Gametogenesis: The process of formation of male or female gamete is
called gametogenesis. There are two types of gametogenesis in Human
beings, Spermatogenesis And Oogenesis.
Oogenesis In Female
The process of formation and maturation of ova is called oogenesis (Gr.
Oon- egg; genesis- origin). It is fundamentally similar in all animals
except for minor difference related to the amount of yolk in the ova
produced.
The process of oogenesis is somewhat more complicated and different
than spermatogenesis. It involves not only the production of the haploid
nucleus but also the acquisition of food reserves and preliminary
organization of the cytoplasm from which the embryo will be
formed.
The ova are derived from the germinal epithelium lining the ovary.
These germinal cells are diploid. Some cells in the germinal epithelium
are larger than others and have bigger nuclei. These are called
primordial germ cells which eventually give rise to the eggs.
The Ovary
The ovary is a solid organ attached by its antero-lateral border to the
broad ligament. In the region of attachment, known as the hilus are
found smooth muscle fibers, numerous large blood vessels and
lymphatics.
The tissue of the broad ligament is continued into the ovary, where it
spreads out as the stroma. The free surface of the ovary is covered with
surface epithelium, the modified mesothelium of the visceral
peritoneum.
In young women this is a cuboidal epithelium; it usually becomes
flattened in later life. Immediately beneath the epithelium the stroma
is condensed to form a tunica albuginea.
The stroma consists of cortex and medulla, which are not sharply
delimited from each other. The cortex occupies the greater part of the
ovary.
It is a primitive type of connective tissue, in which large,
spindle-shaped fibroblasts predominate. Coursing in all directions
between them are compactly arranged fine collagenous and reticular
fibers. Scattered throughout the cortex are vesicles of different sizes
: these are the developing Graafian (or ovarian) follicles.
It also contains transformed discharged follicles-the corpora lutea
(singular, corpus luteum) and degenerating atretic
follicles.
If the ovum has perished on its way to the exterior the luteum survives
for about 14 days and then degenerates: this is the corpus luteum of
menstruation or corpus luteum spurium.
If the ovum has been fertilized the corpus luteum enlarges and
persists during pregnancy as the corpus luteum of pregnancy or corpus
luteum verum.
If fertilization of the ovum has not taken place, slight hemorrhage
into the corpus luteum occurs at the next menstruation and this is the
beginning of its involution.
The wide structure shrinks, and after about three months is
represented by fibrous hyaline scar, the corpus albicans. A corpus
luteum may sometimes give rise to a cyst.
The interior of the ovary, or medulla is a vascular and fibrous core.
Abundant blood vessels enter at the hilus and then take spiral course
inwards. The medulla reaches the "surface" only at hilus where it
merges with the mesovarium.
Events In Oogenesis
Oogenesis is a complication operation. It can be divided into three
phases. They are:-
A. Proliferation Phase
The primordial germ cells undergo proliferation by, mitotic divisions
and the resulting cells are called oogonia or egg mother
cells.
The oogonia multiply by repeated mitotic divisions. When the division
stops, the cells are termed primary oocytes which enter a period of
growth. The nucleus of a primary oocytes is diploid.
B. Growth Phase
Egg contributes the greater part of the substances used in development,
therefore, growth plays a much greater role in oogenesis than in
spermatogenesis.
During oogenesis first meiotic division starts and then goes into a
suspended state, while the nucleus and cytoplasm carry out major
synthetic activities. As a result of these activities, the oocyte
increases greatly in size and volume. Besides, important qualitative
changes also occur.
The period of growth in the female gametes is very prolonged and the
increase in size is appreciably high. In frog, a young oocyte may be
about 50 microns in diameter while the fully developed egg is up to 2000
microns in diameter. The diameter of ovum is about 40,000 microns in
birds, but in mammals it is only 200 microns.
The rate of growth of oocytes varies in different animals. In Rana
pippins, the growth takes place over a period of three years. In the
hen, the last rapid growth of oocytes takes place in the 6 to 14 days
preceding ovulation.
During this time the volume of the oocyte increases 200 fold. In
mammals the proliferation of the oogonia is restricted to the
intra-uterine period of life. The growth period of oocytes may be
divided into two stages:-
(i) Pre-Vitello genesis
During this period-of-growth, the nucleus and cytoplasm of primary
oocyte increases tremendously in volume.
(a) Growth of nuclear substances
The nucleus of the oocyte becomes enlarged mainly due to the assembly
of an outsized amount of nuclear sap. Due to this the nuclei of advanced
oocytes appear to be bloated with fluid and are often referred to as
germinal vesicles.
The chromosomes increase in length. In oocytes of animals having large
eggs (notably in the amphibian oocyte), the chromosomes acquire a very
characteristic appearance. Numerous paired threads or loops project
transversely from the main chromosomal axes.
The shaggy appearance given to the chromosomes by these loops has led to
their being called lamp brush chromosomes. The loops of lamp brush
chromosomes represent loci of gene activity i.e., at these sections
messenger RNA is synthesized which subsequently controls the synthesis
of proteins in the cell.
However, the amount of DNA in the chromosomes does not increase in
proportion to the enlargement of the nucleus.
The nucleolus in the germinal vesicle is concerned with the synthesis of
ribosomal RNA. The nucleolus of a growing oocyte increases greatly in
size.
In many animals, for example in amphibians instead of one large
nucleolus, many smaller nucleoli are formed in the germinal vesicle. It
is believed that RNA passes out of the nucleus into the cytoplasm during
the growth of the oocyte.
(b) Growth of cytoplasmic substances
The amount of cytoplasm increases quantitatively during the growth of
the oocyte. Besides, it also changes in quality by the elaboration and
regular distribution of various cell inclusions like mitochondria, Golgi
bodies, endoplasmic reticulum, cortical granules etc.
i. Mitochondria: The mitochondria are fewer in young oocytes but increase in numbers
quite appreciably during the growth of oocytes In some animals, e.g.,
amphibia, birds, they are aggregated in the form of large mitochondrial
clouds (Romanoff, 1960; Wattenberg, 1962; Belinsky and Davis, 1963).
Mitochondria are carriers of oxidative enzymes, therefore, the overall
oxygen consumption increases during the growth of the oocyte.
ii. Golgi bodies:
In younger oocytes, Golgi bodies are found around the centrosome. In
mature oocytes they sometimes disappear (Odor, 1950). This indicates
that Golgi bodies are transformed into some other structures like
cortical granules.
iii. Endoplasmic reticulum:
The cytoplasm of the young oocytes contains numerous vesicles surrounded
by a simple membrane. Ribosomes are often attached to the surface of the
vesicles.
These vesicles are often supposed to be equivalent to or to be a
modification of the endoplasmic reticulum. Oocytes often have a
different kind of membranous structure.
It takes the form of stacks of double membranes, either in parallel or
in spiral arrangement. The membranes usually do not have ribosomes
attached to them but instead are perforated by pores. These pores
closely resemble the pores of the nuclear membrane.
iv. Cortical granules:
In mature oocytes, formation of special structures known as the cortical
granules takes place on the inner side of the plasma lemma. These are
spherical bodies, surrounded, by a simple membrane and containing acid
mucopolysaccharides They vary in diameter from 0.8 μ (in sea urchins) to
2 μ (in frogs).
The acid mucopolysaccharides of the cortical granules synthesize
fertilization membrane during fertilization. The cortical granules are
present in sea urchins, frogs, fishes, bivalve mollusks, some annelids,
and some mammals (e.g., hamster, rabbit and man), but they are absent in
rat, guinea pig, gastropod mollusks, urodele amphibians, insects and
birds.
(ii) Vitello Genesis
Yolk appears in the oocytes in the second period of their growth called
the Vitello genesis period. Yolk is the most usual form of food storage
in the egg In amphibians (Ward 1962 ; Belinsky and Davis, 1963) and
fishes (Yamamoto and Oota, 1967) the Vitello genesis or synthesis of
different yolk components takes place inside modified
mitochondria.
In other vertebrates, the yolk is not synthesized in the oocytes at all
but is produced in the liver of the body of the female. It is then
transported in a soluble form via the blood stream and the follicle
cells to the oocytes when it is finally deposited in the form of yolk
platelets or yolk granules.
Physio-Chemical Nature Of Yolk
Yolk is a complex of variably assembled components, rather than a
definite chemical substance. The principal components are proteins,
phospholipids and fats in different combinations.
Depending on which of these components predominate, the yolk is
distinguished as "protein yolk" or “fatty yolk." These two kinds of yolk
are present side by side in the eggs of many animals. The avian yolk as
a whole contains 48.7% water, 16.6% proteins, 32.6% phospholipids and
fats and 1% carbohydrates.
The fatty portion of avian yolk is mainly neutral fat (50% of the dry
weight), the remaining being phosphatides and cholesterol. In animals,
yolk is found in three forms as given hereunder:
(a) Granular yolk: Protein yolk of many invertebrates like echinoderms and of lower
chordates (Amphioxus, Tunicates) consists of fine yolk granules which
are fairly evenly distributed in the cytoplasm of the eggs.
(b) Yolk platelets: In amphibians eggs, the yolk is found in the form of large granules
called yolk platelets. The yolk platelets are ovular and flattened in
shape and lies in one plane. They contain two main protein- aqueous
substance's phosvitin and lipovitellin.
Phosvitin is a highly phosphorylated protein (phosphorus 8.4%), whereas
lipovitellin contains a considerable amount of bound lipid (17.5%). In
the yolk platelets 2 molecules of phosvitin are associated with 1
molecule of lipovitellin in a structural unit.
Electron micrographs of amphibian yolk reveal that these units are
arranged in the platelet in a crystalline lattice work with regular
hexagonal packing (Wallace,1963).
In addition to the yolk platelets the amphibian egg contains, lipid and
glycogen. Lipid is found in the cytoplasm in the mitochondria, which
consists of an internal core of lipid surround by a thin protein layer.
Cyclostomes, elasmobranchs, ganoids and the lungfishes have eggs with a
distribution of food reserves much the same as amphibians.
(c) Yolk spheres: The yolk of birds, reptiles and bony fishes lies in a compact mass in
the interior of the egg. The cytoplasm is restricted to a thin layer on
the surface, with a thickened cap on the upper side. Most of the yolk is
liquid, but about 23% is in the form of solid "yolk spheres."
Function of Yolk
i. Yolk is the most usual form of food storage in the egg.
ii. It influences the differentiation of ooplasm and the pattern of
cleavage.
iii. The size of the egg is determined by the amount of yolk
present in it.
iv. Yolk exercises an important influence on the morphogenetic
movements of blastomeres during gastrulation.
v. The nature of development whether indirect with larval forms or
direct with juvenile stages is governed by the amount of yolk
present in the egg.
Role Of Follicle Cells In Oogenesis
The growing oocytes are surrounded by special nutritive cells which
immensely help their growth in various ways. Their are two main
types of nutritive cells, viz., follicle cells and nurse
cells.
(a) Follicle cells: In mammals particularly and some other vertebrates, the oocytes are
surrounded during growth and maturation phases by special cells of the
ovary, the follicle cells.
These are derived from the germinal epithelium of the ovary. Initially,
the young oocyte is surrounded by a single layer of follicle cells but
later. the number of follicle cells increases greatly, the cells become
arranged in several rows. (In a mammal, the follicle cells and the
developing oocyte together constitute a Graafian follicle.
As the egg approaches maturity, an eccentric cavity called antrum
appears in the mass of the follicle cells. This cavity is filled with a
fluid known as liquor folliculi which is secreted presumably but the
cells of the follicle. In the beginning, there is a simple apposition of
the follicle cells and the oocyte.
The cytoplasmic membranes of the adjoining cells remain separated by a
narrow gap of about 80 Å. The plasma membrane of oocyte and that of
follicle cells show close connections at some points in the form of
desmosomes, At a later stage a wider space appears between the follicle
cells and the oocyte.
However, the follicle cells maintain their contact with the oocytes at
the points where the desmosomes were observed in the earlier stage. The
surface of a young oocyte is drawn out into numerous finger-like
microvilli which project into the space between the oocyte and follicle
cells. These microvilli interdigitate with cytoplasmic processes of the
follicle cells.
The stroma consists of cortex and medulla, which are not sharply
delimited from each other. The cortex occupies the greater part of the
ovary.
It is a primitive type of connective tissue, in which large,
spindle-shaped fibroblasts predominate. Coursing in all directions
between them are compactly arranged fine collagenous and reticular
fibers. Scattered throughout the cortex are vesicles of different sizes
: these are the developing Graafian (or ovarian) follicles.
It also contains transformed discharged follicles-the corpora lutea
(singular, corpus luteum) and degenerating atretic
follicles.
If the ovum has perished on its way to the exterior the luteum survives
for about 14 days and then degenerates: this is the corpus luteum of
menstruation or corpus luteum spurium.
If the ovum has been fertilized the corpus luteum enlarges and
persists during pregnancy as the corpus luteum of pregnancy or corpus
luteum verum.
If fertilization of the ovum has not taken place, slight hemorrhage
into the corpus luteum occurs at the next menstruation and this is the
beginning of its involution.
The wide structure shrinks, and after about three months is
represented by fibrous hyaline scar, the corpus albicans. A corpus
luteum may sometimes give rise to a cyst.
The interior of the ovary, or medulla is a vascular and fibrous core.
Abundant blood vessels enter at the hilus and then take spiral course
inwards. The medulla reaches the "surface" only at hilus where it
merges with the mesovarium.
Events In Oogenesis
The nucleus of the oocyte becomes enlarged mainly due to the assembly
of an outsized amount of nuclear sap. Due to this the nuclei of advanced
oocytes appear to be bloated with fluid and are often referred to as
germinal vesicles.
The chromosomes increase in length. In oocytes of animals having large
eggs (notably in the amphibian oocyte), the chromosomes acquire a very
characteristic appearance. Numerous paired threads or loops project
transversely from the main chromosomal axes.
The shaggy appearance given to the chromosomes by these loops has led to
their being called lamp brush chromosomes. The loops of lamp brush
chromosomes represent loci of gene activity i.e., at these sections
messenger RNA is synthesized which subsequently controls the synthesis
of proteins in the cell.
However, the amount of DNA in the chromosomes does not increase in
proportion to the enlargement of the nucleus.
The nucleolus in the germinal vesicle is concerned with the synthesis of
ribosomal RNA. The nucleolus of a growing oocyte increases greatly in
size.
In many animals, for example in amphibians instead of one large
nucleolus, many smaller nucleoli are formed in the germinal vesicle. It
is believed that RNA passes out of the nucleus into the cytoplasm during
the growth of the oocyte.
i. Yolk is the most usual form of food storage in the egg.
ii. It influences the differentiation of ooplasm and the pattern of
cleavage.
iii. The size of the egg is determined by the amount of yolk
present in it.
iv. Yolk exercises an important influence on the morphogenetic
movements of blastomeres during gastrulation.
v. The nature of development whether indirect with larval forms or
direct with juvenile stages is governed by the amount of yolk
present in the egg.
Role Of Follicle Cells In Oogenesis
The growing oocytes are surrounded by special nutritive cells which
immensely help their growth in various ways. Their are two main
types of nutritive cells, viz., follicle cells and nurse
cells.
The presence of the microvilli greatly increases the surface area of
the oocyte. This increase in area facilitates metabolic turnover between
the oocyte and the surrounding cells. It is believed that the follicle
cells actively help in the growth of the oocyte by secreting substances
which are taken up by the oocyte.
The individual microvilli cannot be seen with the optical microscope and
the zone of microvilli appears as a radially striated layer known as the
zona radiata.
The presence of small in pocketing's of the oocyte cytoplasm at the base
of the microvilli points out that the oocyte takes in fluid and
dissolved substances from the space between itself and the follicle
cells by way of "cell drinking or pinocytosis.
(b) Nurse Cells: In some invertebrates like annelids, insects and mollusks the oocyte
is surrounded, in addition to follicle cells, by special nurse cells.
Derived from the egg cell, the nurse cells supplement the function of
follicle cells in providing nutrition to the growing
oocyte.
In Drosophila an oogonial cell divides by four successive mitotic
divisions into 16 cells. One of these cells becomes an oocyte whereas
the other 15 becomes nurse cells and nourish the oocyte. Nutrients
from the cytoplasm of the nurse cells pass into the oocyte through
gaps developed in the cell membranes of two cell types.
This type of relationship is different from that found between oocyte
and follicle cells because in this case no microvilli or cytoplasmic
processes are developed at the interface between the oocyte and the
nurse cell In some insects and annelids the nurse cells are gradually
consumed during the growth of the oocyte.
In mollusks e.g., the snail Helix, the entire nurse cells are
engulfed in the cytoplasm of the oocyte.
C. Maturation Phase: The primary oocyte contains diploid number of chromosomes. Alter the
oocyte completes its growth, it in ready for meiosis or reduction
division during which the diploid chromosome number in reduced to
haploid number.
In this process, the primary oocyte is changed into haploid ovum or.
Egg. This is called maturation. The maturation process of oogenesis is
somewhat different from that of spermatogenesis.
In spermatogenesis, the primary spermatocyte divides into four cells of
equal size which eventually give rise to four sperms, In oogenesis only
one ovum is produced out of the four unequal cells derived from the
primary oocyte.
At the beginning of the maturation phase, the nuclear membrane breaks up
and the contents of the nucleus get mixed up with the surroundings
cytoplasm. The chromosomes which have become greatly contracted and
concentrated toward the center of the germinal vesicle, are carried to the
periphery of the oocyte.
An achromatic spindle is formed at the periphery, which takes up a
position perpendicular to the surface of the oocyte. The bivalents are
stationed on the equatorial plate and subsequently separate into the two
component chromosomes. Next, a bulge appears at the surface of the
oocyte.
The outer pole of the spindle with half of the chromosomes enters into
this cytoplasmic bulge during anaphase of first meiotic division. The
bulge is then pinched off from the rest of the oocyte as a small cell
called the first polar body or polocyte.
It receives only a very small quantity of cytoplasm, while all the rest
goes to the oocyte which is now distinguished as the secondary oocyte. The
secondary oocyte is of the same size as that of primary oocyte. Thus, as a
consequence of the first meiotic division the primary oocyte divides into
a large and a very small cell, each with a haploid number of
chromosomes.
In the second meiotic division an achromatic spindle is again formed at
the surface. When division takes place, half of the chromatids are given
off, along with a small quantity of cytoplasm to form a secondary polar
body or podocyte.
The larger cell, which receives the major part of the cytoplasm together
with one half of the chromatids, represents the fully mature ovum. As the
secondary oocyte is dividing, the first polar body divides into two
cells.
Therefore, in the maturation of egg, four cells are produced from one
oocyte: one cell, the egg, is a functional gamete, the second and third
cells are produced by the division of the first polar body and the 4th
cell is the 2nd polar body.
All the polar bodies: disintegrate later because they have very little
cytoplasm with no food reserves.
The unequal cytokinesis (cytoplasmic division) during oogenesis has the
great significance for the egg. These unequal divisions allow one cell out
of the 4 daughter cells to inherit most of the cytoplasm and reserve food
material which is essential for the developing embryo.
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