Repro: L6,7,8 flashcards

Main stages of germ cell development in females

Primordial germ cells (PGCs) → Oogonia → Oocytes → Follicles → Egg (ovum)

they are all the same cells at different stages:
PGCs undergo mitosis: numbers increase as they multiply and is called an oogonia

Oocyte: when the oogonia stops undergoing mitosis and undergoes meiosis

Egg: when grows to maturity and ovulated

What happens to PGCs in migration

undergo mitosis and migrate to gonads via Growth factors such as TGFβ, SCF (stem cell factor), and bFGF (basic fibroblast growth factor).

end up as oogonia

What do oogonia do in the ovary

undergo mitosis and become surrounded by somatic cells, forming germ cell nests.

what are germ cell nests

Clusters of interconnected oogonia that share cytoplasmic bridges due to incomplete cytokinesis.

Cytoplasmic bridges: communication and sharing of resources, coordinating germ cell development.

What happens in oogonia > oocytes

oogonia stop dividing and enter meiosis and are now called oocytes

oocytes are finite

most oocytes die by apoptosis before follicle formation bc they have insufficient granulosa support or failure to reach meioti

What is meiotic arrest

A prolonged pause in meiosis that occurs in oocytes during fetal development and lasts until ovulation - oocyte remains activeb y:

• Acquiring more components as it grows to resume meiosis around ovulation

• Acquiring developmental competence to support embryo development and a healthy individual

essential for oocytes to remain viable for years and synchronizes maturation with follicle development and hormonal cycles.

primordial follicles

form mid-pregnancy in foetal ovary (humans) or at birth in mice

structure is A primary oocyte surrounded by a single layer of flattened pre-granulosa cells.

The zona pellucida — a glycoprotein layer that persists around the egg even after ovulation.

 

What is the primordial follicle pool

 A finite reserve of resting follicles formed before birth; no new follicles are generated afterward.

Follicles either grow or die. (atresia) through reproductive life

The resting pool are quiescent follicles and if a follicle leaves they will grow and differentiate into more advanced follicles

What is folliculogenesis?

Process where primordial follicles grow and develop into mature, ovulatory (Graafian) follicles

main stages are: Primordial → Primary → Secondary (Pre-antral) → Antral → Pre-ovulatory (Graafian).

What is the strucure of granulosa cells associated with the oocyte as it ages

cumulus: loosely associated, some come out with oocyte in ovulation

corona radiate: attach to outside the zona pellucida

Stalk: suspend oocyte within follicular fluid

What hormones are involved with granulosa cells

They respond to FSH and produce Oestrogen and inhibin A/B

They influence the HPG axis (determines fertility and menstrual cycle

produce AMH as a marker of ovarian reserve

What are the main cell types in the follicle

• Oocyte (germ cell)

• Granulosa cells (somatic support cells)

• Theca cells (hormone-producing outer layer)

What is the first visible sign of follicle activation

granulosa cells: squamous > cuboidal > then multilayerd

oocyte begins to grow

this is irreversible

Regulated by PTEN signalling pathway which keeps primordial follicles resting state

What happens if PTEN signalling is knocked out?

all follicles grow simultaneously = depletion of the ovarian reserve

What is a primary follicle

Follicle with growing oocyte and layer of cuboidal granulosa cells

What is a pre-antral follicle

follicle with multiple layers of granulosa cells and a basement membrane

forms the zona pellucida, a glycoprotein-rich ECM between oocute and granulosa cells

theca cell layer surrounds the granulosa layer externally

avascular inside the basement membrane

What are theca cells

Recruited from the ovarian stroma

essential for follicle growth

differentiate into theca interna and externa

the last to die in atresia

associated with PCOS (LH/FSH imbalance)

Theca interna

Respond to LH > produce androgens

> communicate hormonal messaes to the oocyte

only part of oocyte that has blood vessels (vascular)

Theca externa

Fibrous, for structural support

What is an antral follicle

formation of a fluid-fileld cavity (antrum) with the granulosa layers

antrum accumulates and supplies oocyte with nutrients and metabolic waste

What happens to granulosa and oocyte as the antral follicle forms

oocyte grows and produces mRNAs needed for early embryonic development

granulosa cells proliferate and form specialised subtypes

What are the 2 main types of granulosa in mature follicles

Cumulus granulosa: surround and communicate with oocyte

Mural granulosa: line the follicle wall, steroidogenically active, interface with the theca layer - produce steroids and nutrient and hormone exchange betweeen follicle and bloodstream

connected by gap junctions for transfer of ions and signalling molecules

What is the pre-ovulatory follicle

Large, fully mature follicle ready to release an oocyte during ovulation

stopped here until puberty

What occurs before ovulation

before ovulating, the follicle expands > oocyte resumes meiosis > cumulus and mural granulosa cell functions diverge

What occurs during ovulation

The oocyte + cumulus complex is released; meiosis continues to produce a haploid oocyte and a polar body.

The remaining follicle forms the corpus luteum which secretes hormones (prog) to support pregnancy

What is follicular atresia

degeneration and death of follicles that don’t complete development

90% of follicles undergo atresia

Hormone availability and competition for FSH determines if cell will undergo atresia

AMH and follicles

Produced by growing pre antral follicles, prevents premature follicle activation

High AMH levels inhibit recruitment of new follicles from resting pool

High AMH = fewer follicles activated, resting pool preserved

Low AMH: more follicles activate, reserve depleted faster

PTEN is better for suppressing follicle activation

Which hormones control ovarian follicle development?

GnRH from the hypothalamus stimulates secretion of FSH and LH from anterior pituitary

FSH: Stimulates granulosa cell proliferation and antral growth, Promotes estrogen synthesis in granulosa cells, Required for follicle survival beyond early stages.

LH: Stimulates theca cells to produce androgens (precursors to estrogen), Required for ovulation and corpus luteum formation.

Is FSH required for early follicle growth?

no

Does endocrine suppression (oral contraceptive) affect the primordial follicle pool

 No — the primordial pool declines independently of circulating hormones.

When do follicles become hormonally dependant

At the antral stage, when they begin expressing FSH receptors on granulosa cells.

FSH stimulates aromatase expression in granulosa cells - converts theca-derived androgens into oestrogen

If follicles are FSH deprived, They undergo atresia and degenerate.

What does oestrogen do in follicles?

It promotes granulosa cell proliferation and positive feedback on follicle growth.

What is LH essential for?

Ovulation - triggers oocyte maturation and follicle rupture.

 

What is the LH surge

High levels of oestrogen from the dominant follicle trigger positive feedback on the hypothalamus and pituitary.

• Oocyte completes meiosis I.

• Follicular wall ruptures → ovulation.

• Remaining granulosa and theca cells are luteinised to form the corpus luteum.

What occurs during luteinisation

Focus on production of progesterone over oestrogen

granulosa cells increase 10x in size

Corpus luteum become highly vascularised

Why is corpus luteum yellow

accumulation of glycogen granules and lutein pigments

What is follicle dominance?

The process where one (or a few) follicles are selected to continue development and ovulate, while others regress.

Occurs at mid-antral stage

Characteristics of dominant follicles?

• More FSH receptors.

• Better vascularisation of the theca layer (↑ FSH delivery).

• Mural granulosa cells begin expressing LH receptors.

 It has developed LH receptors and autocrine/paracrine signals that sustain growth even with lower FSH.

What hormone negatively feedbacks on FSH production

Ostrogen and inhibin from granulosa cells.

Why do different animals ovulate different amounts of oocytes?

Humans: only one follicle becomes dominant due to tight FSH regulation.

Sheep (2 oocyte): threshold for FSH decline allows two follicles to survive.

mice (6-10): Mice have weaker FSH suppression and multiple follicles can remain dominant.

What determines litter size in mammals

The number of dominant follicles that successfully ovulate.

Do follicles communicate with each other?

Yes — local paracrine signaling allows follicles to coordinate growth and regulate the number of dominant follicles.

Ensures the appropriate number of follicles mature each cycle to match species reproductive strategy.

What is the ovarian surface epithelium

thin layer outside the ovary

ruptures then repaires each month after ovulation - highly linked to cancer bc of the constant cell turnover (mutations)

Lots of stem-cell-like cells in this to help with repeated repair and rupture

What other structure is associated with ovarian cancers?

Fallopian tube

What is used to study ovarian mesenchymal stem cells

Pdgfr-alpha

isolated and studied with Flow cytometry (FACS) and single-cell sequencing

oMSC paracrine and protective effects:

• Anti-apoptotic (protect granulosa cells)

• Anti-inflammatory (reduce harmful inflammation)

• Angiogenic (promote new blood vessels)

• Stimulate follicles (activate dormant primordial follicles)

• Anti-oxidant (reduce oxidative stress and ovarian aging)

What is the Extracellular matrix in the ovary

A 3D scaffold for follicles and stromal cells

made of water, proteoglycans, fibrous proteins

remodelled for follicle growth every month

becomes fibrous with age

Which pathway mediates tension signalling for follicle activation?

Hippo pathway

Where are perivascular cells found

Around blood vessels

What parts of the ovary is avascular or vasclar

cortex is avascular

medulla is vascularised which is essential for follicle growth and hormone transport

Identifying granulosa and theca diagramatically

granulosa: purple

theca: green

What is the Zona pellucida?

A Halo around the oocyte that is produced by the oocyte

Made up of glycoproteins secreted by oocyte and granulosa

ZP1,ZP2,ZP3 are structural components - repeated dimers

What is ZP3

Essential for preventing polyspermy after fertilisation

Changes conformation once sperm enters to block other sperm

Tried as birth control but it was not reversible and reduced the primordial follicle pool - scrapped

Oocyte activity and mRNA storage

Oocyte relies on stored mRNA - doesnt perform new transcription

Stored mRNA:

• Regulates protein synthesis

• Acts as a switch to turn gene expression on/off

• Maintains meiotic arrest

Mitochondria in oocytes

inherited from mother

quality decreases with age due to being reactive to ROS + high mutation rate

bottleneck: Random segregation of healthy vs mutant mitochondria — no control over which primary oocytes get which

What is the lipid content in oocytes

Very high lipid levels (makes them dark in species like cows)

What are vesicles in oocytes?

Occupy 15–35% of volume; often move to the center during maturation; unclear function

What are cortical granules?

Secretory organelles from Golgi; react with ZP3 to block polyspermy

How do oocytes and granulosa cells support each other

• Oocytes help granulosa cells expand

• Granulosa cells help oocytes maintain meiotic arrest and support maturation

What are gap junctions

• Specialised intercellular channels for exchanging ions, metabolites, signalling molecules

• Maintain coordination between granulosa cells and the oocyte

What are the 2 key connexins (gap juncs)

• Connexin 43 (Cx43)

  • Found between granulosa cells

  • Enables communication across follicular cell layers

• Connexin 37 (Cx37)

  • Connects granulosa cells to the oocyte

  • Essential for oocyte-granulosa communciation

Function of gap junctions

• Supports metabolic cooperation - oocyte depends on the granulosa for nutrients and energy

• Maintain high cAMP in oocyte - prevents premature meiotic resumption

• Facilitate signalling via oocyte-derived factors like GDF9 and BMP15 - promotes granulosa differentiation and function

 

What happens to ga[ junctions in ovulation

• Supports metabolic cooperation - oocyte depends on the granulosa for nutrients and energy

• Maintain high cAMP in oocyte - prevents premature meiotic resumption

• Facilitate signalling via oocyte-derived factors like GDF9 and BMP15 - promotes granulosa differentiation and function

What are transzonal projections

• Long, thing cytoplasmic extensions from granulosas - penetrate the ZP and contact the oocyte membrane

• Contain gap junctions at their tips - reinforces the granulosa-oocyte communication

• Unique structure

• Bridge between oocyte and granulosa

• Transfer of small molecules, growth factors, signals

• Support oocyte metabolism and maintain meiotic arrest

 

What is genomic imprinting?

Epigenetic silencing where only one parental allele (maternal or paternal) is expressed

Maternal vs Paternal imprinting

Maternal: Mother’s gene inactive, father’s active

Paternal: Father’s gene inactive, mother’s active

imprinting occurs during sperm or egg development

What happens to the embryos genome after fertilisation (into zygote)

Global demethylation of DNA (except imprinted genes remain protected)

PGCs must remove imprinting patterns to later re-establish new imprinting during gametogenesis - ensures parent-specific gene expression

Cycle of imprinting

1. Fertilisation:

   • Zygote inherits imprints from both parents.

   • Some genes show methylation from one parent only.

2. Early Embryo:

   • Most of the genome becomes demethylated (methylation removed).

   • Imprinted genes remain methylated and retain their specific pattern.

3. Primordial Germ Cells (PGCs):

   • Later in embryonic development, PGCs undergo demethylation, including removal of previous imprints.

   • This “resets” the genome so that new sex-specific imprints can be established.

4. Gametogenesis:

   • As germ cells mature:

     • Female germ cells → acquire maternal imprints.

     • Male germ cells → acquire paternal imprints.

5. Cycle repeats with the next generation.

 

What stage are oocytes in meiotic arrest?

Dictyate stage of prophase I

meiotic arrest is maintained by GPCRs on the oocyte producing high cAMP levels

How does ovulation trigger meiosis to resume?

 At ovulation, granulosa-oocyte contact decreases, GPCR activity stops, cAMP falls, and meiosis resumes

What weakens the follicle wall before ovulation?

Hyaluronan secreted by cumulus cells

phases of ovulation

1. Follicle Expansion — Hyaluronic acid causes swelling; essential for ovulation

2. Follicle Contraction — Smooth muscle in outer follicle contracts; inhibition stops ovulation

3. Follicle Rupture — Follicle surface bulges and ruptures, releasing egg and follicular fluid

What contraction of a cell is essential during ovulation?

muscle cell contraction is essential for egg release

oocyte survives 24 hours after ovulation