Oogenesis and Follicular Development

Question 1

Where are follicles found?

Answer 1

in the ovaries

Question 2

Where does the first phase of oogenesis take place?

Answer 2

foetal life

Question 3

Where do primordial germ cells develop from?

Answer 3

• endodermal endothelium of the embryonic yolk sac 3 weeks post-conception
• around 5th week post-conception, PGCs migrate to gonadal ridges, undergoing mitosis simultaneously to increase number of germ cells

Question 4

Describe the differentiation of the migrating PGCs

Answer 4

• indifferent primordial gonad can become either ovary or testes
• genotypic sex of embryo directs sexual development
• males: medulla develops, cortex regresses
• females: cortex develops, medulla regresses

Question 5

Describe the process of meiosis

Answer 5

• division of a germ cell involving 2 fissions of the nucleus
• gives rise to 4 gametes
• each has half the number of chromosomes from original cell
• male: production of 4 sperm
• female: production of 1 mature egg (due to periods of arrest of the process)

Question 6

When does the second phase of oogenesis occur?

Answer 6

ovulation

Question 7

What hormones are involved in the second phase of oogenesis and what are their roles?

Answer 7

• LH and FSH
• enlarge and mature oocytes
• differentiate and proliferate granulosa and theca cells
• form and accumulate fluid
• LH stimulates continuation of meiosis

Question 8

Describe the events that take place at the 2nd phase of oogenesis

Answer 8

• first division of meiosis is completed, haploid cell separates into 2 cells
• cytoplasm unequally shared forming a large secondary oocyte and polar body (which has no further role)
• meiosis arrests again at metaphase II and secondary oocyte is ovulated
• second division of meiosis is completed only in fertilised oocytes

Question 9

Describe the process of follicular development

Answer 9

• primordial follicles consist of an oocyte surrounded by a single layer of cells
• cells divide to form stromal layer and surround granulosa cells
• formation of theca cells lying between granulosa and stromal cells, oocyte surroudned by zona pellucida
• follicular antrum develops filled with follicular fluid
• mature oocyte suspended in follicular fluid attached by stalk to granulosa cell layer
• follicle ruptures to release oocyte, antrum fills with blood forming corpus haemorrhagicum to become corpus luteum
• regression of corpus luteum leads to formation of corpus albicans

Question 10

What stimulates follicles to start to develop?

Answer 10

LH and FSH

Question 11

What is the morphology and function of small (primordial) follicles?

Answer 11

• single layer of follicular cells (granulosa cells)
• secrete anti-Mullerian hormone (AMH): levels reflect ovarian follicular reserve and can be measured to assess ovarian ageing

Question 12

What is the morphology of medium (primary) follicles and how does this come about?

Answer 12

• 3 layers of cells around oocyte
• small portion of small follicles recruited to begin period of slow growth
• follicular (granulosa) cells divide

Question 13

What is the process of development of large follicles?

Answer 13

• FSH stimulates rapid development of medium follicles over 14 days
• either results in ovulation or atresia
• zona pellucida encloses oocytes and masks antigens
• mitotic division of follicular cells forms many layers
• antrum develops and fills with fluid
• LH activates theca interna to synthesise androstenedione (precursor for oestradiol by granulosa cells)