Menstrual cycle

Question 1

Give a very brief overview of the stages of the reproductive cycle in females

Answer 1

Ovulation occurs intermittently
Preceded by a period of preparation
Followed by a period of waiting.

Question 2

Give a brief overview of the preparation stage

Answer 2

Follicles grow in ovary
Uterus prepared for sperm transport and implantation of conceptus
Changes to facilitate sexual interactions
Known as the Follicular phase in ovaries
Known as the proliferative phase in uterus

Question 3

Briefly describe Ovulation

Answer 3

Brief period of fertility (about 36 hours)
Release of ovum
Formation of corpus luteum

Question 4

Briefly describe the waiting stage

Answer 4

Corpus luteum is in ovary
Changes in preparation for pregnancy
Luteal phase in ovaries
Secretory phase in uterus

Question 5

What is different in the reproductive cycle of higher primates compared to other species?

Answer 5

Ovulation spontaneous
Corpus luteum also forms spontaneously
Preparatory and waiting phases of similar duration
Ended by shedding of part of endometrium

Question 6

How is the menstrual cycle controlled?

Answer 6

Coordinated by hormones

• Gonadotrophins
• Gonadal steroids
  
  • Oestrogen
  • Progesterone

Question 7

What are the actions of gonadotrophins in the follicular phase?

Answer 7

FSH binds to granulosa cells
LH binds to Thecal cells (theca interna)
They stimulate the development of follicles

Question 8

What are the actions of gonadotrophins in pre-ovulation?

Answer 8

LH surge stimulates ovulation

Question 9

What are the actions of gonadotrophins in the luteal phase?

Answer 9

LH maintains corpus luteum

Question 10

What are the actions of gonadal steroids in the follicular phase?

Answer 10

Oestrogens stimulate

• Fallopian tube function
• Thickening of endometrium
• Growth and motility of myometrium
• Thin alkaline cervical mucus
• Vaginal changes
• Changes in skin, hair, metabolism
• Calcium metabolism

Question 11

What are the actions of gonadal steroids in the luteal phase?

Answer 11

Progesterone acts on oestrogen primed cells

• Further thickening of endometrium into secretory form
• Thickening of myometrium, but reduction of motility
• Thick, acid cervical mucus
• Changes in mammary tissue
• Increased body temperature
• Metabolic changes
• Electrolyte changes

Question 12

Where are gonadotrophins secreted from?

And how is this stimulated?

Answer 12

Pituitary gonadotrophs

Stimulated by pulsatile release of GnRH from hypothalamus

Question 13

Describe the start of the menstrual cycle with details of hormones involved

Answer 13

Corpus luteum is absent
Follicles only partly developed
Secrete very little steroid or inhibin
Oestrogen and progesterone levels are low
Little inhibition at hypothalamus or pituitary
FSH levels rise

Question 14

What are the effects of FSH at the beginning of the menstrual cycle?

Answer 14

FSH binds to granulosa cells
Follicular development continues
Theca interna develops
Secretes oestrogen stimulated by LH
Secretes inhibin

Question 15

Describe the state of hormones at the mid follicular stage.

Answer 15

Oestrogen levels are rising

Inhibin levels are rising

• Selective inhibition of FSH
• No new follicles can develop

Oestrogen exerts positive feedback at hypothalamus and pituitary

LH levels
- But not FSH

Question 16

What happens in the pre-ovulatory phase?

Answer 16

The rising oestrogen increases chance of an LH surge.

Question 17

What influences the precise timing of the LH surge

Answer 17

Environmental factors - lots of evidence for effects of stress

A circadian generator which “fires” late afternoon everyday - little evidence for this

It is the coincidence of “firing” and high oestrogen which generates an LH surge

Therefore precise timing of ovulation is affected by the environment

Question 18

In terms of the menstrual cycle, describe what happens after ovulation.

Answer 18

Corpus luteum forms spontaneously

Begins to secrete progesterone and oestrogen

Maintains suppression of FSH

LH also supressed due to positive feedback inhibited by progesterone

Question 19

In terms of the menstrual cycle describe the luteal phase.

Answer 19

Corpus luteum grows

Secretes more steroids

After 14 days precisely it dies

Question 20

Describe the end of the menstrual cycle

Answer 20

Death of the corpus luteum

Leads to a rapid fall in steroid

Stimulates menses

Relieves inhibition on FSH

Question 21

Tell me some interesting features of the timing of the menstrual cycle.

Answer 21

The interval from ovulation to menses is very constant - always 14 days

The variation in cycle length is due to the variation in the timing of ovulation