Reproductive System - IV Flashcards
describe oogenesis
formation and development of the oocyte (female gamete) from oogonia (stem cell)
- requires mitosis and meiosis
- lifetime supply of gametes produced before birth
- initiated before birth and continues between puberty and menopause.
- cyclic: 1 ovulation every 28 days (on average)
- oocytes develop within ovarian follicles (1 oocyte per follicle)
describe the process of oogenesis that occurs before birth
- oogonium (diploid, 2n=46) in gonad
- population of oogonia increase by mitosis (stops before birth)
- oogonia differentiate to form primary oocytes (diploid, 2n=46)
- encased in primordial follicle
- many primary oocytes undergo atresia - primary oocytes start meiosis
- meiosis I halts at prophase I
- halts until puberty begins
describe the process of oogenesis that occurs from menarche until menopause
- at puberty females have ~300,000 oocytes
- under GnRH influence, small number of follicles recruited each ovarian/menstrual cycle
- only one oocyte will complete development and ovulate (within dominant follicle)
- primary oocyte completes meiosis I
- forms a secondary oocyte and first polar body (both haploid, n=23)
describe the process of oogenesis that occurs from menarche until menopause if you get pregnant
- secondary oocyte starts meiosis II
- halts at metaphase II
- suspended until fertilisation - meiosis II resumes when the sperm penetrates plasma membrane of the ovum at fertilisation
- if not fertilised, will degenerate (called atresia) and therefore never complete meiosis
what are some differences between the development of the follicle (and names) and the oocyte
Before birth:
- oogonium doing mitosis
- primordial follicle formed
Infancy and childhood (ovary functionally inactive):
- primary oocyte
- primordial follicle
Each month from puberty to menopause:
- primary oocyte still stopped in prophase I, while follicle goes to primary follicle, secondary follicle and then vesicular (astral) follicle (so a mature follicle)
- then meiosis I completed and you get ovulation, which is when the secondary oocyte gets ruptured from the follicle
- secondary oocyte completes meiosis II when fertilised, and ruptured follicle goes on to become the corpus luteum
NOTE: one primary oocyte ultimately forms one ova. three polar bodies also form - these undergo atresia (degeneration)
describe how oogenesis occurs within developing follicle
- oocyte develops within developing follicle
- follicles are miltilayered
- granulosa cells (cells on the inside - basement membrane between them)
- theca cells (cells around the outside) - granulose cells produce estradiol
- ovulation: oocyte and corona radiata (cells surrounding oocyte) released into peritoneal cavity
list the female reproductive hormones
- GnRH (hypothalamus): release of FSH and LH
- FSH (anterior pituitary): stimulates the growth of ovarian follicles
- LH (anterior pituitary): surge of LH involved in ovulation, formation of corpus luteum.
- estradiol (developing follicles): assists follicle growth (with FSH); bone and muscle growth; endometrial growth; secondary sex characteristics; feedback to anterior pituitary
- inhibin (granulose cells): negative feedback to anterior pituitary to suppress FSH
- progesterone (corpus luteum): negatively feeds back to suppress GnRH (and therefore LH and FSH); endometrial maturation; maintain pregnant state.
describe what ovarian/menstrual cycles are for/their purpose
the regular cyclic changes in the ovary and uterus that prepare an oocyte/ova for fertilisation and the endometrium for embyro implantation
define menarche and menopause, when they occur and briefly what happens in them
Menstrual cycles occur on an approximately monthly basis (average 28 days) between menarche and menopause
Menarche: first menstrual period
- occurs at age 12-12yr on average
- part of puberty - orchestrated by increase in sex steroid production (estrogens) by the gonads
Menopause: cessation of menstruation
- typically occurs in early 50s
- reduction of estradiol and progesterone due to absence of or lack of response by follicles
- anterior pituitary feedback no longer active (therefore FSH/LH high)
describe what happens in the follicular (preovulatory) phase: day 1-14
- increased FSH from the anterior pituitary
- stimulates follicular growth - growing follicles secrete estradiol and inhibin
- reduces FSH from anterior pituitary (negative feedback)
- growing follicles undergo atresia, except dominant follicle - dominant follicle secretes large amounts of estradiol
- high estradiol stimulates surge of LH (positive feedback)
- follicle ruptures and ovulation occurs
- oocyte enters peritoneal space/collected into uterine tube
describe what happens in the luteal (postovulatory) phase: day 15-28
- ovulated follicle collapses and forms corpus luteum
- secretes progesterone, estradiol and inhibin - these descries FSH and LH secretion
- negative feedback on hypothalamus - if fertilisation and implantation do not occur, the corpus luteum involutes (luteolysis)
- fall in progesterone and estradiol
- removed negative feedback on FSH and LH… cycle starts again
describe the feedback loops happening in the menstrual cycle
Follicular phase:
- Low levels of estradiol
- this provides negative feedback to FSH and LHL
Leading up to ovulation:
- lots of estradiol
- this change from low levels to high levels of estradiol creates a positive feedback loop (increasing signalling to hypothalamus and ant. pit.)
Luteal phase:
- Negative feedback loops created from secretion of progesterone (keeps LH and FSH low)
describe what happens in the menstrual and proliferative phase (day 1-14)
These two phases occur within the follicular phase of the ovarian cycle.
- endometrium breaks down and bleeds during menstruation
- estradiol stimulates endometrial growth from approx. days 6-14
- rapid tissue growth, including growth of glands and vasculature
- menstrual phase is the shedding
- proliferative phase is the regrowing
describe what happens in the secretory phase (days 15-28)
- after ovulation (day 14) - corpus luteum secretes progesterone
- progesterone promotes endometrial maturation
- glands become secretory
- spiral arterioles grow and coil
What happens if fertilisation and implantation do not occur?
If fertilisation and implantation do not occur:
- corpus luteum atrophies
- progesterone levels fall
- spiral arterioles contract
- endometrial tissue breaks down and bleeding occurs
- shed tissue and blood removed via cervix and vagina (menstruation/menses) ie. cycle occurs again
