3. Oogenesis and Follicular Development Flashcards
What makes up the fundamental female reproductive unit?
Single ovarian follicle, composed of one germ cell (oocyte), surrounded by endocrine cells
What is menarche?
Beginning of menstrual cycles avg 11-13yrs
What determines age at puberty?
– genetics – nutrition – geographic loca:on – exposure to light – body composi:on, fat deposition – exercise
What controls the menstrual cycle?
Gonadotropins and gonadal hormones
What are the 3 phases of the ovarian cycle?
- Follicular phase av 15days (ranges from 9-23 days)
- Ovulatory phase: 1-3 days (and culminates with ovulation)
- Luteal phase: 13 days (is less variable than follicular)
What are the three phases of the endometrial cycle?
Menstruation
Proliferative
Secretory
What are 4 function of the ovaries?
- OOGENESIS ie production of gametes during the foetal period
- MATURATION of oocyte
- EXPULSION of the mature oocyte (ovulation)
- SECRETION of female sex steroid HORMONES (oestrogen & progesterone) and peptide hormone inhibin.
What is oogenesis?
The process when a developing egg (oocyte) differentiates into a mature egg (ovum)
Involves completion of meiosis which occurs at the time of fertilisation
Progression from oogonia to primordial follicle?
Oogonia produced by mitotic division (max # about 7m)
At 8-10wks of gestation, prophase of 1st meiosis starts. Oogonia becomes primary oocyte.
Primary oocyte is surrounded by pre-granulose cells. Called primordial follicle
What is mitosis?
Process of cell division that results in two genetically identical daughter cells developing from a single parent cell
What is meiosis?
Division of a germ cell involving two fissions of the nucleus and giving rise to four gametes. Each possessing half the number of chromosomes of the original cell
Describe the first phase of oogenesis that occurs during foetal life?
- Oogonia develops in the embryonic yolk sac 3 weeks post conception.
- Migrates to ovary
- Colonise the cortex
- Undergo mitosis
- At 8-10 weeks meiosis begins
- 1-2 million oocytes around the time of birth (reduces to 400,000 around puberty)
- Remaining oocytes are arrested in meiotic prophase until last oocytes are ovulated
Most common cytogenetic abnormalities associated with age…
Aneuploidy
Down syndrome
Describe the 2nd phase of oogenesis at ovulation?
Meiosis resumes (stimulated by LH)
- The first division of meiosis is completed and the haploid nuclei now separates to form 2 cells
- The cytoplasm is unequally shared forming a large secondary oocyte and a polar body (with no further role)
- Meiosis arrests again at metaphase II and the secondary oocyte is ovulated
- 2nd divison of meiosis is only completed in those oocytes that are fertilised
Spermatgenesis vs oogenesis
- In females, mitotic proliferation of oogonia occurs prior to birth.
- In males, spermatogonia proliferate only ader puberty.
- In females, meiotic divisions of oocyte produces only one mature ovum.
- In males, meiotic divisions of primary spermatocyte produces 4 mature spermatozoa.
- In females, second meiotic division is completed only upon fertilisation.
- In males, the products of meiosis (spermatids) undergo substantial differentiation in the maturing process.
Oogenesis: Continuous? Temp? Begins? Stem cells? Mobile gametes?
Discontinuous
Normal body temp
Meiosis begins before birth (initial stimulus is not steroidal)
Results in production of finite numbers of oocytes (ovary has no stem cells)
Results in production of immobile gametes
Spermatogenesis: Continuous? Temp? Begins? Stem cells? Mobile gametes?
Continuous
Lower than body temp
Meiosis begins at puberty (Indirectly dependent on progesterone)
Results in production of infinite numbers of sperm (testis has a stem cell population)
Results in production of motile gametes
Eggs exist in ovaries in structures known as….
follicles
Follicle development
- Primordial follicle = single layer of granulosa cells around oocyte.
- Oocyte size increases, multiple layers of granulosa cells + separation of oocyte from granulosa cells by zona pellucida.
- BUT cytoplamic processes cross the zona pelucida and form gap junctions with oocyte and nutrients and chemical messengers are passed to oocyte.
- Follicle grows by mitosis of granulosa cells and some differentiate to become theca
- Antrum begins to form from amongst granulosa cells from fluid they secrete
Features of small follicles (primordial)
Most numerous
Non-growing (stock pile)
Oocyte nucleus is in meiotic prophase
Single layer of follicular cells (granulosa cells)
What hormone is secreted by primordial follicles?
Use?
Secrete anti-Mullerian hormone (AMH).
Level reflects the ovarian follicular reserve and therefore can be measured to assess ovarian ageing
At puberty she has ≈ _____ oocytes
• she may experience ___ cycles
• she loses approx. ___ per cycle
At puberty she has ≈ 300,000 oocytes
She may experience 450 cycles
She loses approx. 650 per cycle
What are the stages of medium follicle development?
Throughout life, cohorts of small follicles recruited to begin a period of slow growth
Follicular cells divide, forming 3 layers around the oocyte
Growth in independent of hormones
Takes 85 days (3 cycles) to reach three layers of follicular cells
Other names for large follicles?
Secondary
Antral
Graafian
Preovulatory
Stages of development of large follicles?
FSH stimulates rapid development of medium follicles over 14 days, leads either to ovulation or to atresia.
- Zona pellucida develops and encloses the oocyte, masking its antigens
- Rapid mitotic division in follicular cells forms many layers
- Antrum develops and fills with fluid
LH activates the theca interna to synthesise androstenedione (the precursor for estadriol 17-beta synthesis by granulosa cells.)
Initial growth is _________ of hormones and takes ___ days
Final growth is _________ on FSH and takes ______ days
Initial growth is independent of hormones and takes 85 days
Final growth is dependent on FSH and takes 10 – 14 days
What are the chemical components of ovulation?
LH surge FSH (some LH) Prostaglandins E and F "Stigma" Process facilitation by intrafollicular pressure and contraction of smooth muscle in theca
Influence of LH surge on ovulation
Induces prostaglandin endoperoxide synthase in granulosa cells (sets up
pseudoinflammatory response)
Influence of FSH (and some LH) on ovulation?
Stimulates release of plasminogen activator from granulosa cells (converts
plasminogen to plasmin)
Influence of prostaglandins E and F on ovulation?
Release lysosomal enzymes that digest follicular wall – not completely
understood
Influence of “stigma” on ovulation?
Forms on surface of follicle, balloons out, forms vesicle and ruptures.
Oocyte is expelled
Process of corpus lute formation?
• Mature follicle discharges its antral fluid and egg.
-Collapses around antrum and undergoes rapid
transformation.
• Granulosa cells enlarge, and form gland-like structure ie CORPUS LUTEUM
• Corpus luteum secretes:
-Oestrogen, progesterone, inhibin
• If no egg fertilisation, CL developments reaches maximum within approx 10 days
-Rapidly degenerates by apoptosis
