Oogenesis

 Oogenesis is the period of growth
differentiation and maturation
occurring in the female gonads or
ovaries during which the egg or
ovum acquires its developmental
potential
 Ovum is a unique cell which has all
the properties to develop into a
new individual when segregated
from the organism
 VON BAER in 1827,discovered the
mammalian ovum and studied
embryonic development of many
animals

• In development at the time of gastrulation a small group
of cells are "put aside" to later form oocytes and
spermatozoa. This population of cells is described as the
primordial germ cells (PGCs).
•These cells also migrate initially into the posterior
endoderm that forms the hindgut and from there into the
genital ridge that will be the site of the developing gonad
,from this time there occurs a multiplication phase
leading to the formation of oogonia which is diploid in
number.
•The following cycle depicts different phases of Oogenesis
during meiotic events, it also shows follicular
development of an ovary.

Oogenesis
meiotic
previtellogenesis
vitellogenesis
premeiotic

 The primordial germ cell migrate from the endoderm.
 Multiply to form oogonia initiating the multiplication
phase.
 Thus leading to the formation of primary oocytes.

 This is a differentiation phase
where primary and secondary
oocytes formation takes place.
 It includes various sub-stages
namely:-
> Previtellogenesis and
> Vitellogenesis
 Previtellogenesis further includes
stages involved before formation of
the yolk where as vitellogenesis
refers to the stages involved in
formation of yolk.

GROWTH PHASE OF PRIMARY
OOCYTES
Magnitude of growth
Rate of oocytes growth
Mode of growth of oocytes

 OOCYTE grows to enormous proportions, normally to
become the largest cell of the animal body.
 Most of the PGC’s are approximately 10 microns in
diameter, which is the size of an average body cell.
 In case of mammals, some species may reach a diameter of
200 micron, in frog the increase is up to 2000 microns
where as in birds the diameter of the ovum is as large as
40,000 microns.
 This growth is not so much in active cytoplasm as
compared to an increase in the reserves such as yolk.

 Rate of growth of primary oocytes may be slow or
higher for example in case of drosophila it needs only 3
days for oocytes growth, whereas the mouse needs 16
days and the frog needs 3 years.
 In case of humans its around 27 days.

 During growth phase of primary oocyte qualitative and
quantitative changes take place both in the nucleus
and cytoplasm.
 It includes:-
1. growth of nuclear substances.
2. formation of mitochondrial clouds
3. formation of cortical granules

In oocytes not surrounded by nurse cells, oocyte nuclei swells
and chromosomes begin to elongate, such chromosomes are
called lampbrush chromosomes whereas in oocyte
surrounded by the nurse cells the lamp brush chromosomes
are absent and the nuclei of the nurse cell enlarges, become
metabolically active and synthesises the gene products which
are stored in oocyte cytoplasm.

LAMPBRUSH
CHROMOSOMES
• Homologous and bivalent
• Paternal and maternal chromosomes are
held together by chiasmata(sites of
crossing over)
• Long axis of each chromosome is made
up of two chromatids, with tight coils
called chromomeres, and the regions
which are uncoiled are called lampbrush
loops.
• This shape enhances the production of
mRNA, which produce proteins of
storage value

Gene Amplification
• One set of gene is replicated selectively in
rDNA(ribosomal DNA) of NOR(nucleolar
organiser) of oocytes in nucleolus, such
that large number of ribosomes are
manufactured.
• Eg:- Xenopus eggs are very large cells(1.3
mm in diameter) that accumulates large
number of ribosomes(10^12)

 Large number of mitochondria accumulates in the
oocyte, more than required for respiratory metabolism.
 In young oocytes mitochondria is present at the
periphery, but as it grows it is dispersed in the
cytoplasm.
 During growth, mitochondria segregates and forms
“mitochondrial clouds”.
 Mitochondria arises either from autonomous replication
of the circular DNA or from nurse cells, migrating down
the nutritive chords into oocyte cytoplasm

 membrane bound spherical bodies of diameter from 0.8
micron to 2 micron.
 formed from cisternae of Golgi apparatus, initially they
lie inside the cup-shaped space formed by Golgi
membrane then move to the periphery and get arranged
in layer close to the ectoplasm.
 contain mucopolysaccharides required for the synthesis
of fertilization membrane during fertilization.

 synthesis and deposition of yolk.
 Yolk - major nutritional reserve of the oocyte
- required for the nourishment of embryo
- composed of proteins, phospholipids and
natural fats.
 types of yolk - protein yolk(present in the form of
granules or yolk platelets)
- fatty yolk(present in the form of fat
droplets i.e. lipoproteins or
lipochondria)

 Protein yolk:-Yolk that has more of protein than lipids.
 Fatty yolk:- yolk that has more of fat contents than proteins
 Granular yolk:- yolk in forms of fine granules, evenly distributed
in cytoplasm of oocyte
 Yolk platelets:- yolk in form of large granules, oval in shape,
flattened in one plane
-two main proteinaceous substances:-
-phosphitin
-lipovitellin
-Phosphitin-highly phosphorylated protein
-Lipovitellin-protein with large molecule and
bound lipids
-2 mol of phosphitin+ 1 mol of lipovitellin =
1phospholipoprotein/ vitellogenin

1. Yolk is morphologically designated as the
molecule that takes part in the nourishment of
an embryo.
2. Yolk is made up of phospholipoprotein called
vitellogenin.
3. Vitellogenin is of crystalized form and is made
of 2 molecules of phosphotin + 1 molecule of
lipovitellin.
4. Formation of yolk invertebrates occurs in liver
under the influence of estrogen.
5. In this case the vitellogenin is present in
dephosphorylated form and is soluble by the
action of protein kinase and ATP get
phosphorylated and is stored in the insoluble
form in the ovary.
6. In fishes and amphibians it is present in solid
form where as its present in liquid or semisolid
form in reptiles and birds.

1. MICROLECITHAL OR OLIGOLECITHAL
EGGS – These are small-sized eggs containing
very small amount of yolk. These type of eggs are
found in Hydra and sea urchin.
2. MESOLECITHAL EGGS- These contains
moderate amount of yolk and are found in
annelid worms, molluscs and amphibians.
3. MEGALECITHAL, MACROLECITHAL OR
POLYLECITAL EGGS- These eggs contain
enormous amount of yolk and are found in
insects, reptiles and birds.

1. HOMOLECITHAL OR ISOLECITHAL-In this the
amount of yolk is so little that it is found scattered
almost uniformly throughout the egg cytoplasm.
2. TELOLECITHAL EGGS- These types of eggs have a
polarized distribution of yolk in the ooplasm and are
found in the mesolecitthal and macrolecithal eggs.
3. CENTROLECITHAL- In some hydrozoa and insects,
the yolk is concentrated in the center and the active
cytoplasm forms a thin peripheral layer around the
yolk

1. What do you mean by oogenesis,depict using a flowchart.
2. Explain follicular development of ovary?
3. What are PGC’s?
4. Name various sub stages of meiotic phase?
5. What are the factors involved in the growth phase of primary oocyte in
previtellogenesis?
6. Define the following
1. Lamp brush chromosomes
2. Mitochondrial clouds
3. Cortical granules
4. Lipochondria
5. Vitellogenin
7. What is composition of yolk?
8. What is the phenomenon involved in yolk formation?
9. Write a short note on egg types based on :-
1. Amount of yolk
2. Distribution of yolk
QUESTIONAIRE

 In the absence of lampbrush chromosomes which cells surround the
oocyte.
 Sperm mother cells
 PGCs
 Nurse cells
 Oogonia
 In which phase primary oocytes get arrested
 Metaphase I
 Anaphase I
 Telophase I
 Prophase I
• Primary oocytes leads to formation of how many secondary oocytes.
 4
 3
 2
 1

Oogenesis

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