08-03-23 - Oogenesis and Follicular Development Flashcards
Learning outcomes
- Recognise that oogenesis results in the formation of a finite number of oocytes
- Recognise that oocytes exist in a state of suspended meiotic division until ovulation
- Explain the development, structure and endocrinology of and associated with small, medium and large follicles
- Describe the development of the corpus luteum
- Summarise the entire the process of oogenesis
Female reproductive organs diagram
Internal Structures of the Uterus, Ovary and Fallopian Tube diagram
When does oogenesis start?
Where do primordial germ cells (PGC) develop?
- The 1st phase of oogenesis occurs during foetal life
- Primordial germ cells (PGC) develop in the endodermal endothelium of the embryonic yolk sac 3 weeks post conception
- They do not originate in the gonad
Oogenesis: 1st Phase.
What happens at around 5 weeks post-conception?
What will these areas develop into? What do PGC undergo as they migrate?
- Oogenesis: 1st Phase
- Around 5th week post conception PGC migrate from yolk sac to gonadal ridges, which is part of the urogenital ridges
- These are the areas within the developing foetus that will become the ovaries/testes
- PGC undergo mitosis as they migrate
Oogenesis: 1st Phase.
Where do migrating PGC colonise?
What can the primordial gonad develop into?
What directs sexual development?
How does this differ for males and females?
When does meiosis begin?
When do oocytes development arrest?
- Oogenesis: 1st Phase
- Migrating PGC colonise the cortex
- Indifferent / primordial gonad can differentiate into either an ovary or a testis
- The genotypic sex of the embryo directs the sexual development:
1) In males – medulla develops and cortex regresses
2) In females – cortex develops and medulla regresses - From 8th-20th week meiosis begins (formation of gametes) – oocytes arrested in prophase 1 of meiosis
What is mitosis?
What is meiosis?
- Mitosis is the process of cell division that results in two genetically identical daughter cells developing from a single parent cell
- Meiosis is the division of a germ cell involving two fissions of the nucleus giving rise to four gametes, or sex cells, each possessing half the number of chromosomes of the original cell
Where are LH and FSH produced?
What are 3 roles of Luteinizing hormone (LH) and Follicle stimulating hormone (FSH) and intrinsic factors in the ovary control?
- LH and FSH are produced by gonadotropic cells of the anterior pituitary gland
- 3 roles of Luteinizing hormone (LH) and Follicle stimulating hormone (FSH) and intrinsic factors in the ovary control:
1) Enlargement and maturation of oocytes
2) Differentiation and proliferation of granulosa and theca cells
3) Formation and accumulation of fluid – allows for expansion of graafian follicle
* A graafian follicle is a mature liquid-filled cavity in a mammalian ovary that ruptures during ovulation to release an egg.
How does meiosis of oocytes resume?
What happens when the first division of meiosis is completed?
When does meiosis arrest again?
When is the 2nd division of meiosis completed?
- Meiosis of oocytes resumes through stimulation by LH (only for egg cell selected to be graafian follicle)
- The first division of meiosis is completed, and the haploid nuclei separate to form 2 cells
- The cytoplasm is unequally shared - forming a large secondary oocyte and a polar body (PB) which has no further role
- Meiosis arrests again at metaphase II and the secondary oocyte is ovulated
- 2nd division of meiosis is only completed in those oocytes that are fertilised
Oogenesis overview
Follicular development overview diagram
Medium Power Picture of a Section Through the Cortex of an Adult Ovary
Where are Small Follicles (Primordial) located?
How numerous are they?
What form do they exist in?
What are different parts of primordial follicles?
What do primordial follicles secrete? What do levels of AMH reflect?
How many primordial follicles are left by menopause?
- Small Follicles (Primordial) are Located in ovarian cortex
- They are the most numerous follicles at any time
- Primordial follicles exist in a non-growing resting pool (stockpile)
- Primordial follicles have an oogonium number surrounded by a Single layer of follicular cells (granulosa cells)
- Primordial follicles Secrete anti-Mullerian hormone (AMH)
- Levels reflect the ovarian follicular reserve and therefore can be measured to assess ovarian ageing
- Primordial follicles are progressively depleted so by menopause there are essentially none left.
How many Medium Follicles (Primary) are there at puberty?
How many cycles will women experience?
How many primary follicles are lost per cycle?
What happens to cohorts of small follicles throughout life?
What will happen to these follicular cells?
What is this growth independent of?
How long does it take to reach three layers of follicular cells?
- There are 400,000 primary follicles at puberty
- Women may experience 480-540 cycles
- They will lose about 650 primary follicles by puberty
- Throughout life, cohorts of small follicles recruited to begin a period of slow growth
- Follicular (granulosa) cells divide, forming 3 layers around the oocyte
- Their growth is independent of hormones
- It takes 85 days (3 cycles) to reach three layers of follicular cell
Large Follicles (Secondary, Antral, Graafian, Preovulatory).
What does FSG simulate in medium follicles?
What does this lead to?
Describe the 3 steps in the development of large follicles.
What is the role of LH?
- Large Follicles (Secondary, Antral, Graafian, Preovulatory)
- FSH stimulates rapid development of medium follicles over 14 days: leads either to ovulation or to atresia
- Rapid mitotic division in follicular cells forms many layers
- 3 steps in the development of large follicles:
1) Zona pellucida ( = egg shell) develops and enclosing the oocyte and masking its antigens
2) Rapid mitotic division in follicular cells forms many layers
3) Antrum develops and fills with fluid
- LH activates the theca interna to synthesise androstenedione, the precursor for estradiol 17β synthesis by granulosa cells
Hormonal changes during menstrual cycle diagram
Interaction of Ovarian and Endometrial Cycles diagram
Describe the role of the following substances in the Physiological Process of Ovulation:
1) LH surge
2) FSH (some LH)
3) Prostaglandins E and F
4) “Stigma”
What is the process of ovulation facilitated by?
- Role of the following substances in the Physiological Process of Ovulation:
1) LH surge
* Induces prostaglandin endoperoxide synthase in granulosa cells (sets up pseudoinflammatory response)
2) FSH (some LH)
* Stimulates release of plasminogen activator from granulosa cells (converts plasminogen to plasmin)
3) Prostaglandins E and F
* Release lysosomal enzymes that digest follicular wall – not completely understood
4) “Stigma”
* Forms on surface of follicle, balloons out, forms vesicle and ruptures - oocyte expelled
- The process of ovulation is facilitated by intrafollicular pressure and contraction of smooth muscle in theca
Oogenesis vs Spermatogenesis
