Lecture 21: Ovarian and Estrous Cycle Flashcards
What happens with the formation and activation of primordial follicles?
After initial differentiation of the gonad
-A large number of PGC’s (primary germ cells now made oocytes) will die
-Squamous granulose cells start to differentiate
-Oocytes organized in clusters (cysts) surrounded by the squamous granulose
Oocytes start meiosis and arrest in prophase 1
Timing differ between species
-Rodents: cluster (cysts) assembly occurs at birth
-In humans and cows, this process occurs mid gestation
Primordial follicles do not proliferate, population represents a females total reproductive potential
Process of primordial follicle formulation is gonadotropin (LH; FSH) independent in mammals
What is a primary follicle?
-Define by a simple cuboidal cells surrounding the oocyte
-Meiosis is arrested in the diplotene stage of prophase 1
-During this arrest, chromosomes decondense and are actively transcribed = tremendous growth of oocytes
-A membrane is seen surrounding the oocyte which will become the zona pellucida as ZP protein genes are expressed (specific will ind to sperm and allow recognition)
-Factors secreted by the oocyte stimulate granulosa cells
-In turn, granulose cells from primary follicle secrete factors which stimulate oocyte growth (oocyte and granulose cells communicate)
What happens during oocyte growth?
-Replication of cytoplasmic organelles especially mitochondria and their DNA
-Increase in ribosome, mRNA and proteins
-Lots of nutrients stored in granules and vesicles
-Golgi apparatus enlarge and moves to the periphery (export ZP glycoproteins; cortisol granules)
-Centriole disappears (the male one will be used)
What are the interactions between he oocyte and granulosa?
-Granulosa cells start expressing FSH receptor and are thus now responsive to FSH (critical point for future growth and recruitment)
-Communication via gap-junctions between granulosa cells and oocyte critical to prevent meiosis to proceed
What is a secondary follicle?
-Growing phase no cycle (basal level of FSH stimulate growth from primary to secondary)
-Large increase in granulosa cells layers, surrounded by a basal lamina
-Theca cells start to multiply on the outside of the basal lamina
-Internal theca
-External theca with muscular layer
-Simultaneously blood vessels develop within the theca
-Nutrients and waste will have to diffuse from the theca layer (no blood vessels within the follicle)
What are antral follicles?
-Cavitation: appearance of a fluid filled cavity
-4 stages: small, medium, large and preovulatory
-Both gransulosa and theca cells also multiply
-Theca externa: muscular innervated layer
-Theca interna: produces androgens under the control go LH
What is the follicular wave?
-Group of small antral follicles (less than 1mm) grow and mature in synchrony = recruitment
-Growing follicles secrete estradiol and inhibin in increasing amounts
-Some follicle stop growing and undergo atresia, the others keep growing = selection
-Inhibin levels increase = inhibits FSH production
-Dominant follicles have high density of FHS receptors and high sensitivity to FSH: continue growing to Graafian follicle= dominance
What is the process of resumption of meiosis?
-Occurs during the ovulation sequence
-First step= germinal escape breaks down
-LH stimulates local production of growth factors including IGF
-LH surge desensitizes granulosa cells = decrease in cAMP
-Second step= completion of first meiotic division with expulsion of the first polar body
-Finally: initiation of the second division with arrest in metaphase 2 until fertilization
What is cytoplasmic maturation?
-Essential for the monospermic fertilization, processing of sperm, preparation for development to preimplantation
Includes:
-Ability to release intracellular Ca upon fertilization (triggers exocytosis of cortical granules) seal off zona p after fertilization
-Production of proteins which will prepare male pronucleus
-Accumulation of mRNA, proteins, substrates, and nutrients (that oocyte need to have for further maturation)
What happens during ovulation?
High concentration of LH (from surge) leads to:
-Accumulation of fluid = increase pressure in the follicle
-Secretion of collagenase= loosen the ovarian connective tissue
-Increased blood flow in the ovary, fluid accumulates further
(increase pressure, loosen tissue, increase blood flow)
-Follicular wall bursts, oocyte released from granulosa cells, captured by the infundibulum, moved to ampula by ciliated epithelium
What happens in the luteal phase?
-After ovulation: corpus hemorrhagicum (blood vessels rupture) and granulosa and theca cells mix
-Corpus luteum: mixture of large (ex-granulosa) and small (ex-theca) cells
-Produce large amount of progesterone (to quiet down) : negative feedback on GnRH
-Invasion with blood vessels
How long are most species fertile after ovulation?
-Average of 24-15 hours
-Highlights the once follicle is ovulated at end of estrus need to have fertilization occurring very rapidly after that or not fertilized and go through another cycle
-Technically, oocyte should encounter spermatozoa in the oviduct within 15-30 mins of ovulation (sperm should be there ideally before ovulate)
What are some behaviours in cows that indicate estrus?
-Mounting- stimulated by Estradiol-17B
-Standing to be mounted: Requires more Estradiol-17B (higher threshold) ie running out of time
-Chin pressing (testing)
-Head-turning (welcoming)
-Urine, sniffing, FLEHMEN
-Restless, irritable, noisy, escape
What is the series of steps involved for estrus behaviours?
-Same as reflex
1. Sensory (visual, olf, Aud, Tactile)
2. Hypothalamus: estrogen receptors, increased nerve excitability
3. Midbrain:”receiving zone” for hypothalamic peptides, and speeds impulses
4. Medulla: integrates postural adaptation for lordosis and mounting
5. Spinal cord: generates signals to specific muscles for lordosis and mounting
What are factors promoting mounting behaviour?
-Size of sexually-active group (2+)
-Time of day (usually night)
-Footing (dirt, nonslippery)
-Age/health of cow(agility)
-Lack of distractions + human activity
-Temperature weather
