HPO axis + the menstrual cycle

Question 1

What is the HPO axis?

Answer 1

hypothalamic pituitary ovarian axis

controls female reproduction

Question 2

State the 2 phases of the ovarian cycle

Answer 2

follicular phase (including ovulation) [days 1-14]
luteal phase [days 15-28]
[luteal phase duration = unchanged, changes in duration = in follicular phase]

Question 3

Describe the follicular phase

Answer 3

primordial follicles become primary follicles
primary follicle matures into a secondary follicle, zona pellucida + theca folliculi are formed
cells of theca produce oestrogens during maturation process
fluid-filled space around oocyte develops + some granulosa cells develop into a corona radiata surrounding the oocyte
follicle = mature graafian follicle, ready to release secondary oocyte at ovulation

Question 4

Describe ovulation

Answer 4

Usually occurs in alternate ovaries
Follicle ruptures + secondary oocyte expelled into abdominal cavity
Oocyte enters fimbriated end of fallopian tube
Pain in lower abdomen can occur (mittelschmerz)
Usually 1 oocyte released per ovarian cycle
Follicular phase end = ovulation

Question 5

Describe the luteal phase

Answer 5

Ruptured follicle caves in and fills with blood clot
Granulosa + theca cells increase and form an endocrine structure called corpus luteum
Corpus luteum secretes progesterone + oestrogen
Corpus luteum = prepares for fertilisation + for maintaining a pregnancy
If no fertilisation, corpus luteum degenerates after 12-14 days
Area of corpus luteum → scar tissue → called corpus albicans
Fertilisation → corpus luteum continues to function to maintain pregnancy until placental + foetal hormone production is sufficient

Question 6

State the 3 phases of the uterine cycle and it’s main purpose

Answer 6

proliferative
secretory
menstrual

prepares endometrium for possible implantation by fertilised embryo

Question 7

Describe the proliferative phase

Answer 7

Corresponds to follicular phase of ovarian cycle
Starts ~day 5 when bleeding stops
After menstruation, only basal layer of endometrium remains (stratum basalis)
Release of oestrogen → cell proliferation + regeneration of stratum functionalis
Proliferative phase ends with maturation of graafian follicle + ovulation ~day 14
Endometrium ~2mm thick
Cervical mucus changes from thick plug blocking cervix → profuse amounts of thin, slippery mucus that spermatozoa can penetrate

Question 8

Describe the secretory phase

Answer 8

Corresponds with luteal phase of menstrual cycle
14 days duration
Endometrium prepared by oestrogen now influenced by progesterone
Progesterone causes glands to enlarge + secrete glycogen → nourish embryo at implantation
Spiral arteries in endometrium increase in size + become coiled
Endometrium ~5mm thick
Cervical mucus changes to thick plug → blocks cervical canal + protects embryo
No fertilisation = corpus luteum hormones decrease → spiral artery spasm + endometrium degeneration due to nutrient deprivation + lysosome autodigestion
~24 hours later = menstruation

Question 9

Describe the menstrual phase

Answer 9

Spiral arteries dilate + bleed into necrotic stratum functionalis
Menstrual flow = blood, fluid + endometrial fragments
~3-6 days
Clotting of menstrual cycle blood flow prevented in uterus by release of fibrinolysins ensuring endometrium is completely expelled
Outset of menstruation → uterus contracts in response to prostaglandins to expel blood
Contraction pain = dysmenorrhoea

Question 10

What are the functions of progesterone and where is it produced?

Answer 10

ovaries

maintains + thickens uterus lining
inhibits FSH + LH production (only 1 egg mature + release per cycle)

no fertilisation = levels decrease + menstruation occurs

if fertilisation occurs, corpus luteum produces progesterone then placenta develops + produces progesterone - prevents breakdown of uterus lining

Question 11

What are the functions of oestrogen and where is it produced?

Answer 11

ovaries

stimulates endometrium to grow, thicken + develop blood capillaries

post-ovulation = inhibits FSH + LH production

Question 12

What are the functions of luteinising hormone (LH) and where is it produced?

Answer 12

anterior pituitary gland

at its peak, stimulates ovulation
results in formation of corpus luteum
stimulates follicles in ovary to secrete oestrogen

Question 13

What are the functions of follicle-stimulating hormone (FSH) and where is it produced?

Answer 13

anterior pituitary gland

stimulates egg maturation in follicles of ovary

stimulates follicles in ovary to secrete oestrogen