Download - Ch18 Hum Dev
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Human Development & BirthChapter 18
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Fertilization
• The uniting of the sperm & egg.
• Results in formation of a zygote.
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Sperm Anatomy
• Three parts • Head: contains the
nucleus with the father’s DNA.
• Midpiece: Contains energy producing mitochondria.
• Tail: A flagellum that allows the sperm to swim toward the egg.
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Egg Anatomy
• Plasma membrane surrounded by zona pellucida & corona radiata which nourish the egg.
• Zygote receives cytoplasm & organelles only from mother.
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Fertilization Steps
• Only one sperm enters the egg.
• Acrosome releases enzymes that aid the sperm’s entrance into the egg.
• The nuclei of the sperm and egg fuse.
• Changes in the egg’s membrane and zona pellucida prevent more sperm from entering the egg.
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Processes of Development• Cleavage: Mitotic divisions
of the zygote that do not result in an increase in size.
• Growth: Mitotic divisions accompanied by increase in size.
• Morphogenesis: Migration of cells that results in changing shape of embryo.
• Differentiation: The taking on of a specific structure and function of cells. Nervous system is first to differentiate.
Primitive brainNeural tube
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Extra embryonic Membranes
• Not part of embryo.• Chorion: fetal half of
placenta.• Placenta: organ that
provides fetus with nourishment and oxygen & takes away waste.
• Yolk sac: first site of blood formation.
• Allantois: becomes umbilical blood vessels.
• Amnion: contains cushioning fluid.
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Development
• Events that occur from fertilization to birth.
• Gestation calculated by adding 280 days from the start of the last menstruation.
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Pre-embryonic Development-1st Week
• Zygote: divides into a compact ball of embryonic cells called morula.
• Trophoblast: a layer of cells that surrounds an inner mass of cells—becomes the chorion.
• Cells of the blastocyst and morula have the capability of becoming any tissue.
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Twinning
• Identical twins: Result when either the cells of the morula separate or the inner cell mass splits. Identical chromosomes.
• Fraternal twins: Two different eggs fertilized by two different sperm.
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Cloning• Therapeutic cloning:
Experimental procedure where cells of embryo are separated and treated to become particular tissues. These are called stem cells.
• To try a simple, interactive cloning activity see: http://www.science.org.au/nova/043/043act.htm
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Pre-embryonic Development-2nd Week
• Embryo begins to implant itself in the uterus.
• If embryo implants itself in a location other than the uterus an ectopic pregnancy results. These are not successful pregnancies.
• Embryonic disk, yolk sac, & amnionic cavity form.
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2nd week Continued• Primary germ layers
form:• Ectoderm (outer layer)
becomes epidermis, nervous system, and epithelial lining of digestive system.
• Endoderm (inner layer) becomes organ linings of other organs.
• Mesoderm (middle layer) becomes skeletal, muscular system.
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Third Week
• Nervous system and heart begin to develop.
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Fourth Week
• At 4 weeks body stalk connects tail with chorion.
• Chorionic villi are treelike projection from chorion.
• Umbilical blood vessels appear; umbilical cord fully formed
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Fifth Week
• Limb buds appear.• Head enlarges.• Eyes, ears, and
nose can be seen.
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6th through 8th Weeks
• Easily recognized as human.
• Neck region develops.
• Reflex actions present.
• All organ systems established.
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Placenta
• Two portions—fetal portion (chorionic) and maternal portion (uterine tissue).
• Surface covered with chorionic villi.
• Exchange of materials takes place across placental membrane.
• Maternal & fetal blood rarely mix.• Functions as the digestive
system, lungs, and kidneys of the fetus.
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3rd & 4th Month• Head growth slows
down; body increases in length.
• Fingernails, nipples, eyelashes, eyebrows, & hair on head appear.
• Cartilage begins to be replaced by bone.
• Sexual differentiation begins.
• Fetal heartbeat can be heard during the fourth month.
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5th-7th Month
• Mother can feel movement.
• Skin covered by a fine down called lanugo.
• Coated with white, cheese like substance called vernix caseosa (protects the skin).
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8th-9th Month
• Fetus usually rotates so head is pointed toward cervix.
• Weight gain largely due to accumulation of fat beneath skin.
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Premature Birth
• Difficulties associated with premature birth:
• Respiratory distress syndrome
• Retinopathy• Intracranial
hemorrhage• Jaundice• Infection• Circulatory disorders
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Development of Sex Organs
• Gonads begin developing during the seventh week of development.
• Development depends on the action of hormones.
• Testosterone stimulates ducts to become male genital ducts.
• No Y chromosome-no testosterone-ovaries develop instead of testes.
• Near the end of development testes descend into scrotum.
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Preventable Birth Defects• German measles (Rubella):
blindness, deafness, mental retardation, heart malformations & other problems-can be prevented by vaccination.
• Spina bifida (spinal cord or spinal fluid bulge) Most can be avoided by adequate folate intake.
• Anencephaly (absence of a brain) Most can be avoided by adequate folate intake.
Anencephaly
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Preventable Birth Defects Continued…
• Fetal alcohol syndrome-decreased birth rate, height, head size, malformation of head & face. Mental retardation is common.
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Birth
• True labor marked by uterine contractions that occur regularly every 15-20 minutes and last 40 seconds or longer.
• Expulsion of mucous plug from uterus.
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Parturition: Stage 1
• Cervical canal pulled upward toward the baby’s head.
• Amniotic membrane ruptures if it has not already done so.
• Cervix dilates completely.
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Parturition: Stage 2
• Contractions occur every 1-2 minutes and last about 1 minute.
• Mother feels the need to push.
• Baby’s head descends into vagina.
• Ends with the delivery of the baby and the cutting and tying of the umbilical cord.
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Parturition: Stage 3
• Placenta becomes dislodged from the uterus and expelled from the vagina.
• Expulsion of placenta marks the end of parturition.
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Effects of Pregnancy on the Mother
• Loss of appetite, nausea, vomiting, and fatigue early.
• Weight gain due to breast & uterine enlargement.
• Enlarged uterus compresses the ureters & bladder.
• Sometimes pregnancy induced diabetes can occur.
• Stretch marks appear due to increased steroid levels.
• Darkening in some areas of the skin is common.
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Credits:
• Slide 1: http://embryology.med.unsw.edu.au/• Slide 4:
http://www.mercksource.com/ppdocs/us/common/dorlands/dorland/images/fig_o_0024.jpg
• Slide 4: cgee.hamline.edu/.../ dp_cycles_micro_dex.htm • Slide 5: http://www.lsc.org/online_science/prenatal/development.html• Slide 6:
http://embryology.med.unsw.edu.au/wwwhuman/Stages/Stage10.htm• Slide 7: http://www.sunyniagara.cc.ny.us/val/embryonic-mem.jpg• Slide 16: http://www.lsc.org/online_science/prenatal/development.html• Slide 17: http://www.lsc.org/online_science/prenatal/development.htm• Slide 18: http://www.ricktraceyinc.com/placenta2.jpg• Slide 22: http://preemie.info/cms/modules/mylinks/viewcat.php?cid=8• Slide 25: medlib.med.utah.edu/ WebPath/PEDHTML/PED029.html