Lecture 12 Flashcards
1
Q
What is puberty marked by?
A
- Maturation of the genital organs, - Development of second sexual characteristics, - Accelerated growth (2in:6lb/year to 3in:17.5lb/year), - Occurrence of menarche (in the female)
2
Q
What key thing happens in puberty?
A
Marks you becoming capable of reproducing
3
Q
What are the 2 physiological processes?
A
- Gonadarche –> growth and maturation of the gonads, - Adrenarche –> maturation of the adrenal cortex
4
Q
What are the tanner stages?
A
- Series of 5 stages (1 being least mature, 5 being most mature), - Shows maturation through puberty, - Similar for men and women, - Changes to hair, breasts, organs etc
5
Q
When do males typically start puberty?
A
9-14, but average is 12
6
Q
When do females typically start puberty?
A
8-13, but average is 11
7
Q
What is happening to the age when you start puberty?
A
Evidence to show that the start age is shifting earlier and earlier
8
Q
What drives puberty? What does it do?
A
- Gonadal-hypothalamic-pituitary (GHP) axis, - Drives development of sex hormones, - Drives development of tissues
9
Q
Give some examples of hormones released by the pituitary in puberty
A
- GnRH, - FSH, - LH, - Prolactin, - Oxytocin
10
Q
Give some examples of hormones produced by the ovary
A
- Oestradiol, - Testosterone, - Progesterone, - Inhibitin B
11
Q
Describe the key features of the mammalian ovary
A
- Follicle cells (oocytes) develop, - Have oocyte (egg cells) surrounded by ring of granule cells, - Granule cells develop to become theca cells –> Allows more trophic support to be given to egg, - Follicle that develops the most becomes the primary follicle and ruptures to release the egg during menstruation –> The oocyte and some granule cells get released into fallopian tubes, - Rest of cell collapses into the corpus luteum –> If egg cell gets fertilised, corpus luteum stays and gets larger to support follicle developing into embryo, –> If egg cell isn’t fertilised, collapses and becomes corpus albicans and is essentially scar tissue
12
Q
What is FSH? What does it do?
A
- FSH = Follicle Stimulating Hormone, - Stimulates the development of follicles
13
Q
What is LH? What does it do?
A
- LH = Luteinising Hormone, - Stimulates the development of the corpus luteum
14
Q
What do FSH and LH do together?
A
- Stimulate the secretion of oestradiol, - Stimulate development and secretion of gonadal steroid and peptides that feedback to the hypothalamus
15
Q
What is GnRH?
A
GnRH = Gonadotropin Releasing Hormone
16
Q
What drives the beginning of puberty?
A
- Starts with changes to cells in hypothalamus –> Cells producing GnRH start increasing production and causes phasic release, - Get enough release of GnRH to stimulate release of LH and FSH from the anterior pituitary, - Get waves of LH and FSH that get down to ovaries and drive development of follicles within the ovary –> Get more granulosa and theca cells –> Ability of ovaries to start secreting sex hormones increases
17
Q
What triggers puberty to start?
A
Not really sure, few different hypotheses
18
Q
What has happened to the start age of puberty in the last century? Why might this be?
A
- Large change in onset between early 19th (16-17 years) and the late 20th (13 years) century, - Why? –> Environmental factors, improved healthcare, –> Photoperiod –> control physical environment –> extend day, exist in pseudo-long days but little direct evidence (days are artificially longer for us as we have lights, whereas before people would sleep when it got dark), –> Major improvement in living standards and nutrition in last 100 years, - Improvement in food quality and availability makes everyone healthier and more able to start puberty
19
Q
How do low protein diets affect the menstrual cycle? What does this show?
A
- Low protein diets offset or even stop oestrous cycle, e.g anorexia nervosa, - Big effect of diet on puberty
20
Q
What can childhood obesity do to puberty?
A
In females, childhood obesity can lead to earlier menstruation
21
Q
What are the 2 hypotheses for the mechanisms underlying puberty?
A
- Gonadostat hypothesis, - Hypothalamic maturation hypothesis
22
Q
What is the gonadostat hypothesis?
A
- Before puberty, have low levels of sex hormones –> Have negative feedback regulation of FSH and LH secretion operating at a low threshold or set point, –> Very sensitive to low levels of steroids, - Puberty –> set point = to become less sensitive leads to an increase of gonadotrophins and sex steroids, –> Once you reach a set concentration of pituitary hormones, everything starts, - Many questions remain, e.g are these actually driving it or are they secondary responses?
23
Q
What is the hypothalamic maturation hypothesis?
A
- Puberty only requires hypothalamic GnRH, –> Once hypothalamus matures more, releases more GnRH, –> Puberty starts when hypothalamus develops far enough to develop threshold level of GnRH, - There is a direct link between CNS:pituitary:hypothalamus GnRH neurons, - Supports other experimental studies using animal models
24
Q
What is the menstrual cycle? What are the hormones involved?
A
Monthly rhythmical changes in hormones resulting in secondary changes to ovarian function and the lining of the uterus and the breasts, - Hormones involved: –> GnRH, FSH, LH, –> Oestradiol and progesterone, - Involves ovulation and changes to the endometrium
25
What happens at the beginning of the menstrual cycle?
- Start shedding uterine lining, - At this point, all hormonal levels are low --> Have low level pulses of GnRH from hypothalamus
26
Describe the fluctuations of hormones and the uterine lining during the menstrual cycle. Start with higher levels of GnRH being secreted from the hypothalamus
- Get higher levels of GnRH secreted, - Start getting more production of FSH which stimulates production of more oestrogen, - This helps follicles to develop and get larger, - Positive feedback system where increased hormones leads to increased size of tissues that produce hormones so can secrete more hormones, - FSH/oestrogen levels keep increasing until reach threshold where LH starts to be produced in much higher quantity (surge), - This triggers ovulation --> Get rupture of follicle cells --> Cells surrounding follicle (granulosa/theca) now need to get uterine lining ready for if pregnancy occurs, - Cells become corpus luteum --> This is good at producing progesterone, --> Cells were making oestrogen, now make progesterone so levels of oestrogen drop and levels of progesterone increase, - As corpus luteum develops, increases its ability to produce oestrogen so that level starts to increase, - Surge in progesterone production drives thickening of uterine wall, creating more tissue there that could hopefully support implantation of fertilised egg, - Get spiral arteries developing in thickening tissues --> Rich blood supply for egg, - After ~14 days, corpus luteum degenerates --> Progesterone and oestrogen levels drop --> Stops production of more tissue cells --> Drives shedding of uterine lining
27
What are the FSH/LH feedback mechanisms in place in the menstrual cycle?
- Pulsatile secretion of gonadotrophin from pituitary stimulates the synthesis and secretion of LH and FSH, - LH and FSH stimulate development and secretion of gonadal steroid and peptides that feedback to the hypothalamus, - Following the midcycle FSH:LH surge and ovulating, the feedback becomes inhibitory --> Oestrogen and progesterone become inhibitory and make all cells quiescent (inactive)
28
Why do the fluctuations of the hormones in the menstrual cycle occur?
- Occurs to prepare for fertilised egg and pregnancy, - Cleverly timed balance of ovulation and preparedness of the endometrium for possible implantation of fertilised egg, - As follicles grow, they pump out more oestradiol, - Following ovulation, the corpus luteum will produce progesterone --> Altering the endometrium structure to prepare for implantation
29
What does preantral mean?
Before follicle develops
30
What do preantral follicle cells look like in the presence of FSH?
- Thickening of granulosa cells, - Supporting structure gets larger and larger and start developing theca cells
31
What do early antral, corpus luteum and corpus albicans follicle cells look like during the ovarian cycle?
- Thickening of granulosa cells, - Supporting structure gets larger and larger and start developing theca cells in clear rings around the follicle, - Start getting enlargement and intake of fluid to bulk out size of oocyte, - As we continue to develop, granulosa/theca cells get pushed out to periphery --> Get massive fluid filled cavity in the middle that dominates the space, - Within 24 hours, follicle ruptures and turns into corpus luteum --> Then regresses to become corpus albicans which is scar tissue on ovary and doesn't produce anything
32
What is the 2 cell hypothesis for steroid hormone synthesis?
- Rare for a single cell to generate oestrogen directly from cholesterol, so oestrogen biosynthesis is dependent on cooperation between 2 cell types, - Theca cells use cholesterol to produce androstenedione and testosterone, - These precursors are shuffled to granulosa cells which express enzymes needed to produce and express estradiol and estrone
33
What does the 2 cell hypothesis allow?
- Allows a greater degree of control of biosynthesis, - Allows the influence of different factors on different cells working independently
34
What converts cholesterol towards androstenedione?
CYP11A1 and CYP17A1
35
What converts androstenedione and testosterone to estrone and estradiol in the granulosa cells?
CYP19A1
36
What is the cellular structure of the corpus luteum (which is produced after follicle rupture)?
- Granulosa cells become large luteal cells, - Theca cells become small luteal cells, - Both of these do the same thing --> Both produce progesterone
37
How is our oocyte reserve produced?
- In embryonic development, get surge of production of egg cells by ~ week 20 --> Get atrophy of these egg cells, - Hit pretty steady level at ~ week 28 onwards, - At time of birth, that is oocyte reserve --> won't ever make more of these in post-natal development --> Get your entire reserve of oocytes that you will ever have (2-3 million oocytes)
38
What happens to your oocyte reserve after birth?
- Number of egg cells stays stable until puberty --> Start releasing these with menstrual cycles so ovarian reserve starts to fall, --> Ability to produce/release sex hormones from the ovaries also reduces, - At some point, release enough oocytes that now don't have enough oestrogen production in ovaries to support further ongoings of menstrual cycle --> Hit menopause (hundreds of thousands of oocytes)
39
How many eggs start developing in every menstrual cycle?
Only 1 oocyte is typically released in menstruation but many will start developing and won't reach point of being released and start atrophying before anything actually happens
