Gametogenesis and Fertilization
Gametogenesis and Fertilization
Gametogenesis and Fertilization
Introduction
SPERMATOGENESIS
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells
in the seminiferous tubules of the testis. Begins at puberty.
In human being a pair of testes is located below the abdominal cavity inside the muscular
scrotum. Inside the testes there are numerous tubular structures, called seminiferous
tubule. The inner surface of these tubules are lined by germinal cells (spermatogonia),
which are the raw material for the formation of spermatozoa.
The process occur in two steps-
a) Formation of Spermatid
b) Spermeiogenesis
Formation of Spermatid
i) Multiplication phase (mitotic phase)- In this phase the germ cells present on the
basement layer of inner side of the seminiferous tubule undergo repeated mitotic division.
These newly formed cells are called spermatogonia or sperm mother cells. Two types of
spermatogonia are produced i.e Type A spermatogonia and Type B spermatogonia. Type
A spermatogonia maintain the germ cell pool through self-renewal while Type B
spermatogonia undergo further differentiation.
ii) Growth phase- The spermatogonia formed as a result of repeated mitotic division,
stops division and grows in size by accumulating nutrient and prepare for next phase.
These cells are now called primary spermatocytes. During the growth phase the entire
cell with the nucleus enlarges in size.
iii) Maturation or meiotic division phase- In this phase the diploid primary spermatocytes
undergo first reduction division(meiosis 1) and produce two haploid secondary
spermatocytes. Then the secondary spermatocytes with half the number of chromosome
undergo second maturation division and form four haploid cells. These are called
spermatids.
Spermeiogenesis
The Golgi apparatus within the spermatid becomes active, producing vesicles that
fuse to form a cap-like structure known as the acrosome.
The acrosome is located at the anterior end (head) of the developing sperm cell and
contains enzymes that play a crucial role in penetrating the egg during fertilization.
Nuclear Changes
The nucleus of the spermatid undergoes condensation, leading to a reduction in size
and compaction of genetic material.
The excess cytoplasm is shed, leaving a streamlined and condensed nucleus.
Formation of Flagellum (Tail)
The centrioles located near the nucleus organize microtubules to form the axial
filament, which extends into a long tail.
The tail is essential for providing motility to the sperm, enabling it to swim toward the
egg.
Mitochondrial Sheath Development
Mitochondria accumulate around the axial filament, forming the midpiece of the sperm.
The midpiece is crucial for providing the energy (in the form of ATP) required for the
sperm's movement.
Elimination of Excess Cytoplasm
Excess cytoplasm is eliminated, streamlining the sperm and reducing its overall size
and weight.
This process ensures that the mature sperm is highly specialized and focused on its
reproductive function.
Maturation and Release
The mature spermatozoa detach from the Sertoli cells (nurse cells) in the seminiferous
tubules of the testes.
The mature sperm move into the epididymis, a coiled tube connected to the testes,
for further maturation, storage, and acquiring the ability to swim.
Spermatozoa Storage:
The mature spermatozoa are stored in the epididymis until they are released during
ejaculation.
The epididymis also plays a role in concentrating and transporting the sperm.
How the sperm is adapted to perform its function:
the lashing movement of the tail enables the sperm to move/propulsion in fluid medium
towards the egg
1. Multiplication Phase:
The primary germinal cells of the ovary with diploid number of chromosomes (2n) divide
several times mitotically so as to form a large number of daughter cells known as oogonia.
This form groups of oogonia called follicles or eggnest. Addition of oogonia results in the
growth of the ovaries.
2. Growth Phase:
The oogonium does not divide but increases in size to form a primary oocyte. The growth
is associated with both nuclear and cytoplasmic growth. The nuclear growth is due to
accumulation of large amount of nuclear sap and is termed as germinal vesicle. The
cytoplasmic growth is associated with increase in number of mitochondria, endoplasmic
reticulum and Golgi complex and accumulation of reserve food material called yolk or
vitellin.
3. Maturation phase:
The primary oocyte undergoes two successive divisions by meiosis. The first division is
meiosis-I . Two unequal daughter cells are produced. The large cell is called secondary
oocyte containing haploid (n) set of chromosomes . The smaller cell is called first polar
body or polocyte containing ‘n’ number of chromosomes and little or no cytoplasm.
The secondary oocyte and first polar body then undergo second maturation division by
meiosis-II. As a result of this division one large ovum is formed containing entire amount
of cytoplasm and ‘n’ number of chromosomes and a second polar body like the first polar
body.
Simultaneously, the first polar body may divide into two polar bodies or may not divide at
all. Thus only one functional ovum is formed and the two or three polar bodies soon
degenerate. In vertebrates the first polar body is formed after the primary oocyte is
released from ovary and has entered into the oviduct. The second polar body is formed
only when the sperm enters into ovum during fertilization.
Ovum structure
Process where the nucleus of a male gamete fuses with the nucleus of a female
gamete to form a zygote.
This takes place in the upper part of the oviduct after copulation. Sperms are drawn
up by suction through the cervix into the uterus. They swim up to the oviduct using
their tails.
Very many sperms are released but only one is required to fertilise the ovum.
Stages of Fertilization
Sperm preparation/conditioning
At the beginning of the process, the sperm undergoes a series of changes, as freshly
ejaculated sperm is unable or poorly able to fertilize. The sperm must
undergo capacitation in the female's reproductive tract, which increases its motility and
hyperpolarizes its membrane, preparing it for the acrosome reaction.
Capacitation
Starts in uterus and continues in to fallopian tube. Follicular fluid enhances the
process.
Fertilizin is glycoprotein, produced by cells of zona pellucida. It Makes sperm
sticky and to adhere to egg surface (Corona radiata)
Glycoproteins are removed from plasma membrane over acrosome.
Capacitated sperms show no morphological change, but more active.
Completion of capacitation permits acrosome reaction to occur.
Acrosome reaction and penetration
When the acrosome reaction occurs, a number of proteolytic enzymes are released.
These enzymes is responsible for digesting the hole through the zona pellucida through
which the sperm enters the perivitelline space.
Reactivation/completion of meiosis II
As a result of the sperm fusing with the egg plasmalemma, the oocyte nucleus, which is
at metaphase of the second meiotic division, completes that division giving rise to another
polar body.
The head of the sperm containing the nucleus detaches from the entire sperm and is
known as male pronucleus. The tail and the second polar body degenerates.
Following completion of the second meiotic division, the nucleus of the ovum becomes
the female pronucleus.
The haploid male and female pronuclei move toward one another, meet, and fuse to form
the diploid nucleus of the zygote.
Activation of Ovum
This refers to a series of morphological, physiological and molecular changes that occur
in the egg in response to fusion of the sperm with the egg.
2.Cortical reaction-rupture of cortical granules that occurs concurrently with the Ca++
release. Contents of granules are released into perivitelline space and cause “hardening”
of the vitelline membrane or zona pellucida. Causes vitelline/fertilization membrane to
rise away from surface of egg in somespecies.
3. In many species,an influx of Na+(sodium) into the egg cytoplasm that causes a change
in membrane potential-fast block to polyspermy.
4.In many species a reorganization of the egg cytoplasm.