Oogenesis

DETAILED MECHANISM OF OOGENESIS INCLUDING FOLLICULOGENESIS

1. 1

HAJEE KARUTHA ROWTHER HOWDIA COLLEGE
(An Autonomous institution affiliated to Madurai Kamaraj University, Madurai.)
UTHAMAPALAYAM -625 533

ASSIGNMENT

Course Title : Developmental biology & Evolution
Course Code : 17PZYC31
Assignment on : Oogenesis
Submitted to
Ms.Athira Sugumaran
Assistant Professor
Department of zoology
Hajee Karutha Rowther Howdia College,Uthamapalayam


Submitted by
H.Kalimani
II-M.Sc Zoology
Hajee Karutha Rowther Howdia College, Uthamapalayam



Submission Date: 23/08/2020

2. 2

Contents
1. Definition of Oogenesis …………………………………………………………….3
2. Introduction to Oogenesis…………………………………………………………....3
3. Mechanism of Oogenesis…………………………………………………………….3
4. Multiplication phase…………………………………………………...……… ….....3
5. Germ cells in the human ovary over the life span …………………………… ………4
6. Growth phase…………………………………………………………………… .……5
7. Previtellogenesis growth period………………………………………………………6
8. Growth of cytoplasmic substances…………………………………………… ……..7
9. Growth of cytoplasmic substances……………………………………… …….…….7
10. Folliculogenesis ………………………………………………………… …………...8
11. Development phases of Follicles…………………………………………………….11
12. Vitellogenesis growth period…………………………………………………….…13
13. Maturation phase………………………………………………………………..…..15
14. Polar Bodies………………………………………………………………….……..18
15. Life of Ovum…………………………………………………………….………….19
16. Reference…………………………………………………………………………….20

3. 3

OOGENESIS


Definition of Oogenesis
Oogenesis is the growth process of female gametes in female reproductive system. It is the type
of gametogenesis through which the primary egg cell (or ovum) becomes a mature ovum.

Introduction to Oogenesis
Oogenesis is differs from spermatogenesis. The gamete formed by oogenesis contains all
materials needed to initiate and maintain metabolism and development. Therefore, in addition to
forming a haploid nucleus, oogenesis also builds up a store of cytoplasmic enzymes, mRNA,
organelles and metabolic substrates. While the egg devlops complex cytoplasm. The mechanisms
of oogenesis vary among species. This variation should notbe surprising, since patterns of
reproduction vary so greatlyamong species. In those species that produce thousands of ova the
germ cells, called oogonia, are self-renewing stem cells that endure for the lifetime of the
organism. In those species that produce fewer eggs, the oogonia divide to form a limited number
of egg precursor cells.

Mechanism of Oogenesis
Oogenesis is a intricated process. It occurs in three different phases.

I. Multiplication phase
II. Growth phase
III. Maturation phase

I. Multiplication phase
Oocytes are the descendants of primordial germ cells that originate in the wall of the yolk sac
in the embryo and migrate to the gonadal region. They divide repeatedly by mitosis and the
resultant cells are called oogonia or egg mother cells. The oogonai again divide repeatedly by
mitosis. When the division stops, the cells are named as primary oocytes. The nucleus of primary
oocyte is diploid.

4. 4


Germ cells in the human ovary over the life span

In the human embryo, the thousand or so oogonia divide rapidly from the second to the seventh
month of gestation to form roughly 7 million germ cells. (Figure 1) After the seventh month of
embryonic development, however, the number of germ cells drops precipitously. Most oogonia
die during this period, while the surviving population, under the influence of retinoic acid, enter
then next step and initiate the first meiotic division.

They become primary oocytes. This first meiotic division does not proceed very
far, and the primary oocytes and remain in the diplotene stage of first meiotic prophase. This
prolonged diplotene stage is sometimes referred to as the dictyate resting stage. This may last
from 12 to 50 years. primary oocytes continue to die.



FIGURE 1 Changes in the number of germ cells in the human ovary over the life span.




Of the millions of primary oocytes present at her birth, only about 400 mature during a woman's
lifetime. With the onset of puberty, groups of oocytes periodically resume meiosis. At that time,
luteneizing hormone (LH) from the pituitary gland releases this block and permits these
oocytes to resume meiotic division. They complete first meiotic division and proceed to second
meiotic metaphase. This LH surge causes the oocyte to mature (Figure 2)


The oocyte begins to synthesize the proteins that make it competent to fuse
with the sperm cell and that enable the first cell divisions of the early embryo. This maturation

5. 5

involves the cross-talk of paracrine factors between the oocyte and its follicular cells, both of
which are maturing during this phase. The follicle cells activate the translation of stored oocyte
mRNA encoding proteins such as the sperm binding proteins that will be used for fertilization
and the cyclins that control embryonic cell division. After the secondary oocyte is released from
the ovary, meiosis will resume only if fertilization occurs. At fertilization, calcium ions are
released in the egg, and these calcium ions release the inhibitory block and allow the haploid
nucleus to form.




FIGURE 2 Overall oogenesis



II. Growth phase

Growth is an important phenomenon in oogenesis. During growth nutrients and other materials
necessary for the development of the embryo are synthesized. As these substances accumulate in
the cytoplasm, the oocyte increases considerably in size. The duration of growth of oocyte is
prolonged. In the new born human baby all her oocytes are already formed. The first ovum is
released at the time of puberty. So, it has a growth period of 12 to 14 years. The last egg released
at the age of 45-50. So the last egg has a growth period of 45-50 years.

6. 6

The progressive growth increase in nuclear as well as cytoplasmic substances of oocytes may be
divided into two stages (Figure 3)

1. Previtellogenesis growth period

2. Vitellogenesis growth period



FIGURE 3 Growth of oocyte



1. Previtellogenesis growth period

During this phase no synthesis and accumulation of food reserve material, the yolk takes place
but tremendous increase in the volume of nucleus and cytoplasm of primary oocyte occurs.
There is qualitative and quantitative increase in the amount of cytoplasm. The mitochondria
increase in number, the network of endoplasmic reticulum with ribosomes becomes more
complicated, the, Golgi bodies manufacture cortical granules, besides performing their normal
function.

There are two function happened during previtellogenesis growth period:

i. Growth of nuclear substances
ii. Growth of cytoplasmic substances

7. 7



i. Growth of cytoplasmic substances

During this phase due to the production of the large amount of nuclear sap, the nucleus of the
growing oocyte increase in size. This large sized oocyte inflated with the fluid is now called
germinal vesicle.
The nucleus of the occyte enters the prophase of meiotic division. Synapsis occurs between
homologous chromosomes but the subsequent stages of meiosis are postponed and each
chromosomes increases in its length, but the amount of DNA in each choromosomes does not
increase in proportion to the enlargement of the nucleus. It is believed that the loop of
chromosomes represent actual site for the main activity of the genes, i.e. transcription of mRNA,
which inturn controls translation process in which synthesis of proteins in the cell cytoplasm
takes place.
During the growth period of ooctye, all mRNA molecules are not utilized during translation but
some are inactivated by the wrapping of proteins around them and stored as informosomes to be
used during early cleavage of egg, when chromosomal DNA remains more actively engaged in
its own transcription of mRNA (messenger RNA) r- RNA (ribosomal RNA) or t-RNA (transfer
RNA).
The RNAs are transcribed by r-DNA of ‘nucleolar organizer region’ of chromosomes. The
nucleolus has a significant role in the storage and maturation of the ribosomal RNAs. It also
synthesizes all the proteins required for the biogenesis of ribosomes. Therefore, during growth
period of primary oocytes, the nucleolus increases greatly in size and becomes very conspicuous.
The increased transcriptional activity (i.e. RNA synthesis) of chromosomal genes during growth
period of oocytes, is called gene amplification or redundancy. When MRNA molecules are
transcribed from DNA then it is known as transcriptional amplification. Each MRNA molecule
in turn can be translocated several times into the corresponding proteins known as translational
amplification.
This high rate of gene amplification or gene activity is correlated with the fact that gene
reduction (meiosis) does not take place until after the growth of the oocytes has been completed.
As a result the oocytes remain tetraploid for a long time.

ii.Growth of cytoplasmic substances

The amount of cytoplasm of oocyte increases both quantitatively and qualitatively during the
Previtellogenesis growth period of oocyte. Young oocytes, in many animals, show a very simple
organisation due to poor cytoplasmic inclusions and possess none of the specialized structures
found in the adult oocyte and mature egg. The cytoplasm is finely granular having granules of
ribonucleo-protein and DNA.

8. 8

Mitochondria, the carriers of oxidative enzymes are fairly scarce in young oocytes but may
increase in number very considerably during the growth of primary oocyte because overall
oxygen consumption increases during this time. Mitrochondria possess its own circular DNA. So
in a growing oocyte, the amount of mitrochondrial DNA far exceeds the amount of nuclear
DNA.
The young oocytes have the granular endoplasmic reticulum in the form of numerous, small
vesicles. Annulated lamellae are also found in the cytoplasm of growing oocyte. These
membranous structures appear in the form of stakes of cistemae, either in parallel or in spiral
arrangement.
Sometimes, annulated lamellae are associated with ribosomes and RNA in high concentrations,
and there is also an ATPase activity in the pore complexes of these lamellae. The lamella, thus
serves as a storage site of RNA in cytoplasm and they are found to break down and disappear
during late oogenesis.
In young oocyte the Golgi bodies are found around the Centrosome. In mature oocytes they form
a large spherical mass in some mammals, or sometimes may disappear completely. The Golgi
complex of oocyte is believed to synthesize cortical granules besides performing its normal
function.
In the cortical region, cortical granules are present. These are membrane bound spherical bodies
contain acid mucopolysaccharides. These mucopolysaccharides are used during fertilization, in
the formation of fertilization membrane.
They are present in bivalve molluses, some annelids, fishes, frogs and some mammals (rabbit
and man), but are absent in some insects, gastropodes urodeles, birds and some mammals (rat
and guine apig). These granules are synthesized’ by cisternae of Golgi complex in the interior of
the oocyte and later they move to the periphery where they are arranged in a layer close to the
plasma membrane of oocyte.
The growing oocytes are surrounded by special kinds of nutritive cells. These cells immensely
assist the growth of oocytes in various ways. There are two types of nutritive cells, namely
follicle cells and nurse cells. In the ovary of chordates, the developing oocyte is surrounding by
follicle cells. Other invertebrates addition to the follicular cells it also contain nurse cells.

Folliculogenesis
Definition: folliculogenesis is the maturation of the ovarian follicle, a densely packed
shell of somatic cells that contains an immature oocyte. Folliculogenesis describes the
progression of a number of small primordial follicles into large preovulatory follicles.
Folliculogenesis ends when the remaining follicles in the ovaries are incapable of responding to
the hormonal cues that previously recruited some follicles to mature. This depletion in follicle
supply signals the beginning of menopause.

9. 9


The primary role of the follicle is oocyte support. From birth, the ovaries of the human female
contain a number of immature, primordial follicles. These follicles each contain a similarly
immature primary oocyte. At puberty clutches of follicles begin folliculogenesis, entering a
growth pattern that ends in death (apoptosis) or in ovulation (the process where the oocyte leaves
the follicle).

During follicular development, primordial follicles undergo a series of critical
changes in character( Figure 4, 5) , both histologically and hormonally. First they change into
primary follicles and later into secondary follicles. The follicles then transition to tertiary, or
antral, follicles. At this stage in development, they become dependent on hormones, particularly
FSH which causes a substantial increase in their growth rate. The late tertiary or pre-ovulatory
follicle ruptures and discharges the oocyte (that has become a secondary oocyte), ending
folliculogenesis.



FIGURE 4

10. 10


FIGURE 5 Folliculogenesis stages (a) , secondary follicle (b)

11. 11

Development phases of Follicles

Folliculogenesis is continuous, meaning that at any time the ovary contains follicles in many
stages of development. The majority of follicles die and never complete development. A few
develop fully to produce a secondary oocyte which is released by rupture of the follicle in a
process called ovulation.
The growing follicle passes through the following distinct stages:

Primordial phase
At 18–22 weeks post-conception, the cortex of the female ovary (foetal female ovary) contains
its peak number of follicles (about 4 to 5 million in the average case, but individual peak
populations range from 6 to 7 million). These primordial follicles contain immature oocytes
surrounded by flat, squamous granulosa cells (support cells) that are segregated from the
oocyte's environment by the basal lamina. They are quiescent, showing little to no biological
activity. Because primordial follicles can be dormant for up to 50 years in the human, the length
of the ovarian cycle does not include this time.
The supply of follicles decreases slightly before birth, and to 180,000 by puberty for the average
case (populations at puberty range from 25,000 to 1.5 million). By virtue of the "inefficient"
nature of folliculogenesis, only 400 of these follicles will ever reach the preovulatory stage.
At menopause, only 1,000 follicles remain. It seems likely that early menopause occurs for
women with low populations at birth, and late menopause occurs for women with high
populations at birth, but there is as yet no clinical evidence for this.The process by which
primordial cells 'wake up' is known as initial recruitment.


Primary phase

During ovarian follicle activation, the granulosa cells of the primordial follicles change from a
flat to a cuboidal structure, marking the beginning of the primary follicle. The oocyte genome is
activated and genes become transcribed. Rudimentary paracrine signalling pathways that are
vital for communication between the follicle and oocyte are formed. Both the oocyte and the
follicle grow dramatically, increasing to almost 0.1 mm in diameter. Primary follicles develop
receptors to follicle stimulating hormone (FSH) at this time, but they are gonadotropin-
independent until the stage.

Second phase
Stroma-like theca cells are recruited by oocyte-secreted signals. They surround the follicle's
outermost layer, the basal lamina, and undergo cytodifferentiation to become the theca
externa and theca interna. An intricate network of capillary vessels forms between these two
thecal layers and begins to circulate blood to and from the follicle.

12. 12

The late-term secondary follicle is marked histologically and structurally by a fully grown oocyte
surrounded by a zona pellucida, approximately nine layers of granulosa cells, a basal lamina, a
theca interna, a capillary net, and a theca externa. The development of the antrum also starts
taking place in secondary follicle stage.

Antrum formation
The formation of a fluid-filled cavity adjacent to the oocyte called the antrum designates the
follicle as an antral follicle, in contrast to a so-called preantral follicle that still lacks an antrum.
An antral follicle is also called a Graafian follicle.
Definitions differ as to which stage this shift occurs in, with some designating follicles in
the secondary stage as antral,
]
and others designating them as preantral.




FIGURE 6 A mature Graafian follicle

13. 13

Structure of ovarian follicle

An ovarian follicle is a roughly spheroid cellular aggregation set found in the ovaries. It secretes
hormones that influence stages of the menstrual cycle. Women begin puberty with about 400,000
follicles, each with the potential to release an egg cell (ovum) at ovulation for fertilization. These
eggs are developed once every menstrual cycle.

Ovarian follicles are the basic units of female reproductive biology. Each of them contains a
single oocyte (immature ovum or egg cell). These structures (Figure 6) are periodically initiated
to grow and develop, culminating in ovulation of usually a single competent oocyte in
humans. They also consist of granulosa cells and theca of follicle.


Oocyte: Once a month, one of the ovaries releases a mature egg (ovum), known as an oocyte.
The nucleus of such an oocyte is called a germinal vesicle.

Cumulus oophorus: Cumulus oophorus is a cluster of cells (called cumulus cells) that surround
the oocyte both in the ovarian follicle and after ovulation.

Mambrana granulosa: It contains numerous granulosa cells.

Granulosa cell: Granulose cells or follicular cells are cells that surround the oocyte within the
follicle; their numbers increase directly in response to heightened levels of
circulating gonadotropins or decrease in response to testosterone. They also produce peptides
involved in ovarian hormone synthesis regulation. Follicle-stimulating hormone (FSH) induces
granulosa cells to express luteinizing hormone (LH) receptors on their surfaces; when circulating
LH binds to these receptors, proliferation stops.

Theca of follicle: The granulosa cells, in turn, are enclosed in a thin layer of extracellular
matrix – the follicular basement membrane or basal lamina (fibro-vascular). Outside the basal
lamina, the layers theca interna and theca externa are found.



2. Vitellogenesis growth period

The process of formation and deposition of yolk in the oocyte is called vitellogenesis. Yolk is
the nutritive material of the ovum. It is present in the form of platelets or granules.
Chemically yolk is lipoprotein composed of proteins, phospholipids and neutral fats along with a
small amount of glycogen. The yolk is synthesised in the liver of the female parent in soluble
form.(Figure 7) Through circulation it is transported to the follicle cells that surround the
maturing ovum, and is deposited in the form of yolk platelets and granules in the ooplasm. The
mitochondria and Golgi complex are said to bring about the conversion of the soluble form of
yolk into insoluble granules or platelets.

14. 14



FIGURE 7 Vitellogenin production



In mammalian vitellogenesis, vitellogenin is the major protein, produced by the Vit gene and
regulated by oestrogen. The yolk consists of lipids (triglycerides, cholesterol etc.) and proteins,
mainly vitellogenin.
The term vitellogenesis comes from the Latin vitellus ("egg yolk").

In vertebrates, a very small quantity of the material for the formation of yolk (hardly 1%) is
synthesised by the oocyte cytoplasm itself.
Thus a major part of the material forming the yolk is exogenous (formed outside the oocyte).

15. 15



Processing of yolk by the oocyte

The liver does not produce the yolk directly. But it synthesizes a precursor called vitellogenin
for the yolk. When it is brought to the oocyte, the vitellogenin is transformed into actual yolk.
The Golgi complex and the endoplasmic reticulum transport the yolk-components to the
mitochondria. In mitochondria, the soluble yolk-components are made insoluble by a
mitochondrial enzyme called proteinkinase. This enzyme crystallizes the soluble yolk into
insoluble yolk granules or yolk platelets. During this process of crystallization, the mitochondrial
cristae become dislodged and their membrane ultimately become arranged in concentric layers
while the whole mitochondrial space is occupied by the main body of the yolk platelet.

3.Maturation phase

The primary oocyte contains a diploid number of chromosomes. The diploid chromosome
number is reduced to haploid number by meiosis or reduction division and the primary oocyte is
chaned into the secondary then ootid and the into ovum or egg (Figure 8). This is called
maturation.

The majority of the oocytes within the adult human ovary are maintained in the diplotene stage
of the first meiotic prophase, often referred to as the dictyate state. Each oocyte is enveloped by a
primordial follicle consisting of a single layer of epithelial granulosa cells and a less organized
layer of mesenchymal thecal cells.

Periodically, a group of primordial follicles enters a stage of follicular growth. During this time,
the oocyte undergoes a 5OO-fold increase in volume (corresponding to an increase
in oocyte diameter from 10 !lm in a primordial follicle to 80 /lm in a fully developed follicle).
Concomitant with oocyte growth is an increase in the number of granulosa cells, which fonn
concentric layers around the oocyte. TItis proliferation of granulosa cells is mediated by
paracrine factor, GDF9, a member of the TGF-β family.

Throughout this growth period, the oocyte remains in the dietyate stage. The fully grown follicle
thus contains a large oocyte surrounded by several layers of granulosa cells. The innennost of
these cells will stay with the ovulated egg, forming the cumulus, which surrounds the egg in the
oviduct. In addition, during the growth of the follicle, an antrum (cavity) forms and becomes
filled with a complex mixture of proteins, hormones, and other molecules. Just as the maturing
oocyte synthesizes paraerine factors that allow the follicle cells to proliferate, the follicle cells
secrete growth and dillerentiation factors (TGF-β2, VEGF, leptin, Fgf2) that allow the oocyte to
grow and bring blood vessels into the follicular region. The oocytes are maintained in the
dictyate stage by the ovarian follicle cells. Signals from the follicles activate a G­protein linked
receptor that stimulates adenyl cyclase to elevate levels of cAMP in the oocyte. The release from
this dictyate stage and the reinitiation of meiosis is driven by lutenizing hormone (LH) from the
pituitary.

16. 16






FIGURE 8



Meanwhile, the growing oocyte is actively transcribing genes whose products are necessary for
cell metabolism, for oocyte-specific processes, or for early development before the zygote-
derived nuclei begin to function. In mice, for instancei the growing diplotene oocyte is actively
transcribing the genes for zona pellucid proteins ZP1, ZP2, and ZP3. Moreover, these genes are
transcribed only in the oocyte and not in any other cell type, as the proteins essential for
fertilization are being synthesized.

17. 17




Ootidogenesis:

The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid.
This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological
definition, a cell whose primary function is to divide by the process of meiosis.
However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all
oocytes, still primary oocytes, have halted at this stage of development, called the dictyate.
After menarche, these cells then continue to develop, although only a few do so every menstrual
cycle.

Meiosis I:

Meiosis of ootidogenesis begins during embryonic development, but halts in the diplotene stage
of prophase I until puberty. The mouse oocyte in the dictyate (prolonged diplotene) stage
actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate
(leptotene, zygotene and pachytene) stages of meiosis. For those primary oocytes that continue to
develop in each menstrual cycle, however, synapsis occurs and tetrads form,
enabling chromosomal crossover to occur. As a result of meiosis I, the primary oocyte has now
developed into the secondary oocyte and the first polar body.

Meiosis II:

Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this
process is also halted at the metaphase II stage until fertilization, if such should ever occur. If the
egg is not fertilized, it is disintegrated and released (menstruation) and the secondary oocyte does
not complete meiosis II (and doesn't become an ovum). When meiosis II has completed, an ootid
and another polar body have now been created.polar body is small in size.

Maturation into ovum:

Both polar bodies disintegrate at the end of Meiosis II, leaving only the ootid, which then
eventually undergoes maturation into a mature ovum (Figure 9).
The function of forming polar bodies is to discard the extra haploid sets of chromosomes that
have resulted as a consequence of meiosis.

18. 18



FIGURE 9 A mature Ovum




Polar Bodies

Oogenic meiosis in mammals differs from spermatogenic meiosis not only in its timing but in the
placement of the metaphase plate. When the primary oocyte divides, its nuclear envelope, breaks
down, and the metaphase spindle migrates to the periphery of the cell. This asymmetric
cytokinesis is directed through a cytoskeletal network composed chiefly of filamentous actin that
cradles the mitotic spindle and brings it to the oocyte cortex by myosin-mediated contraction. At
the cortex, an oocyte-specific tubulin mediates the separation of chromosomes, and mutations in
this tubulin have been found to cause infertility. At telophase, one of the two daughter cells
contains hardly any cytoplasm, while the other daughter cell retains nearly the entire volume of
cellular constituents (Figure 10). The smaller cell becomes the first polar body, and the larger
cell is referred to as the secondary oocyte. A similar unequal cytokinesis takes place during the
second division of meiosis. Most of the cytoplasm is retained by the mature egg (the ovum), and
a second polar body forms but receives little more than a haploid nucleus. (In humans, the first
polar body usually does not divide. It undergoes apoptosis around 20 hours after the first meiotic
division.) Thus, oogenic meiosis conserves the volume of oocyte cytoplasm in a single cell rather
than splitting it equally among four progeny.

19. 19



FIGURE 10 (1)Meiotic anaphase I, wherein the spindle migrates to the periphery of the egg and releases a
small polar body. (2) Meiotic metaphase II, wherein the second polar body is given off (the first polar body has
also divided)






Life of Ovum
In some species, such as sea urchins and frogs, the female routinely produces hundreds or
thousands of eggs at a time, whereas in other species, such as humans and most other mammals,
only a few eggs are produced during an indivdual's lifetime.

20. 20

Reference

1) Developmental Biology by Scott F. Gibert (6
th
Edition)
2) Developmental Biology by Scott F. Gibert (8
th
Edition)
3) Developmental Biology by Scott F. Gibert 11
th
Edition)
4) https://en.wikipedia.org/wiki/
5) https://www.biologydiscussion.com/animals-2/eggs-and-oogenesis-in-animal-1956-words-
biology/645