Gonads Flashcards
what are the gonads?
gamete producing organs
- testes
- ovaries
function of the gonads?
Gametogenesis:
- Spermatogenesis
- Oogenesis
Steroidogenesis:
- males: predominantly androgens (+ some of female sex steroids)
- females: predominantly oestrogen and progestogens (+ some of males sex steroids)
where do germ cells come from?
the primordial germ cell
number of spermatogonia stay high throughout lifetime decreasing only slightly.
number of oogonia are maximal at 24 weeks of gestation then atresia causes degeneration of the oogonia pool to about 2 million at birth and only 400,000 by puberty.
how many eggs does a female normally release during her lifetime?
350-400
what is menopause?
no more eggs are released.
process of spermatogenesis
GERM CELL develops into SPERMATOGONIA (44XY)
AT PUBERTY:
- Spermatogonia divide by mitosis to form PRIMARY SPERMATOCYTES and spermatogonia (so constant pool of primary cells are kept) in a 50:50 ratio of spermatogonia to spermatocytes.
- Primary spermatocytes then divide by meiosis to form the haploid SECONDARY SPERMATOCYTES (22Y or 22X)
- Secondary Spermatocytes then undergo the second meiotic division to form SPERMATIDS (haploid)
- spermatids then mature to form the acrosome etc into SPERMATOZOA
process of oogenesis
Germ cells form OOGONIA (44XX)
- Oogonia divide by mitosis to form PRIMARY OOCYTES.
- Primary Oocytes then undergo meiosis I to form cells around it forming a follicle and arrest until puberty for about 12-50 years (uptil menopause)
- AT PUBERTY: meiosis I is established and forms SECONDARY OOCYTES (larger daughter cell 22X) and a polar body (a smaller daughter cell which is degraded as it cannot be fertilised)
- Secondary Oocytes undergo second meiotic division and form OVUM and second polar body
how do sperm leave the testes?
- spermatogenesis occurs in the coiled seminiferous tubules
- these tubules converge onto the rete testes which direct the sperm into the vasa efferentia
- this leads to the epididymis where nutrient are secreted into and the sperm cells gain motility
- the mature sperm are then ejected using the vas deferens smooth muscle via the urethra.
why do the testes hang outside the body?
few degrees lower than body temp needed for spermatogenesis. they usually drop down into scrotum just before birth.
where does spermatogenesis occur and how?
SERTOLI cells form the tubule. tight junctions maintain a blood testes barrier but spermatogonia can pass through this tight junction into the sertoli cell where they mature into sperm cells. these are then released into the tubule
what other cells are present in the testes?
leydig cells, where steroidogenesis occurs.
what do sertoli cells do?
- they contain FSH and Androgen receptors
- In response to FSH they produce inhibin and androgen binding hormone.
- needed for the development for spermatocytes
what do leydig cells do?
- lies outside seminiferous tubules
- have LH receptors
- produce androgens when LH receptor stimulated
how are the sex steroids synthesised?
cholesterol, pregnenolone, progesterone, 17OH progesterone, androstenedione:
In the Testes:
- androstenedione converted to testosterone by 17betahydroxysteroid dehydrogenase
- testosterone is converted to Dihydrotestosterone using 5 alpha reductase in epididymis, prostate and peripheral tissue
In the Ovaries:
- Testosterone is converted into oestrodiol by aromatase which is activated by FSH
how long does the menstrual cycle last usually?
28 days approx, 20 to 35+. Day 1 is the first day of loss of blood and cellular debris from necrotic uterus.
when does the egg get released?
14 days into the cycle
what is the menstrual cycle split into and how are these linked?
ovarian cycle phases: follicular, ovulation, luteal
endometrial cycle phases: proliferative and secretory
Oestrogen produced in follicular phase causes proliferation of endometrium (lining of uterus)
Progesterone and oestrogen produced in luteal phase causes secretory phase of endometrium by decreasing proliferation and reducing number of oestrogen receptors .
describe the ovarian cycle
- pre-antral follicle formed in ovaries without use of gonadotrophin
- If FSH levels are raised, early-antral follicle formed
- If FSH levels too low the follicle will undergo atresia
- these mature into late-antral follicle, with ovum in centre
- graafian follicle then formed leading to ovulation (release of egg)
- this leaves the corpus luteum
describe structure of the ovarian follicle
two layers of cell line the follicle:
Outer layer cells are called Thecal Cells which produce androgens in response to LH
Inner layer cells are called Granulosa cells which convert androgens produced from thecal cells into oestrogens
what is the corpus luteum?
remnant after ovum released, it contains LH and FSH receptors which allow it to release oestrogen and progesterone.
describe the proliferative phase of the endometrial cycle
after 6 days of menstruation, starts due to oestrogen produced from follicle this causes endometrium to thicken and glands to turn from straight to coiled and enlarge with increased blood supply.
describe the secretory phase of the endometrial cycle
after ovulation (~14 days) dominant progesterone influence from follicle causes glands to produce mucus mixed with blood. endometrium sheds and becomes necrotic if not fertilised.
DHT is produced by Reduction of Testosterone by which enzyme and where does this occur?
5alpha-reductase
prostate, testes, skin, brain, adenohypophysis: gives phenotypic features for men.
Oestrogen is produced by aromatisation of testosterone by which enzyme and where does this occur?
Aromatase
adrenals, testes, liver, skin, brain, adipose
how is testosterone transported?
in blood: bound to Sex Hormone Binding Globulin and Albumin
in Seminiferous fluid: Androgen binding globulin
Actions of androgens in Foetus
development of male genitalia
foetal growth
this androgen production is triggered by HcG.
Actions of androgens in Adult
- Final steps of spermatogenesis
- Growth and development of secondary male characteristics
- Protein and Bone anabolism
- Male sexual behaviours
- Pubertal growth spurt
how do effects of androgens occur chemically?
by conversion to DHT or Oestrogen eg in brain.
what is an oestrogen?
any substance natural or synthetic that induces proliferation/mitosis in the endometrium
eg: 17beta-oestradiol, oestriol, oestrone
biological effects of oestrogen
- thickening of endometrium
- triggers LH surge in ovulation
- vaginal secretions in intercourse
- stimulates growth of breast ducts
- skin, decreases sebaceous gland secretion
less significant effects of oestrogen?
- stimulates osteoblasts
- lowers lipid levels
- influences release of prolactin and thyrotrophin
- behavioural effects
- increased salt retention
- increases plasma protein synthesis
what are progestogens?
any substance that induces secretion of endometrium
eg Progesterone and 17a-progesterone
effects of progestogens
- increases basal body temperature
- decreases reabsorption of sodium
- growth of alveolar gland system of breast
describe the hypothalamo-pituitary-testicular axis
GnRH production from the hypothalamus is pulsatile. This triggers LH and FSH to be released from the APG. LH causes testosterone production from leydig cells and FSH causes spermatogenesis in sertoli cells. Sertoli cells also produce inhibin. Both inhibin and testosterone negatively feedback to the hypothalamus and pituitary.
what is virilzation?
development of male characteristics due to testosterone
describe the hypothalamo-pituitary-ovarian axis
GnRH from hypothalamus causes LH and FSH release from pituitary. This causes oestrogen production in the ovaries and follicular production (associated with inhibin production). Inhibin and oestrodiol then negatively feedback onto the hypothalamus and APG.
what are the phases of the menstruation cycle in order?
1) Early follicular phase
2) Early-mid follicular phase
3) Mid follicular phase
4) Late follicular phase
5) Luteal phase
what happens in the early follicular phase?
follicles grown under normal levels of LH and FSH and start to produce predominantly oestrodiol.
what happens in the early-mid follicular phase?
one of the follicles starts to outcompete the others meaning even more oestrodiol produced as the big follicle grows. LH and FSH levels are stable.
what happens in the mid follicular phase?
As the graafian (big) follicle gets larger it produces even more oestrogen. In the graafian follicle, this triggers a local positive autocrine and autocrine response where by the oestrogen produced causes more proliferation of granulosa cells which produce even more oestrogen.
This high oestrogen causes negative feedback on the hypothalamus and pituitary meaning there is a reduction of LH and FSH secreted. this causes regression of the remaining follicles hence only the graafian follicle grows.
what is unique about the graafian follicle?
it does not require FSH to grow.
what happens in the late follicular phase?
There is a positive feedback neurone in the hypothalamus that, if oestrogen levels have exceeded a threshold, this causes an LH and GnRH surge. this causes the ovum to be released (ovulation) into the fallopian tubes.
what happens in the luteal phase?
- the corpus luteum forms after the ovum has been released
- it starts releasing inhibin, oestrogen and large amounts of PROGESTERONE which causes leutolysis and menstruation over a few days
what is primary amenorrhoea?
woman has never had a period
what is secondary amenorrhoea?
woman has period which stops
what is oligomenorrhoea?
infrequent period cycles
what is the definition of infertility?
where a couple cannot get pregnant after 12 months of regular unprotected sex.
causes of infertility?
- pituitary failure
- prolactinoma (as prolactin inhibits FSH and LH release)
- Testicular failure (Kleinfelters)/ asospermia
- Ovarian failure (Turners)
- polycystic ovarian syndrome
what is polycystic ovarian syndrome associated with?
- hyperandrogenaemia (eg hirsutism etc)
- infrequent periods
- lots of enlarged ovary follicles
