pregnancy, growth and development
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Pregnancy, Growth and Development. Chapter 23. Conception. A secondary oocyte can be fertilized for about 24 hours after ovulation Sperm remain viable for up to 48 hours within the female reproductive tract - PowerPoint PPT PresentationTRANSCRIPT
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Pregnancy, Growth and Development
Chapter 23
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Conception
• A secondary oocyte can be fertilized for about 24 hours after ovulation
• Sperm remain viable for up to 48 hours within the female reproductive tract
• This gives a three day “window” for intercourse to result in fertilization: two days before to one day after ovulation
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• Fertilization usually takes place in the outer one-third of the uterine tube, but can take place in the abdominal cavity
• Sperm swim up the female reproductive tract, aided by muscular contractions of the uterus stimulated by prostaglandins in the semen.
• The oocyte may also secrete a chemical that attracts sperm
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• Sperm undergo a functional change in the female tract – called capacitation
• During this process the membrane around the acrosome becomes fragile, and its enzymes are released.
• It requires the combined action of many sperm to allow one sperm to penetrate the oocyte.
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• When the first sperm enters the egg, the cell depolarizes causing the release of calcium ions inside the cell.
• This stimulates the release of granules that cause changes in the zona pellucida to prevent entry of other sperm.
• Secondary oocyte completes division, and nuclei of ovum and sperm unite to form a zygote.
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Twins
• Dizygotic or fraternal twins occur when two separate eggs are ovulated. May be of different sexes.
• Monozygotic or identical twins occur when a single egg is fertilized but dividing cells break into two groups and develop into two individuals. Genetically identical (clones)
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• Zygote undergoes rapid mitotic cell division, but these do not increase the size of the zygote – called cleavage divisions
• Cleavage produces a solid sphere of cells, still surrounded by zona pellucida – now called a morula.
• At 4.5 to 5 days, cells have developed into a hollow ball of cells – blastocyst.
• It is at this stage that it enters the uterus.
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• Blastocyst has an outer layer of cells called the trophoblast, an inner cell mass, and a fluid filled cavity called the blastocele.
• The trophoblast and part of the inner cell mass will form the membranes of the fetal portion of the placenta, the rest of the inner mass forms the embryo.
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Implantation• The blastocyst remains free in the uterus a
short time, during which the zona pellucida disintegrates.
• Blastocyst nourished by glycogen from glands of the endometrium.
• At about 6 days after ovulation blastocyst implants – orients cell mass toward endometrium, and secretes enzymes which allow it to penetrate (digest) the endometrial wall. This nourishes the blastocyst for about a week after implantation.
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• Implantation can also occur in uterine tube, cervix, or the abdominal cavity.
• Implantation anywhere outside the uterus is called an ectopic pregnancy.
• It is possible for fetus to grow in the abdominal cavity, but growth inside the uterine tube causes the tube to rupture, resulting in severe bleeding.
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• As early as 8 -12 days after fertilization, the blastocyst begins to secrete human chorionic gonadotropin or hCG.
• hCG keeps the corpus luteum active until the placenta can produce estrogens and progesterone.
• The presence of hCG is the basis for pregnancy tests.
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• Inner cell mass forms two cavities:– The yolk sac– Amniotic cavity
• In humans the yolk sac produces blood cells and future sex cells
• The amniotic cavity becomes the cavity in which the embryo floats. Fluid is produced from fetal urine, and secretions from the skin, respiratory tract, and amniotic membranes.
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Primary germ layers
• In between the yolk sac and the amniotic cavity is the embryonic disc, which gives rise to the primary germ layers:
– Endoderm
– Mesoderm
– Ectoderm
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Gestation period• Divided into three trimesters.
• During first trimester individual starts out as a zygote, then morula, blastocyst, and after implantation, is called an embryo.
• Embryonic phase of development lasts from fertilization until the 8th week of gestation, when it becomes a fetus.
• By day 35 the heart is beating, and eye and limb buds are present.
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• By month four, the rudiments of all organ systems are formed and functioning, and from then on, fetal development is primarily a matter of growth.
• By the end of the third month the placenta is functioning.
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The placenta• The chorion develops into the fetal part of the
placenta.
• The chorionic villi connect the fetal circulation to the placenta
• Composed of both fetal and maternal tissues
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Functions of the placenta:1 Transfer gasses
2 Transport nutrients
3 Excretion of wastes
4 Hormone production – temporary endocrine organ – estrogen and progesterone
5 Formation of a barrier – incomplete, nonselective – alcohol, steroids, narcotics, anesthetics, some antibiotics and some organisms can cross
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Quickening• The first movement of the fetus felt by the
mother, usually occurring during the fourth or fifth month of pregnancy
• By month seven the fetus is quite active
• During the last month the fetus becomes less active (usually due to space considerations.)
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• At the end of pregnancy both the mother and the uterus become “irritable”
• The uterus undergoes Braxton-Hicks contractions: intermittent, painless contractions which can come 10 to 20 minutes apart.
• Become more frequent as gestation progresses, and can be mistaken for onset of labor
• Cervix begins to thin and dilate
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Labor (parturition)• Stage one – the period from the onset of
true labor contractions until the cervix is completely dilated at 10 cm.
• The uterine contractions cause the cervix to dilate, and the amniotic sac may rupture.
• Usually lasts 6 – 24 hours depending on the number of previous deliveries.
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Stage 2• Period from maximal cervical dilation until
the birth of the baby
• Lasts minutes to an hour
• Contractions become more intense and frequent.
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Stage 3
• The expulsion of the placenta
• Usually occurs within 15 minutes after the birth of the baby, but can range from 5 to 60 minutes.
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The End !!
• That’s it !
• You’ve made it !
• Study well !
• Good luck !