LG: What happens after fertilization?

1. LG: What happens after fertilization?

2. EMBRYO From one cell to many • A fertilized egg goes through an early stage of development called an embryo.

3. The value of sexual reproduction • Sexual reproduction introduces genetic variation • genetic recombination during meiosis • crossing over • mixing of alleles across homologous chromosomes • random fertilization • which sperm fertilizes which egg? • Driving evolution • Variation for evolution

4. Variation from random fertilization • Sperm + Egg = ? • any 2 parents will produce a zygote with over 70 trillion (223 x 223) possible diploid combinations

5. Michael & KirkDouglas Baldwin brothers Martin & Charlie Sheen, Emilio Estevez Sexual reproduction creates variations Sexual reproduction allows us to maintain both genetic similarity & differences.

6. Regents Question

7. Fertilization: Four Major Steps • Sperm contacts the egg • Sperm or its nucleus enters the egg • Egg becomes activated and developmentalchanges begin • Sperm and egg nuclei fuse

8. Fertilization

9. The Nuclei Fuse Together

10. What happens now? • Development of the zygote • The zygote undergoes a series of mitotic cell divisions called cleavage. • The stages of development are: Fertilized ovum (zygote)  2-cell stage  4-cell stage  8-cell stage  Morula  Blastula  Early Gastrula  Late Gastrula

11. Twins • Dizygotic or fraternal twinsoccur 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)

12. Cleavage (divide via mitosis) forms the 2 cell stage

13. They split again to form the 4 cell stage

14. And again to form the 8 cell stage…

15. And eventually form a Morula • Morula – solid ball of cells formed from cleavage

16. Next it becomes a blastula • Blastula – hollow ball of cells formed from cleavage

17. And next, a gastrula • Gastrula – a hollow ball of cells with an “in pushing” and 3 layers (germ layers)

18. The Regents Diagram… • Sperm and ovum (fertilization) • Zygote (fertilized ovum) • 2-cell stage • 4-cell stage • Morula • Blastula • Gastrula Cleavage: process of cell division (Mitosis) during development

19. Regents Question

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23. Differentiation (Organogenesis) • Organogenesis is the formation of the organs (Organo = organs, genesis = creation) • Arises from the layering of cells that occurs during gastrula stage • The layers are germ layers; they have specific fates in the developing embryo: • Endoderm • The innermost layerformthe gut • Mesoderm • In the middle form the muscles, circulatory system, blood and many different organs • Ectoderm • The outermostformthe skin and nervous system

24. Inner: intestines Outer: skin cells Middle: bone cells Specialized cells perform specific functions. • Cells develop into their mature forms through the process of cell [differentiation]. • Cells differ because different combinations of genes are expressed.

25. Late Gastrula Endoderm Ectoderm Mesoderm

26. Differentiation of Germ Layers

27. Early Human Development Summary • Meiosis makes sperm in males and ovum in females • Sperm and ovum unite nuclei to form a zygote • Zygote undergoes cleavage and becomes gastrula with 3 germ layers

28. Extraembryonic Membranes

29. Human Prenatal Development • Gestation lasts 266 days from fertilization to birth • Development begins in the oviduct • About 24 hours after fertilization, the zygote has divided to form a 2-celled embryo • The embryo passes down the oviduct by cilia and peristalsis • By the 5th day, the embryo enters the uterus • The embryo floats free for several days, nourished by fluids in the uterine wall • At this point, it is called a blastocyst

30. Implantation • The embryo implants in the wall of the uteruson about the 7th day of development

31. 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.

32. 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.

33. Development of the Placenta

34. 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

35. UMBILICAL CORD Connecting link between fetus and placenta. • Transports oxygen and nutrients to fetus from the placenta and returns waste products from the fetus to the placenta. • Contains: 2 arteries and 1 vein • Contains NO pain receptors.

36. Regents Question • Identify the structure in the uterus where the exchange of material between the mother and the developing fetus takes place. _____________________________________

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39. Regents Question

40. Placenta • Food & gases diffuse across blood vessels

41. 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.

42. TRIMESTERS Trimester 1st = week’s 1-13 (2.5-3 inches, 0.5-1oz.) 2nd = week’s 14 – 26 (11-14 inches, 1.75-2 lbs.) 3rd = week’s 27 and on (38-40 WEEKS) (7-7.5 inches, 7-7.5 lbs.)

44. Human fetal development 4 weeks 7 weeks

45. Regents Question

48. Human Fetus at Ten Weeks

49. Human fetal development 10 weeks

50. Human fetal development 12 weeks 20 weeks