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FERTILISATION (TRANSIT OF SPERMS (during ejaculation, hundreds of millions…
FERTILISATION
TRANSIT OF SPERMS
during ejaculation, hundreds of millions of sperm are released into the vagina
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the race into the uterine tubes, is the most typical site for sperm to encounter oocyte
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if the sperm do not encounter an oocyte immediately, they can survive in the uterine tubes for another 3-5 days
during the journey, fluids in the female reproductive tract prepare the sperm for fertilisation through capacitation
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if they reach the oocyte before capacitation is complete, they will be unable to penetrate the oocyte's thick outer layer
DESCRIPTION
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the reproductive cells are haploid containing half of the genetic material needed to form a human being
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CONCEPTION
DIVISION
In the uterine tube, the haploid ovum is fertilized and becomes a diploid zygote
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Blastomere division produces a morula, which is a solid ball of 16 cells
Cell division continues and fluid accumulates between the cells of the morula, producing the 58-cell early blastocyst on day 4 post-fertilization
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After 2 days of free floating in the uterine fluid, the secretory-phase endometrial glands produce proteases that liberate the blastocyst from the zona pellucida
Hormones and cytokines produced by the blastocyst now directly influence the endometrium and promote receptivity; the embryo absorbs nutrients from the uterine secretions
THE FETAL PERIOD
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Weeks 10-12: fetus can swallow, small intestine can undergo peristalsis and glucose transport
Week 12: ossification centres appear in bones, genitalia begin to differentiate, skin and nails and hair appear
Weeks 20-24: neuronal migration and proliferation are completed, and neurologic development shifts towards increasing neuronal organization and differentiation
Week 24: bronchioles and alveoli are nearly completely developed, but terminal alveolar sacs are typically not yet formed (therefore breathing is unlikely)
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EMBRYONIC PERIOD
begins 2-3 weeks after ovulation, and lasts 8 weeks
At the beginning of the embryonic period, the embryonic disc is well-defined, the body stalk is differentiated, and a true intervillous space exists
In week 3, fetal blood vessels develop in the chorionic villi
In week 4, true maternal-fetal circulation and a rudimentary fetal cardiovascular system are established
By the end of week 6, the heart is completely formed, fingers and toes are present, and the ears are present as elevations on the head
INVASION
blastocyst penetrates the endometrium, upper third of myometrium, and uterine vasculature
Whole process is mediated by receptors on the blastocyst and endometrium and by estrogen and progesterone release from the corpus luteum (therefore, uterus is only receptive to implantation between days 20 and 24 of the menstrual cycle)
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APPOSITION
blastocyst is apposed, in most cases, to the upper posterior uterine wall
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By day 8 after fertilization, trophoblast cells have differentiated into the syncytiotrophoblast (multinucleated syncytium that makes up the outer layer of the blastocyst) and the cytotrophoblast (inner layer of mononuclear cells)
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Villous trophoblast cells give rise to chorionic villi and are responsible for oxygen and nutrient transport between mother and fetus