Physiology of pregnancy Flashcards
what is reproduction
= ability to produce a new generation of individuals of the same species.
In humans the individual develops from the fusion of 2 different sex cells= gametes:
- Males: spermatozoa (sperm)
- Females: ova (ovum, egg)
Gametes are produced by gonads:
- Males: testes
- Females: ovaries
Nuclei of sex cells are haploid: contain a single set of 23 unpaired chromosomes
what is fertilisation
= fusion of haploid sperm and haploid egg -> creates single, diploid cell= zygote (develops to embryo)
- Posesses a full set of chromosome (23 pairs): half originate frpm sperm and half from ovum (-> paternal and maternal chromosomes)
Fertilisation occurs in fallopian tube
Ovaries release mature, fertilizable ova at regular intervals throughout the reproductive years of a woman’s life
The female reproductive cycle involves 2 linked cycles:
- Ovarian: produces mature eggs
- Uterine: prepares the internal environment for a fertilised egg
Sperm attaches to secondary oocyte- enters oocyte and joins with its pronucleus to form a new single nucleus -> end product is the sigle cell =zygote
Oviduct cilia beat and move the zygote along, developing as it moves- zygote divides to form 2 cells (18-36h) and divide into 4,8…
ovarian cycle
Period of oestrogen dominance characterizes the first half of cycle (follicular phase):
- 1 ovarian follicle reaches full maturity (= Graafin follicle) -> ovulation (on day 14 of the menstrual cycle)
Estrogen (steroid hormone) is secreted by follicular cells
uterine cycle
Period of progesterone dominance characterizes the second half of the ovarian cycle (=luteal phase)
The uterus is maintained in a state favorable for implantation and development of blastocyst (=early embryo)
- The uterine cycle involves the ‘build-up’ of the uterine endometrium from ~day 5.
- Endometrial development is maximal 5 days post-ovulation; it remains prepared for blastocyst implantation for a further 9 days:
- In absence of fertilisation & implantation the corpus luteum degenerates (luteolysis): endometrium ‘sloughs off’, and menstruation occurs.
- oestrogen stimulates th endometrium to proliferate
- Progesterone stimulates vascularisation & mucus secretion
Hyothalamo-pituitary gonadal control
Reproductive function is largely controlled by a chain of hormones:
1st in chain: hypophysiotropic GnRH (gonadotropin-releasing hormone).
- GnRH stimulates the release of anterior pituitary gonadotropins: FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
FSH and LH act upon the gonads:
Maturation of sperm or ova.
Stimulation of sex hormone secretion.
- Inhibin mainly feedbacks to inhibts the release of FSH
hormonal control of ovarian and uterine cycles
Puberty: hypothalamus ↑ GnRH release -> stimulates release of gonadotropins (FSH and LH) from anterior pituitary.
FSH & LH cause growth of ovarian tissue; ovarian (sex steroid) hormones are produced (e.g. oestrogen).
First 12 days (~28-day ovarian cycle): oestrogen inhibits LH & FSH release.
12-14 days: oestrogen exerts positive feedback ->LH surge: triggers ovulation (ruptured follicle develops into a corpus luteum).
persistence of corpus luteum during pregnancy- the role of gonadotropin
Maintenance of the structure and function of corpus luteum beyond the end of the menstrual cycle is dependent on the secretion of human chorionic gonadotropin (hCG): a glycoprotein hormone.
After fertilisation, hCG is secreted by trophoblast cells (once they start their endometrial invasion ):
- Prevents both degeneration of the corpus luteum and strongly stimulates its steroid sex hormone secretion.
- hCG level is monitored in pregnancy tests. Following fertilisation, its presence in urine 2 weeks after ovulation is used as a reliable and simple test for pregnancy.
- hCG enables the corpus luteum to continue to secrete progesterone (and oestrogen) required for the continuation of pregnancy:
-> Maintains the specialised uterine endometrial layers until the pregnancy can be supported by progesterone of placental origin
- Placenta takes over during the 2nd trimster (7th/8th week)
Embryonic development: zygote to blastocyst
- day 14-21 of menstrual cycle
Trophoblast cells play a crucial role in implanting embryo into uterine wall: - Syncytiotrophoblast actually accomplishes implantation.
Trophoblast cells contribute to the formation of the placenta: - Chorion develops from the trophoblast and extraembryonic mesoderm layer (chorion: embryo’s portion of placenta).
