Video notes- ovulation and fertilization PDF

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OVULATION VIDEO NOTES

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The Reproductive cycles begins with ovulation ◦ the release of a ripe egg, or ovum, from the ovary ◦ hundreds of thousands of follicles sit nestled inside the ovary ◦ each follicle is a sphere of cells with an immature egg at its center

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the typical 28 day menstrual cycle begins on the first day of menstrual bleeding ◦ during the first 7 days of the cycle a few follicles start to grow ◦ these maturing follicles secrete estrogen hormone into the blood stream to prepare the lining of the uterus for prgnancy

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around day 7 ◦ all of the follicles stop growing and begin to degenerate ▪ except for one!!

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this dominant follicle continues to grow

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nourishing the developing egg inside it

around day 12 ◦ the follicle secretes a surge of estrogen into the blood stream ▪ when the estrogen reaches the hypothalamus and the pituitary gland in the brain... ▪ the anterior or front part of the pituitary gland floods the blood stream with Luteinizing Hormone (LH)

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around day 14 ◦ LH causes the follicle to undergo a sudden growth spurt ◦ right before ovulation, the egg detaches from inside of the follicle ◦ this now bulging follicle releases chemicals ▪ signaling one of the two fallopian tubes to move in closer and envelope the follicle ◦ the follicle swells until – IT BURST OPEN ▪ ejecting the ovum and fluid from the follicle into the abdominal cavity ▪ in response, the fimbriae (tiny finger-like projections at the end of fallopian tubes) sweep across the ovulation site and pick up the ovum

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microscopic cilia on the fimbrial surface guide the ovum to the entrance of the fallopian tube

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muscular contractions along the wall of the fallopian tube gently propel the ovum toward the uterus

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the ovum lives for 12-24 hours (the interval when fertilization can occur) ◦ if fertilization does not occur: ▪ the ovum will disintegrate ▪ will be shed along with the uterine lining during menstruation ▪ marking the end of one reproductive cycle and the beginning of another. FERTILIZATION VIDEO NOTES

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during sexual intercourse ◦ about 300 million sperm enter the vagina ◦ soon afterward millions of them will either flow out of the vagina or die in its acidic environment ◦ however, many survive because of the protective elements provided in the fluid surrounding them

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next the sperm must pass through the cervix and opening into the uterus ◦ usually the cervix remains tightly closed, but during the time a woman ovulates it is open for a few days

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the sperm swim through the cervical mucus ◦ which is thinned to a more watery consistency for easier passage

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once inside the cervix ◦ the sperm continue swimming toward the uterus ▪ though millions will die trying to make it through the mucus ▪ some sperm remain behind caught in the folds of the cervix, but they may later continue the journey as a backup to the first group

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inside the uterus ◦ muscular uterine contractions assist the sperm on their journey toward the egg ◦ however, resident cells from the women’s immune system mistaking the sperm for foreign invaders, destroy thousands more

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next half the sperm head for the empty fallopian tube while the other half swim toward the tube containing the unfertilized egg ◦ now only a few thousand remain

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inside the fallopian tube, ◦ tiny cilia pushed the egg toward the uterus ◦ to continue the sperm must surge against this motion to reach the egg ▪ some sperm get trapped in the cilia and die ◦ during this part of the journey chemicals in the reproductive tract cause the membranes covering the heads of the sperm to change ▪

as a result the sperm become hyperactive

▪ swimming harder and faster toward their destination ◦ the sperm has now reached the egg ▪ only a few dozen of the original 300 million sperm remain •

the egg is covered with a layer of cells called the corona radiata ◦ the sperm must push through this layer to reach the outer layer of the egg – the zona pellucida

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when sperm reach the zona pellucida, ◦ they attach to specialized sperm receptors on the surface ▪ which triggers their acrosomes to release digestive enzymes •

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enabling the sperm to burrow into the layer

inside the zona pellucida is a narrow fluid filled space just outside the egg cell membrane ◦ the first sperm to make contact will fertilize the egg

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after a perilous journey and against incredible odds – a single sperm attaches to the egg cell membrane ◦ within a few minutes their outer membranes fuse and the egg pulls the sperm inside ◦ this event causes changes in the egg membrane ▪ preventing other sperm from attaching to the egg

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next the egg releases chemicals ◦ that push other sperm away from the egg ◦ create an impenetrable fertilization membrane ◦ as the chemical reaction spreads outward, the zona pellucida hardens ▪ trapping any sperm unlucky enough to be caught inside ▪ outside the egg, sperm are no longer able to attach to the zona pellucida

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INSIDE THE EGG:

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tightly packed male genetic material spreads out ◦ a new membrane forms around the genetic material creating the male pronucleus ◦ inside the genetic material reforms into 23 chromosomes

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the female genetic material awakened by the fusion of the sperm with the egg finishes dividing ◦ resulting in the female pronucleus – also contains 23 chromosomes

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as the male and female pronuclei form ◦ spider web-like threads (microtubules) pull them toward each other ◦ the two sets of chromosomes joined together ▪ completing the process of fertilization •

at this moment unique genetic code arises instantly determining ◦ gender, ◦ hair color, ◦ eye color, ◦ hundreds of other characteristics

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this new single cell – zygote – is the beginning of a new human being ◦ and now the cilia and the fallopian tube gently sweep the zygote toward the uterus ▪ where the zygote will implant in the richer uterine lining ▪ growing and maturing for the next nine months until birth...