Folliculogenesis MU AnSci 4314 Repro Phys

Anatomy of ovarian follicle

• Oocyte
• Granulosa cells (cumulus, mural)
• Theca layer (externa smooth M and
internal interstatial)

Primordial Follicle 

Single layer of squamous pregranulosa cells.

Primary Follicle

Single layer of cuboidal granulosa cells.

Secondary Follicle

Two or more layers of cuboidal granulosa cells and no evidence of antrum formation.

Antral Follicles

Tertiary: Early antrum formation and theca layer
Graafian: Continued antrum formation, cell division,
and oocyte growth

Primordial to Primary Follicle Transition

Foxl2 – Forkhead box L2 – transcription factor required for granulosa cell differentiation.
Nobox – oocyte specific homeobox gene
Sohlh1 and 2 - oogenesis basic helix–loop–helix transcription factor that is essential for oogenesis.
Lhx8 – (Lim homeobox gene) is essential for mouse oocyte differentiation and survival.

Primary to Secondary Follicle Transition

• Growth differentiation factor 9 (GDF 9)
– Oocyte specific
– Derived by Sohlh1 and 2 and Nobox
– Required for primary to secondary follicle transition.
• Kitl – Kit ligand - controls oocyte growth, oocyte survival, theca cell differentiation, and protects preantral follicles from apoptosis.
• Kit – (cKit) - Kit ligand recepto

Secondary to Antral Follicle Transition

hypothalamic–pituitary–gonadal (HPG) axis-dependent

• Fsh + Fshr - FSH and FSH receptor at/after the primary follicle stage– endocrine regulators.
• Inha – Inhibin a – inhibits FSH secretion
• Igf1 – Insulin like growth factor 1 – increases responsiveness of follicular cells to gonadotropins
• Ccnd2 – Cyclin D2 – regulated by FSH and is required for granulosa cell proliferation.
• Foxo1/3 – Forkhead box o transcription factor – highly expressed in granulosa cells and can regulate follicle growth or death.
• Smad3 – mediator of transforming growth factor (TGF) beta signaling pathway and required for follicular growth
• Taf4b – transcription factor that has a role in meiotic gene expression.

Antral (tertiary) to Preovulatory (Graafian)
Follicle Transition
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Antral (tertiary) to Preovulatory (Graafian) Follicle Transition

• Esr1 and 2 – estradiol receptor
• Acvr2a – Activin receptor type 2a – Activin can stimulate granulosa cell proliferation and increase estradiol receptors.
• Gja4 – Gap junction protein – Connexin 37
• Connexins – Connexin 37 (somatic-germ cell interactions)
• Mutation – blocks development after antral stage.
– Connexin 43 (somatic cell interactions)
• Mutation – blocks development after primary follicle

Gap Junctions and Paracrine Communication

• Granulosa cells and the Oocyte are connected through gap junctions:
– Made of proteins called connexons composed of six subunits.
– Form pores connecting the cytoplasm of the two cell types.
– Allow for direct “communication”, i.e., transfer of
small molecules: ions and metabolites between the
oocyte and GC.
• Also “communicate” through paracrine secretions
– Larger molecules

The dogma surrounding folliculogenesis?

At birth, or
soon after the female
is equipped with the
number of germ cell
for the rest of her life.

Atresia

Programmed cell death (apoptosis)
• Most follicles (99%) are destined to undergo
atresia

Two Stages of Follicular Recruitment

Initial recruitment: primordial follicular pool recruited
into the pool of growing follicles in a continuous
manner- i.e., independent of stage of cycle
• Gonadotropin independent
– Cyclic recruitment: a cohort of antral follicles are
recruited to grow towards ovulation during a specific estrous cycle
• Gonadotropin dependent

Stages of a Follicular Wave stage 1

Recruitment (cyclic)
– Cohort of small antral follicles initiate growth
following release from the inhibition of the
previously dominant follicle and an increase in
circulating concentrations of FSH.
– Fate: • Selected for continued growth or become atretic. • Atresia

Stages of a Follicular Wave stage 2

Selection
– From the recruited pool, typically one (or more if litter bearing species) follicle will be selected for continued growth.
– At selection, circulating concentrations of FSH have decreased in response to negative feedback of estradiol and inhibin from the growing cohort of
recruited follicles
– As the selected follicle becomes dominant it becomes dependent upon LH.

Stages of Follicular Wave stage 3

Dominance: Selected follicle(s) become
dominant follicle(s)
• Dominant follicle secretes a lot of estradiol and
inhibin which keeps circulating concentrations of
FSH low.
• Low FSH prevents recruitment of the next wave.
Fate: ovulation (low progesterone) or atresia (high
progesterone)