Objectives: This lecture will enable you to



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Dr Henan Dh. Skheel 4th year lecture on obstetrics

2016-2017

Ovulation, fertilization, implantation

Objectives:


This lecture will enable you to:

1. Explain the process of ovulation & list the hormones influencing it

2. Explain the luteinization & the formation of corpus luteum (of menstruation & of pregnancy), their functions & fate

3. Explain the precondition of the sperm for fertilization

4. Describe phases of fertilization

5. Explain the reaction of oocyte to the fertilization & results of fertilization

6. Describe the different stages from zygote to blastocyst

7. Define implantation & its normal sites in the uterus

8. Explain the decidua reaction of the uterus during implantation

9. Explain the abnormal implantation & its results

Ovarian Cycle

• Monthly cycle for female at puberty

Control by hypothalamus

• Release GnRH anterior pit gland gonadotrophins (FSH & LH)

• FSH & LH act on ovaries

• FSH: stimulates the development of ovarian follicles and production of estrogen by follicular cells

• LH: Trigger ovulation and stimulates follicular cells and corpus luteum to produce progesterone

Ovarian Cycle

Beginning of Ovarian Cycle

• FSH stimulates 15-20 primary stage follicles to grow only one reaches full maturity and only one oocyte is discharged others degenerate and become atretic

• FSH stimulates maturation of follicular (granulosa) cells
• Proliferation of follicular cells is mediated by GDF-9

• Granulosa and thecal cells produce estrogens

• Effects of estrogen:

1. Thinning of cervical mucus

2. Uterine endometrium to enter follicular/proliferative phase

3. Stimulate pituitary gland to secrete LH

Ovarian Cycle

Mid-cycle

• LH surge causes:

1. Oocyte complete meiosis I and initiate meiosis II

2. Stimulates production of progesterone by follicular cells

3. Follicular rupture and ovulation

 Ovulation

• Secondary follicle expanse rapidly under the influence of FSH and LH and become preovulatory follicle

• Meiosis II is initiated and arrested at the metaphase II, 3 hours before ovulation

• Follicle is displaced to the surface of ovary where it forms a bulge and stigma

• LH collagenase activity increases causes digestion of collagen fibers surrounding the follicle

• LH increase in prostaglandin causes muscular contraction on the ovarian wall release of oocyte with surrounding cumulus oophorus

Corpus luteum

• The granulosa cells and theca interna remaining in the wall of ruptured follicle are vascularized by surrounding vessels

• Under the influence of LH, the remaining cells develop into yellowish pigment and change into lutean cells (corpus luteum) which secretes progesterone

• Progesterone and estrogen causes endometrial layer to enter secretory phase (preparation for implantation)

. Corpus albicans

• If fertilization does not occur, on day 9 of ovulation, corpus luteum shrinks, degenerate and form a mass of fibrotic scar tissue (corpus albicans) reduce progesterone level menstrual bleed

• If fertilization occur, degeneration of corpus luteum is prevented by hCG (human chorionic gonadotropin) (secreted by syncytiotrophoblast of developing embryo). CL continue to grow at the end of 3rd month of pregnancy CL size is half of ovary secretes progesterone until 4 month of pregnancy

• After that CL regress slowly as secretion of progesterone by trophoblastic component of placenta becomes adequate for maintenance of pregnancy



Oocyte transport

• Fimbriae sweep over the surface of the ovary and catch the oocyte that are released from the ovary

• In the tube, the oocyte move towards the uterine cavity by the contraction of the tube and propelled by cilia

• Fertilized egg reach uterine lumen 3-4 days

Fertilization • Begins with contact between sperms and secondary oocyte and ends at metaphase I of zygote

• Occur in the ampulla of uterine tube

• 200-500 million sperms ejaculated into the vagina, only 200 sperms reach secondary oocyte

• Occur 12-24 hours after ovulation

• Sperm remains viable for 48 hours, secondary oocyte viable for 24 hours after released from the ovary

• Sperm swim from vagina to cervix by propelled whiplike movement of their tail

• Movement from cervix to the tube - help by contraction of uterus and tube

• Sperm need to wait for 7 hours to fertilize the oocyte, must undergo:



  1. Capacitation

2. Acrosome reaction

Capacitation: • Functional changes that causes the tail of sperms to beat vigirously




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