Oogenesis/Folliculogenesis Flashcards
Embryo cleavage stage cells are ___potent
Totipotent
Can form any type of cell, including extra-embryonic membranes
Primordial germ cells
- Pluripotent (can form all body cell types)
- Extra-gonadal origin
- Migrate to gonads (while proliferating)
- Gamete precursors: give rise to oogonia vs spermatogonia in ovaries vs testes
Blastocyst stage = differentiation into 2 cell types:
- Trophoblast
- ICM
What are the 2 differentiated layers of the ICM?
Inner epiblast/embryonic ectoderm
Outer primitive extra-embryonic visceral endoderm
Where do PGCs arise from?
Inner epiblast/embryonic ectoderm of the ICM
When do PGCs within the primary ectoderm detach/migrate into the extra-embryonic yolk sac near the base of the allantois?
2-4 weeks s/p fertilization
When do fetal membranes start forming at the implantation site?
15 days post-fertilization
Migration of PGCs begins at ___ weeks gestation:
- First migrate to yolk sac > pass hindgut > via dorsal mesentery > dorsal wall (split to R/L sides)
By ___ weeks gestation:
- Reach presumptive gonad region (final destination)
- Induce formation of genital ridge (primordial gonads)
*Remember: during migration, PGCs proliferate from 30-50 cells > 1-2K cells at genital ridge
5-6 weeks gestation
6-12 weeks gestation
What do PGCs look like?
What are their molecular markers?
Large
Spherical
Pale cytoplasm (high glycogen granule content)
Molecular markers: TNAP, PAS, OCT4, VASA, c-KIT
PGCs initially form as clusters held together by ___
Adhesion molecules:
- Cadherins
- Integrins
What paracrine factors inspire PGC differentiation from the ICM epiblast?
Bone morphogenic protein (BMP4) from the extra-embryonic ectoderm bind SMAD proteins on PGCs
What do competent PGCs in the epiblast secrete?
- Fragilis –> which then induces
- Blimp1
- Stella
What is the ligand for the integrin receptor on PGCs that aids in migration to the gonads?
Fibronectin (in the extracellular matrix on the migration pathway)
Other ECM molecules that aid in PGC migration: laminin, collagen, tenascin
How does the gonadal ridge itself induce PGC migration?
Release chemoattractants (TGF-beta)
What genetic reprogramming occurs in PGCs during migration to the genital ridge?
- Achieve pluripotent status
- Epigenetic modification (resets genomic imprinting)
What are the primitive sex cords made of?
Primitive sex cords are swellings (primitive gonads) on each side of the mesonephros
Coelomic (germinal) epithelium and mesonephros (embryonic kidney) that migrate alongside PGCs into the gonadal ridge form the sex cords
What determines sex differentiation of the primitive sex cords?
SRY on Y chromosome
Which factors direct the bipotential gonad > ovary?
> testes?
Ovary:
- DAX1
- WNT4
Testes:
- SOX9
- SRY
Describe the primordial follicle
- Primary oocyte: arrested in meiosis I, prophase I, diplotene stage
- Surrounded by single layer of flattened, pre-granulosa cells
**Since mitosis completes in fetal life, at birth, there are NO oogonia (all are primary oocytes)
Oogonia –> Primary oocyte
Complete mitosis in the gonadal ridge during fetal life –>
Oogonia become primary oocytes arrested in meiosis I, prophase I
What is the longest stage of folliculogenesis?
Primordial follicle (may stay in this stage x 50 years)
When is the maximum number of primordial follicles reached, and how many is this?
6-7 million primordial follicles by 16-20 weeks gestation
How many primordial follicles by birth?
1-2 million
How large are primordial follicles?
Primary follicles?
Pre-antral follicles?
Antral follicles?
Graafian follicles?
Primordial follicles: 30-60 um in diameter (contain primary oocytes with diameter 9-25 um)
Primary follicles: (contain primary oocytes with diameter 60-120 um)
Pre-antral follicles: (contain primary oocytes with diameter > 120 um)
Antral follicles: 200-400 um
Graafian follicles: 2 cm
What is the first sign of follicle growth?
- Primordial follicles increase in oocyte size
- Then, granulosa cells become more cuboidal
- Gonadotropin-independent growth
Describe a primary follicle
- Primary oocyte (larger)
- Surrounded by > or = 15 cuboidal granulosa cells
- Separated by basal lamina
Describe a pre-antral (secondary) follicle
- Still primary oocytes (larger)
- Surrounded by > 1 layer of granulosa cells
- Zona pellucida forms
- Gap junctions between oocytes and granulosa cells
- Theca cells form
When do pre-antral follicles start forming
20-24 weeks gestation
Continuous process until menopause
When does the zona pellucida form (which follicle type)
Pre-antral (secondary) follicle
When do gap junctions form between granulosa cells and oocytes?
Pre-antral (secondary) follicle
When do theca cells form?
Pre-antral (secondary) follicle
When does the oocyte reach its maximal size?
Late pre-antral (secondary) follicle stage
What factors are important in pre-antral (secondary) follicle development?
FSH (promotes granulosa cell growth and inhibits apoptosis)
IGF-1 produced by granulosa cells, helps amplify follicular response to FSH
Describe an antral follicle
- Antrum: fluid accumulates and coalesces between granulosa cells
- > 600 granulosa cells! > 50 million by end of development!
- “Cumulus” differentiation
When do the granulosa cells differentiate into cumulus?
Antral follicle
When does theca differentiate into interna vs externa. Define the layers
Development of antral follicles
Theca interna: androgenic (has LH receptors)
Theca externa: collagen
In which stage of folliculogenesis does estrogen become dominant in follicular fluid?
Antral follicle
Describe a Graafian follicle
- Mature ovarian follicles (completes meiosis I)
- Hyaluronate
- Hypoxanthine and adenosine - hold oocytes in meiotic arrest
- Theca vacuolized and vascularized
- Peak estrogen
- LH surge –> luteinization granulosa cells –> progesterone
When does the theca become vascular?
Graafian follicle
By puberty, ____% of follicles have become atretic
~95%
Primordial > primary follicle growth factors
Gonadotropin-independent
TGF-beta superfamily
Activins+
BMPs+
Inhibins-
AMH-
GDF-9
BMP 15
FOXL2 –> granulosa cell growth
Neurotropins (via tyrosine kinase receptors): NGF, BDNF
C-KIT, Kit ligand (communication between oocytes and GCs): required for antrum development in follicle
What mutation is responsible for POI, eyelid disorder
FOXL2
When is GDF-9 required?
Primary follicle > Secondary (pre-antral) > Antral > pre-ovulatory
Connexins:
- Cx37
- Cx43
Cx37: granulosa cells to oocytes
Cx43: granulosa cells to each other
Oocyte-derived factors that promote folliculogenesis prenatally (primordial, primary follicles)
- Fig-alpha (factor in the germline alpha): controls pre-granulosa cells to recognize female gametes and form single layer of primordial follicle, also helps produce ZP proteins in the pre-antral (secondary) follicle
Recruitment of primary follicles (5 oocyte-derived factors): GDF-9, BMP-15, BMP-6, FGF-8, TGF-2
Which paracrine factors released from the oocyte control granulosa cell proliferation?
GDF-9
BMP-15
Promote GCs to differentiate into mural vs cumulus (antral follicle stage)
LH receptor is expressed mainly by __ GCs, but not by ___ cells
LH receptor expressed by mural GCs not by cumulus cells
What induces cumulus expansion pre-ovulation?
Gonadotropins stimulate
Cumulus cells to produce
Hyaluronic acid (glycosaminoglycan)
Also: oocytes release CEEF (cumulus expansion-enabling factor) stimulating
Cumulus cells to produce
Hyaluronic acid
GDF-9
What happens right before ovulation?
- Luteinization of cumulus cells
- Surge of gonadotropin > expansion of cumulus cells (production of hyaluronic acid)
- Prostaglandin production (prostaglandin-endoperoxide synthase 2 or PTGS2/COX2) mediated by GDF-9
Factors secreted by granulosa cells in folliculogenesis
AMH - inhibits folliculogenesis
Activin - promotes GC proliferation, initiates growth of primordial follicles, potentiates FSH action, modulates steroidogenesis in GC and TC
Inhibin - inhibits pituitary FSH secretion, but is a follicular growth signal
Kit ligand - promotes oocyte growth, forms oocyte/GC regulatory loop with GDF-9
Follistatin - binds to activin to neutralize its action
Oocyte-GC Regulatory Loop
In the pre-antral follicle:
- Oocyte secretes GDF-9 –>
- Promotes GC growth/differentiation
- GC secretes Kit ligand –>
- Promotes oocyte development
What is the obligate energy source for oocytes and early zygotes?
Where is it secreted from?
What else is consumed to provide energy source for oocytes?
Pyruvate from granulosa cells
Oxygen –> forms ATP which can pass through gap junctions to oocyte
What process produces energy in granulosa cells?
Glycolysis
What morphological changes occur during oocyte growth?
- Increased elongated mitochondria
- Increased smooth (lipid synthesis) & rough ER (protein synthesis)
- Golgi apparatus move to periphery, become vesicular
- Nuage clouds rich in ribonucleoproteins (RNPs)
- Nucleus expands to 4 um in diameter
What morphological changes occur in the dominant follicle?
- Decreased size of Golgi apparatus
- Superficial accumulation of cortical granules
- Undulated nuclear membrane
- Nucleolus vacuolates –> ring-like
- Enlarged peri-vitelline space
- Corona radiata cells start to retract connections to oocyte (causes gap junctions to retract)
What arrests oocytes in prophase I?
- Hypoxanthine (in follicular fluid of Graafian/dominant follicle)
- cAMP (from GCs and possibly oocytes)
- LH surge decreases cAMP (due to retraction of gap junctions between GCs and oocytes) and allows resumption of meiosis
Does the oocyte have an LH receptor? How does it respond to LH surge?
No, via the cumulus cells
What is MPF?
Maturation promoting factor
Universal cell cycle engine for driving mitosis and meiosis
Once activated, activates meiotic events:
- Chromosome condensation
- GV breakdown
- Spindle assembly
What is cytostatic factor?
Includes MAP kinase
Keeps oocytes in metaphase II (M2) arrest prior to fertilization
Inactivated by calcium influx upon fertilization
In what stage of meiosis are oocytes when they ovulate?
Meiosis II, Metaphase II
The introduction, development, migration, and survival of primordial germ cells (PGCs) are controlled by signals produced by the neighbor cells. Among the following signals, which one is involved?
a.
Sex-determining region Y (SRY)
b.
Maturation-promoting factor (MPF)
c.
Tissue-nonspecific alkaline phosphatase
d.
Stem cell factor
d. SCF
The trigger for a cell to enter metaphase is:
Select one:
a.
Decrease in gap junctions between an oocyte and corona radiata cells
b.
Fertilization of an ovulated oocyte by sperm
c.
Increase in MPF activity
d.
Increase in cyclic adenosine monophosphate (cAMP)
c. Increase in MPF activity
Example of totipotent cells
Primordial germ cells
How is retinoic acid involved in oogenesis?
Meiosis inducer (oogonia –> primary oocytes)
GV
MI
MII
GV - immature, arrested in prophase I
MI - GV breakdown, resumption of meiosis I
MII - completed meiosis I, arrested in metaphase II, extruded PB, corona radiata
