Reproductive Function and Regulation Flashcards
Provide a brief description of the menstrual cycle
The menstrual cycle incorporates a cyclic series of event designed to prepare the female reproductive system for sexual reproduction
**It begins at puberty **
The cycle is **interupted by pregnancy **and terminated by menopause
Although variable between individuals; the average cycle is 28 days duration
There are three phases of the menstrual cycle:
- Follicular / Proliferative phase
- Ovulation phase
- Luteal Phase
Describe the **Follicular / Proliferative Phase **of the menstrual cycle
Include references to the state of hormones, follicles and uterus
Follicular / Proliferative Phase = Day 0-14
Hormones:
FSH stimulates follicular development
- maturation to secondary + tertiary follicles
- stimulate granulosa cells to produce estrogen
Estrogen produced from granulosa cells
- positive feedback to granulosa cells to further increase estrogen levels
- long negative feedback loop to decrease FSH and LH
LH stimulates thecal cells to produce androgens
- androgens are converted by granulosa cells into estrogen
Follicle:
Follicular enlargement and maturation
- one dominant follicle develops into
- estrogen negative feedback on LH and FSH prevents the development of additional follicles
Uterus:
Endometrium rebuilds as estrogen levels rise
- menses occured at the beginning of proliferative phase
Characterise the ovulation phase of the menstrual cycle
Ovulation Phase = Day 14
Hormones:
Estrogen levels high
- elevated concentration changes the effects of estrogen to have positive feedback on GnRH
-
“leads to an LH and FSH surge”
- FSH surge to a lesser extent because it is inhibited by inhibin and estrogen
-
“leads to an LH and FSH surge”
LH surge
- High LH stimulates the first meiotic division to produce the secondary oocyte
- Promotes follicular rupture and ovulation of egg
- Promotes luteinising hormone
- transforms the follicular cell into corpus luteum
**High inhibin **
- Inhibits FSH to prevent the development of new follicles
Low progesterone
- Produced by granulosa cells
- Positive feedback to GnRH and LH
- contributes to LH surge
Follicle:
Completes 1st meiotic division + ovulates egg + differntiates to corpus luteum
Uterus:
Estrogen readies endometrium for implantation
Cervix:
Mucous secretions abundant
Characterise the **Luteal Phase **of the menstruation cycle
Luteal Phase = Day 15-28
Hormones:
Progesterone significantly elevated
- produced by corpus luteum
- maintains endometrium
- inhibits GnRH, LH and FSH
- increases body temperature
- thicken cervix mucus
Estrogen increased
- produced by corpus luteum
- maintain endometrium
- inhibits GnRH, LH and FSH
**High Inhibin **
- Inhibits FSH and follicle development
Corpus Luteum:
Significant progesterone production + increased estrogen & inhibin
Follicle:
No new development
Uterus:
Prepares for pregnancy
Cervix:
Viscous mucus secretion
Body temperature:
Increases
Depending on the outcomes of fertilisation, what happens to the menstrual period?
No pregnancy:
- The corpus luteum dies due to the absence of placental-produced human chorionic gonadotropin (hCG) which is required to maintain it.
- No hormones produced from corpus luteum
- **increase in FSH and LH causes menses and new follicular development **
Pregnancy:
- maintain high hormones via corpus luteum (maitained by hCG) to retain endometrial lining
- Eventually the corpus luteum will die when the placenta itself can produce the required hormones
Why does menopause lead to infertility?
As a female ages, her ovaries cease to respond to FSH and LH which leads to cessation of egg development -> ultimately the death and loss of all eggs
Levels of estrogen and progesterone decrease without ovarian reception of LH and FSH
Leads to symptoms of hot flushes and osteoporosis
Describe the sequence of events occuring during fertilisation
- Sperm swim “upstream” to the ampulla of the fallopian tube
- Spermal acrosomal reaction releases digestive enzymes which dissolve the zona pellucida and cell junctions of the secondary oocyte
- Sperm and oocyte membranes fuse allowing the sperm nucleus to enter
- The secondary oocyte completes final meiotic division and the two nuclei fuse to form a zygote
- Secondary polar body is ejected
- Cortical reaction blocks polyspermy
What are the respective life spans of a sperm and secondary oocyte?
Sperm = 48 hours
Secondary oocyte = 12-24 hours
The life spans of these two gametes dictate the ‘fertilisation window’
Describe zygote development
Day 1 = fertilisation
Day 2-4 = cell divisions
Day 4-5 = blastocyst migrates to uterus
Day 5-9 = blastocyst implants in uterus
Describe the critical periods of human development in utero
Early Pregnancy (Week 1-2)
- Not susceptible to teratogens
- Susceptible to chromosome abnormalities
- Environmental disturbances interfere with implantation
Embryonic Period (Week 3-8)
- Most susceptible to tetratogens
- All major organ systems develop and are present at week 8
Foetal Period (Week 8 - Term)
- When all organ system are present = foetus
- Rapid growth
- Physiological and functional defects susceptible to developing
- Minor morphological abnormalities can develop
What are the two central functions of the reproductive organs?
- Gametogenesis
- Spermatogenesis in male Sertoli cells
- Oogenesis produce ova in the ovaries
- Secretion of hormones
- Testosterone from male Leydig cells
- Estrogen from Granulosa cells
The female reproductive organs are additionally responsible for the reception of sperm, fertilisation, gestation, parturation and lactation
What is gametogenesis?
Gametogenesis is production of haploid germ cells within a person’s sex organs.
Males undergo spermatogenesis ; females undergo oogenesis
Characterise spermatogenesis
Spermatogenesis begins in puberty
- Spermatogonia mitotically divide in the inner surfaces of the seminiferous tubule produceing two daughter spermatogonium cells: one that maintains proliferating germ cell line and the other that moves towards the lumen as a primary spermatocyte destined for meiosis
- A diploid primary spermatocyte undergoes the first meiotic division to produce two haploid secondary spermatocytes
- Secondary spermatocytes undertake second meiotic division to produce haploid spermatids
- These spermatids differentiate into spermatozoa (obtain head and tail etc.)
- Spermatozoa are stored in the epididymis where they mature into sperm
Characterise oogenesis
Oogenesis begins in utero prior to birth
- Mitotic divisions of germ cell oogonium occur to produce primary oocytes prior to birth and this ceases permanently late in foetal life
- Females are subsequently born with their full complement of gametes
- After the onset of puberty, one primary oocyte reaches maturity and is ovulated each menstrual cycle until menopause
- The first meiotic division is completed immediately prior to ovulation in order to ovulate a secondary oocyte and produce a first polar body
- The secondary oocyte undergoes the second meiotic division immediately following fertilisation -> giving rise to a mature ovum
Note: many primary oocytes develop in each menstrual cycle but only one reaches the right level of maturity to be ovulated -> other developing oocytes die off
This dying off of primary oocytes over the course of a woman’s life contributes to menopause
Illustrate the hormonal regulation of male sexual development
