Intro and Menstrual Cycle Flashcards
What is the purpose of the menstrual cycle?
- Generate oocyte
- Facilitate fertilisation
- Optimise endometrium for implantation
- Protect developing embryo
What is the endocrine control of the menstrual cycle?
Hypothalamus, anterior pituitary and ovaries work together
What is the name of the first menstrual cycle?
Menarche
What does the hypothalamus release during menstrual cycle?
GnRH
How does GnRH act?
Exerts effect on anterior pituitary –> stimulates release of FSH and LH
What does LH stand for?
Luteinising hormone
What does FSH stand for?
Follicular stimulating hormone
What does FSH and LH act on?
Ovary to release oestrogens (this feeds back to hypothalamus and anterior pituitary)
What part of the hypothalamus is GnRH secreted by?
Mid basal hypothalamic neurons
How is GnRH transported to the pituitary?
Via hypophyseal portal blood system
What is GnRH secretion affected by?
bereavement anxiety time zone day/night duty exercise weight loss/gain
What cells of the anterior pituitary secrete FSH and LH?
Basophils
What type of molecules are FSH and LH?
Glycoproteins
What does FSH stimulate?
Stimulates follicular activity, thus promoting estradiol production from granulosa cells
What does LH stimulate?
Triggers release of egg from dominant follicle
Promotes development of the corpus luteum and the production of progesterone
What is the predominant gonadotropin during the 1st half oF the cycle?
FSH
GnRH, LH and FSH production is inhibited by rising E2 levels.
When is there an exception to this?
Near midcycle when high levels of E2 lead to a surge in LH release which triggers ovulation
What E2?
Oestradiol –> Major female sex hormone (strongest of the 3 naturally produced oestrogens)
What are the first class of follicles formed in ovaries?
Primordial follicles
- In fetus at 6 months: 2,000,000
- At birth: 750,000
- At puberty 400,000
Approx 450 follicles develop and produce an egg; remainder degenerate
No new follicles produced
Describe the sequence in which the egg is produced before ovulation
- Primordial follicle
- Primary/preantral follicle
- Secondary/antral follicle
- Preovulatory follicle
- Ovulation
What are the phases of the ovarian cycle?
- Follicular phase
- Ovulation
- Luteal phase
What is the follicular phase?
Marks the beginning of a new cycle as follicles (oocytes surrounded by stromal cells) begin to mature and prepare to release an oocyte.
What are follicular cells also known as?
Granulosa cells
Where do granulosa cells lie?
Directly around the oocyte zone pellucida and are released with the oocyte at ovulation
Describe a primordial follicle
Cconsist of an oocyte surrounded by a single layer of flattened granulosa cells
Describe the follicular development
- Proliferation of granulosa cells: antrum develops (fluid)
- Gap junctions and cytoplasmic processes between oocyte and granulosa cells allows communication and passage of low mol wt materials (e.g.nutrients)
- Proliferation of theca cells
What are theca cells?
The endocrine cells associated with ovarian follicles that play an essential role in fertility by producing the androgen substrate required for oestrogen biosynthesis.
Describe the theca interna cells
Glandular and highly vascularised.
Express receptors for luteinising hormone (LH).
Secrete androgens.
Describe the theca externa
Fibrous capsule
After the follicle ruptures, what do the theca interna cells differentiate into?
The theca lutein cells of the corpus luteum.
What are the androgens secreted by the theca interna cells converted to? Where?
Converted to oestradiol in granulosa cells
Describe the ovarian hormone production at the beginning of the cycle (menses)
There is little ovarian hormone production and the follicle begins to develop independently of gonadotropins or ovarian steroids.
What do the low steroid and inhibin levels at the beginning of the cycle cause? Why?
There is little negative feedback at the HPG axis resulting in an increase in FSH and LH levels. These stimulate follicle growth and oestrogen production.
What do almost mature follicles release? What does this then stimulate?
High levels of oestrogens
Stimulates release of more GnRH and LH
GnRH then promotes release of FSH and even more LH
How many follicles can continue to maturity? What do the other follicles form?
Only one
Polar bodies
As follicular oestrogen eventually becomes high enough to initiate at the HPG axis, what happens?
Causes increasing levels of GnRH and gonadotropins.
However, the effect is only reflected in LH levels (the LH surge).
What is the LH surge due to?
Due to the increased follicular inhibin, selectively inhibiting FSH production at the anterior pituitary.
Granulosa cells become luteinised and express receptors for LH.
What is the HPG axis?
Connection between the hypothalamus, pituitary gland, and gonads.
Describe the production of inhibin
FSH stimulates the secretion of inhibin from the granulosa cells of the ovarian follicles in the ovaries. In turn, inhibin suppresses FSH
What does the LH surge bring about?
Ovulation
What happens to the follicle in response to the LH surge?
It ruptures and the mature oocyte is assisted to the fallopian tube by fimbria.
Here it remains viable for fertilisation for around 24 hours.
Following ovulation, what does the follicle secrete?
Secreting oestrogen and now also progesterone
What is the effect of the secretion of oesotrogen and progesterone by the follicle? What is purpose of this?
-ve feedback on HPG axis
This, together with inhibin (inhibits FSH) stalls the cycle in anticipation of fertilisation.
What is the corpus luteum? What does it produce?
The tissue in the ovary that forms at the site of a ruptured follicle following ovulation.
It produces oestrogens, progesterone and inhibin to maintain conditions for fertilisation and implantation.
If no fertilisation occurs, what happens to the corpus luteum?
Degenerate/fibroses to become corpus albicans
As the corpus luteum regresses, what happens to the hormone levels?
Fall significantly, relieving negative feedback, resetting the HPG axis ready to begin the cycle again.
What is the endometrium?
The lining layer of the uterus
What are the phases of the uterine cycle?
- Proliferative phase (runs alongside follicular phase)
- Secretory phase (runs alongside luteal phase)
- Menses
What does menses mark?
Beginning of new menstrual cycle
Describe the endometrium during the proliferative phase?
Effects of oestradiol:
- Endometrium thickens
- Increased stromal cels
- Increased glands, blood vessels
By ovulation, endometrium is 2-3mm thick
What else does oestradiol initiate during proliferative phase?
Fallopian tube formation, thickening of the endometrium, increased growth and motility of the myometrium and production of a thin alkaline cervical mucus (to facilitate sperm transport).
What else does oestradiol initiate during proliferative phase?
Fallopian tube formation, thickening of the endometrium, increased growth and motility of the myometrium and production of a thin alkaline cervical mucus (to facilitate sperm transport).
What is the predominant hormone during the 2nd half of the cycle?
Progesterone
Describe the secretory phase
Progesterone stimulates further thickening of endometrium
Increased secretion
Increased lipids and glycogen
Increased blood supply
Describe the endometrium at the end of the secretory phase
Optimal conditions for implantation of fertilised egg: stable, vascular, nutrient-rich
What happens during menstruation?
Corpus luteum has broken down and the internal lining of the uterus is shed
How does the uterus lining shed?
Necrotic outer layers of endometrium separate from uterus
Separated tissue and blood initiate uterine contractions –> expel contents
What is produced by the cervix?
Mucus production from columnar glands
Describe the mucus produced by the cervix midcycle? What is purpose of this?
Stringy and runny midcycle
Facilitates sperm access at ovulation
Describe the mucus produced by the cervix in the luteal phase? What is purpose of this?
Tenacious and inelastic in luteal phase
- Prevents microbial ingress protects developing embryo
- Critical to pregnancy (mucus ‘plug’)
What does the combined oral contraceptive contain?
Oestradiol and a progestagen
What is progestagen?
A synthetic progesterone
What does the combined oral contraceptive do?
Inhibits GnRH/FSH/LH to prevent ovulation.
Thin endometrium.
Tenacious and inelastic mucus.
Describe the effects of FSH in the menstrual cycle
Binds to granulosa cells to stimulate follicle growth, permit the conversion of androgens (from theca cells) to oestrogens and stimulate inhibin secretion
Describe the effects of LH in the menstrual cycle
LH acts on theca cells to stimulate production and secretion of androgens
What are the effects of moderate oesotrogen levels on the HPG axis?
Negative feedback
What are the effects of high oesotrogen levels on the HPG axis (in the absence of progesterone)?
Positively feedback on the HPG axis
What are the effects of oesotrogen on the HPG axis in the presence of progesterone?
Negative feedback on the HPG axis
What are the effects of inhibin?
Selectively inhibits FSH at the anterior pituitary
What is only hormone with positive feedback effect on HPG axis?
Oestrogen (LH surge)
What day is ovulation?
Around day 14
What is the Tanner Scale?
Scale of physical development in children, adolescents and adults (puberty scale)
How many stages are there in the Tanner Scale?
5
Describe stage 1 in Tanner Scale
Preadolescent, no sexual hair
No boobs for females
Describe stage 2 in Tanner Scale
Sparse, pigmented, long straight hair along labia and at base of penis
Breast budding (areola and papilla)
Describe stage 3 in Tanner Scale
Darker, coarser, curlier hair
Continued enlargement of breats
Describe stage 4 in Tanner Scale
Adult but decreased distribution (hair)
Areola and papilla form secondary mound
Describe stage 5 in Tanner Scale
Adult in quantity and type with spread to medial thighs
Mature female breast
