolk that is a part of the egg. This occurs in most bony fish, many
reptiles, some cartilaginous fish, most amphibians, two mammals, and all birds.
• Reptiles and insects produce leathery eggs, while birds and turtles produce eggs with high
concentrations of calcium carbonate in the shell, making them hard
• In ovoviparity, fertilized eggs are retained in the female, but the embryo obtains its
nourishment from the egg’s yolk and the young are fully developed when they are hatched.
This occurs in some bony fish (like the guppy Lebistes reticulatus), some sharks, some
lizards, some snakes (such as the garter snake Thamnophis sirtalis), some vipers, and
some invertebrate animals (like the Madagascar hissing cockroach Gromphadorhina
portentosa).
• In viviparity the young develop within the female, receiving nourishment from the mother’s
blood through a placenta. The offspring develops in the female and is born alive. This
occurs in most mammals, some cartilaginous fish, and a few reptiles.
• Internal fertilization has the advantage of protecting the fertilized egg from dehydration on
land. The embryo is isolated within the female, which limits predation on the young.
Internal fertilization enhances the fertilization of eggs by a specific male. Fewer offspring
are produced through this method, but their survival rate is higher than that for external
fertilization.
