fertilization n.
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  1. Fertilization • Sperm are ejaculated from the penis into the vagina just below the cervix and then swim through the uterus into the Fallopian tubes and they join with an egg = internal fertilization • The egg is surrounded by a thick protective layer that contains sperm binding sites • When a sperm binds to the egg, enzymes in a sac on the head of the sperm (acrosome) break through the protective layer of the egg • The sperm enters the egg and their nuclei rupture and join • A fertilized egg = zygote • The zygote begins to divide by mitosis forming a ball of cells that attaches itself to the wall of the uterus

  2. Fallopian tube Zygote 4 cells 2 cells Morula Blastocyst Uterine wall Ovary Fertilization and Implantaton Section 39-4 Day 2 Day 3 Day 1 Day 4 Fertilization Day 0 Day 7 Implantation of blastocyst Egg released by ovary

  3. Early Development • Cleavage is the first major phase of embryonic development, it occurs in the fallopian tubes • It is the rapid succession of cell divisions (Mitotic) • It creates a multicellular embryo from the zygote • NO growth occurs cells divide in given space • After 4 days, the embryo is a ball of about 50 cells = morula • As the morula grows, a hollow fluid-filled cavity forms in the center when this occurs it is now called a blastocyst • After 6-7 days, the blastocyst attaches itself to the wall of the uterus by secreting enzymes that digest a path into the soft tissue = implantation

  4. Cleavage and Blastocyst Formation Figure 20-2

  5. Embryonic Development • Gastrulationis the second major phase of embryonic development • The cells at one end of the blastula move inward • Organs start to form after gastrulation • Embryonic tissue layers begin to differentiate into specific tissues and organ systems

  6. Gastrulation • 3 layers = primary germ layers because all organs and tissue will be formed from them • Ectoderm (outer) – becomes skin and nerves • Mesoderm(middle) – becomes everything else • Endoderm(inner) - becomes digestive, excretory, and respiratory systems • Membranes develop that surround, protect, and nourish the developing embryo = amnion and chorion

  7. Structures of early development

  8. Placenta • After 3 weeks of development, the nervous and digestive systems have begun to form • The chorion has grown into the placenta • Placenta = connection between mother and developing embryo supplying nutrients and oxygen and eliminating carbon dioxide and other wastes (respiration, nourishment, and excretion) • The placenta separates the mother’s and embryo’s blood, but gases, food and wastes can diffuse across it • After 8 weeks of development, embryo = fetus • After 3 months of development, fetus has umbilical cord connecting fetus to placenta, most organs are fully formed (including muscles) and the fetus can reflex • Amnion has grown into amniotic sac, filled with amniotic fluid that cushions and protects developing fetus

  9. Later Development • After 6 months of development, tissues are more complex and specialized, skeleton formed, heartbeat strengthened, soft hair grows over skin, and fetus continues to increase in mass • The fetus may be able to survive outside the uterus with life-supporting equipment • Last 3 months of development increase the fetus’ chances of survival • The fetus doubles in mass, lungs and other organs change to prepare for life outside uterus

  10. Critical Times…

  11. Childbirth • After about 9 months of development, the fetus if ready for birth • Oxytocin from the pituitary gland affects large involuntary muscles in the uterine wall, stimulating contractions = labor • Cervix expands until large enough to pass the head of the baby • Amniotic sac breaks and fluid rushes out of the vagina = “water-breaking” • Uterine contractions become stronger and more frequent, forcing baby out through the vagina • The baby cries or coughs when outside the body, riding the lungs of fluid, so breathing can begin immediately • The blood supply through the umbilical cord dries up and it is cut leaving a scar on the baby (navel/belly-button) • After birth, the placenta and empty amniotic sac are expelled by a final series of uterine contractions called the afterbirth

  12. Labor • Dilation Stage • Begins with onset of true labor • Cervix dilates • Fetus begins to shift toward cervical canal • Highly variable in length, but typically lasts over 8 hours • Frequency of contractions steadily increases • Amnionicmembraneruptures (water breaks)

  13. Labor

  14. Labor • Expulsion Stage • Begins as cervix completes dilation • Contractions reach maximum intensity • Continues until fetus has emerged from vagina: • Typically less than 2 hours • Delivery • Arrival of newborn infant into outside world

  15. Labor Figure 20-11

  16. Labor • Placental Stage • Muscle tension builds in walls of partially empty uterus • Tears connections between endometrium and placenta • Ends within an hour of delivery with ejection of placenta, or afterbirth • Accompanied by a loss of blood

  17. Labor Figure 20-11

  18. After Childbirth • The baby begins to lead an independent existence (respiration, excretion, maintaining homeostasis) • The mother’s pituitary gland secretes the hormone prolactin, which stimulates the production of milk in the breast tissue • Breast milk contains all of the nutrients the baby needs for growth and development during the first few months of life

  19. Multiple Births? • Fraternal twins = when 2 eggs are released and fertilized by 2 sperm during the same cycle • Not identical because different eggs fertilized by different sperm • Identical twins = when 1 zygote splits producing 2 embryos • Identical because contain genetic information from same egg and same sperm

  20. Multiple Births • Conjoined Twins • Siamese twins • Genetically identical twins • Occurs when splitting of blastomeres or of embryonic disc is not completed

  21. Reproductive Technologies have made it possible to: • fertilize and egg outside the female body • called In vitro fertilization.  • introduce semen inside the female body without sex • called artificial insemination. • Implant a fertilized egg into the uterus : • also done by artificial insemination. • Hormone therapy:provide female hormones to woman who can no longer make them. Risky.