Histology: Oocyte Development Flashcards
6 stages of the life history of an oocyte
- Primordial germ cell
- Oogonium (mitotically active)
- Oocyte (entry into meiosis)
- Growth
- Meiotic maturation
- Ovulation and fertilization
What occurs to embryonic oocytes part-way through meiosis
They become arrested and complete meiosis at sexual maturity
Fate of meiotically arrested oocytes near the time of birth
Become enclosed in primordial follicles
Percentage of oocytes that die before puberty
>90%
4 steps of post-natal oogenesis
- Oocyte and follicle exit the primordial stage and initiate growth
- Oocyte grows; follicle enlarges by cell proliferation and follicular fluid
- Oocyte matures and is ovulated
- Empty folicle becomes corpus luteum, producing progesterone needed for implantationg and beyond
- Embryo secretes factors required to maintain the corpus luteum
3 important genes involved in premature ovarian failure
- Pten
- Foxo3a
- Tsc1/2
(Inhibitors of primordial follicular growth)
Genetic treatment for infertility
Hippo signalling disruption and Akt stimulation of ovarian follicles
Volume increase and time required for oocyte growth
Volume increase = ~100 fold
Requires 3 - 4 months
Where is the growing oocyte arrested?
Late G2 (prophase I, diplotene, diakinesis)
Why do oocytes grow?
- To accumulate mRNAs, proteins, ribosomes, mitochondrias etc that the early embryo will need
- To produce a giant cell that can be divided into many smaller cells during the cleavage divisions of early embryogenesis
Follicular cells required for the oocyte to grow
Granulosa cells (without them, there is no growth)
2 factors provided by the granulosa cells to the growing oocyte and how
- Gja4 (gap junctions)
- Kit ligand (secreted)
Without Gja4, what cannot occur in the oocyte?
Oocytes cannot undergo final stage of development
Define the zona pellucida
Barrier to oocyte-graulosa communication
What connects the granulosa to the oocyte?
Transzonal processes (TZPs)
Essential factor provided by the oocyte to the granulosa
Gdf9
3 consequences of lacking Gdf9
- Granulosa cells do not proliferate properly
- Oocytes grow larger than normal
- Oocytes cannot undergo final stage of development (meiotic maturation) –> infertility
Describe the role of thecal and granulosa cells in the production of estradiol
- Thecal cells synthesize testosterone from cholesterol
- Granulosa cells convert testosterone to estradiol
2 distinct populations of granulosa cells (different gene products)
- Mural granulosa
- Cumulus granulosa
At what stage do most follicles become atretic?
After reaching the antral stage (although the oocytes are probably healthy)
Substance that underlies follicular atresia
Limited supply of FSH
Effect of injected FSH
Rescue subordinate future atretic) follicles to increase the number of eggs for assisted reproduction
Physiological trigger of meiotic maturation of the oocyte
Luteinizing hormone (LH) (also triggers ovulation)
When may meiotic maturation of the oocyte occur without LH?
When fully grown oocytes are removed from the follicle and places in culture (in vitro maturation)
Two components of meiotic maturation of the oocyte
- Nuclear maturation
- Cytoplasmic maturation
Events of nuclear maturation
Completion of the first meiotic division and progression to metaphase II
Origin of maternally derived aneuploidy
Nuclear maturation
5 essential processes of cytoplasmis maturation
- Translational activation of some mRNAs
- Degradation of many mRNAs (oocyte-to-embryo transition)
- Migration of cortical granules to cortex
- Reorganization of endoplasmic reticulum – facilitate Ca release at fertilization
- Redistribution of mitochondria – surround meiotic spindle
3 steps from LH to oocyte maturation
- LH
- Mural granulosa LH receptors induced by FSH (note: not present on the cumulus granulosa cells or oocyte)
- EGF-related growth factors
- OGFR or mural and/or cumulus granulosa
- Oocyte maturation
Event in the oocyte that triggers maturation
Decreased cyclic AMP within the oocyte
Explain the way cAMP (and maturation) are regulated by LH
cGMP inhibition by EGFR to reduce cAMP production
