PRIMORDIAL GERM CELLS (PGCs)
DURING 2ND WEEK OF EMBRYO
PGC FORMED IN EPIBLAST
PGC BEGINS TO MIGRATE FROM THE YOLK SAC
MIGRATION ENDS IN DEVELOPING GONADS
DIFFERENTIATE INTO OOGONIA - 7 MILLION
CELL DEATH BEGINS -MAJORITY OF OOGONIA
REMAINING PRIMARY OOCYTES HAVE ENTERED
PROPHASE OF MEIOSIS I – DIPLOTENE STAGE (RESTING PHASE)
• Oogonia divide rapidly from the second to the seventh month of gestation to form
roughly 7 million germ cells. After the seventh month number of germ cells drops
BEGINNING OF PUBERTY
TILL 50 YEARS (MENOPAUSE)
2 MILLION PRIMARY OOCYTES
500 ONLY OVULATED
• Oogonia divide rapidly from the second to the seventh
month of gestation to form roughly 7 million germ cells.
After the seventh month number of germ cells drops
• Gametes formed by oogenesis contains all the materials needed to
initiate and maintain metabolism and development.
Form haploid nucleus,
All the organelles involved in fertilization have to be constructed,
All the mRNAs & proteins have to positioned properly in the oocyte,
All the membrane proteins involved in coordinating the interactions
with sperm have to be synthesized and in place,
The accumulated material in the oocyte cytoplasm includes energy
sources and energy –producing organelles(yolk & mitochondria)
The enzymes and precursors for DNA, RNA and protein synthesis,
Stored messenger RNAs,
Morphogenetic regulatory factors that control early embryogenesis
CELLULAR COMPONENTS STORED IN THE MATURE OOCYTE
OF XENOPUS LAEVIS
APPROXIMATE EXCESS OVER
AMOUNT IN LARVAL CELLS
• Oogenesis vary among species to species.
• E.g. Sea urchin & Frogs - routinely produces
hundreds or thousands of eggs at a time.
• Humans and most mammals - only a few eggs are
produced during the lifetime of an individual.
Most of the biochemical changes takes place in the growth
1) MULTIPLICATION PHASE
• No conspicious changes have been traced
upto the formation of primary oocyte.
1. Oocyte increases in size
• Nutrients & other materials synthesized, then
these substances accumulate in the cytoplasm.
Young oocyte of frog - 50µ in diameter
Mature oocyte of frog- 20-40 times larger.
Hen’s oocyte- 200 times larger.
Mouse oocyte- 43 times larger.
2. Period of growth:
Frog 3 years oogenesis – first 2 years size increase
gradually then 3rd year accumulation of yolk
increases its size.
Human – 12 or 13 years
3. Stages of growth phase:
Previtellogenesis (cytoplasmic & nuclear
materials grow- no yolk synthesis)
ii. Vitellogenesis (synthesis of yolk )
Nucleus increase in size
Nucleoli increase in
Mitochondria increase in
Corticle granules in GB
Follicle Cells/Nurse Cells
• Nuclear sap produced in a large amount –
nucleus increases its size = Germinal vesicle.
• Homologous chromosomes pair together.
• Active mRNA synthesis –
GENE TRANSCRIPTION IN AMPHIBIAN OOCYTES:
• During the diplotene stage, certain
chromosomes stretch out large loops of DNA,
causing the chromosome to resemble a lamp
brush. These lamp brush chromosomes can
be revealed as the sites of RNA synthesis by in
be incubated with
a radioactive RNA
used to visualize
the gene is being
Figure shows diplotene chromosome I of the
newt Triturus cristatus after incubation with
radioactive histone mRNA.It is obvious that a
histone gene (or set of histone genes) is
located on one of these loops of the
Lampbrush chromosome (Old et al. 1977).
• Liver produce inactive/precursor vitellogenin
• Transported through blood to ovary enzymes protein
kinase convert it into active vitellogenin- mitochondria
• The growing diplotene oocyte is actively
transcribing the genes for zona pellucida
proteins ZP1, ZP2, and ZP3.
• The diploid primary oocyte reduced into
haploid & form ovum.
• 1st meiotic division – 1 polar body having small
amount of cytoplasm
- larger cytoplasm having
cell is known as secondary oocyte.
2nd meiotic division – produce 3 polar bodies
and a single ovum
• The time of maturation varies in different
• It may occur after / before fertilization or at
the time of fertilization.
• Sea urchin – before
• Vertebrates – after
• Acidian – at time of fertilization
FSH promotes the growth & development of oocyte
LH triggers ovulation
FSH, LH promote meiotic maturation division of oocyte
and stimulate follicle cells to synthesis vitellogenin.
• During the process of oogenesis, oocytes of
many animal species undergo meiotic arrest
prior to the completion of chromosomal
reduction and it is in this state that they
undergo tremendous growth. The length of
time that oocytes remain in this arrested state
and the nature of the stimulus which
reinitiates meiosis are species dependent