1. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Human Anatomy & Physiology, Sixth Edition Elaine N. Marieb PowerPoint ® Lecture Slides prepared by Vince Austin, University of Kentucky 28 Pregnancy and Human Development Part A

2. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings From Egg to Embryo  Pregnancy – events that occur from fertilization until the infant is born  Conceptus – the developing offspring  Gestation period – from the last menstrual period until birth  Preembryo – conceptus from fertilization until it is two weeks old  Embryo – conceptus during the third through the eighth week  Fetus – conceptus from the ninth week through birth

3. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Relative Size of Human Conceptus Figure 28.1

4. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Accomplishing Fertilization  The oocyte is viable for 12 to 24 hours  Sperm is viable 24 to 72 hours  For fertilization to occur, coitus must occur no more than:  Three days before ovulation  24 hours after ovulation  Fertilization – when a sperm fuses with an egg to form a zygote

5. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Sperm Transport and Capacitation  Fates of ejaculated sperm  Leak out of the vagina immediately after deposition  Destroyed by the acidic vaginal environment  Fail to make it through the cervix  Dispersed in the uterine cavity or destroyed by phagocytic leukocytes  Reach the uterine tubes  Sperm must undergo capacitation before they can penetrate the oocyte

6. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Acrosomal Reaction and Sperm Penetration  An ovulated oocyte is encapsulated by:  The corona radiata and zona pellucida  Extracellular matrix  Sperm binds to the zona pellucida and undergoes the acrosomal reaction  Enzymes are released near the oocyte  Hundreds of acrosomes release their enzymes to digest the zona pellucida

7. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Acrosomal Reaction and Sperm Penetration  Once a sperm makes contact with the oocyte’s membrane:  Beta protein finds and binds to receptors on the oocyte membrane  Alpha protein causes it to insert into the membrane

8. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Acrosomal Reaction and Sperm Penetration Figure 28.2a

9. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Blocks to Polyspermy  Only one sperm is allowed to penetrate the oocyte  Two mechanisms ensure monospermy  Fast block to polyspermy – membrane depolarization prevents sperm from fusing with the oocyte membrane  Slow block to polyspermy – zonal inhibiting proteins (ZIPs):  Destroy sperm receptors  Cause sperm already bound to receptors to detach

10. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Completion of Meiosis II and Fertilization  Upon entry of sperm, the secondary oocyte:  Completes meiosis II  Casts out the second polar body  The ovum nucleus swells, and the two nuclei approach each other  When fully swollen, the two nuclei are called pronuclei  Fertilization – when the pronuclei come together

11. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Figure 28.3 Events Immediately Following Sperm Penetration

12. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Preembryonic Development  The first cleavage produces two daughter cells called blastomeres  Morula – the 16 or more cell stage (72 hours old)  By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)

13. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Preembryonic Development  Blastocyst – a fluid-filled hollow sphere composed of:  A single layer of trophoblasts  An inner cell mass  Trophoblasts take part in placenta formation  The inner cell mass becomes the embryonic disc

14. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings (a) Zygote (fertilized egg) (b) 4-cell stage 2 days (c) Morula 3 days (e) Implanting blastocyst 6 days (d) Early blastocyst 4 days (e) (d) Fertilization (sperm meets egg) Uterine tube Oocyte (egg) (a) Ovulation Ovary (b) (c) Uterus Endometrium Cavity of uterus Cleavage: From Zygote to Blastocyst Figure 28.4 Degenerating zona pellucida Blastocyst cavity Inner cell mass Blastocyst cavity Trophoblast

15. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Implantation  Begins six to seven days after ovulation when the trophoblasts adhere to a properly prepared endometrium  The trophoblasts then proliferate and form two distinct layers  Cytotrophoblast – cells of the inner layer that retain their cell boundaries  Syncytiotrophoblast – cells in the outer layer that lose their plasma membranes and invade the endometrium

16. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Implantation  The implanted blastocyst is covered over by endometrial cells  Implantation is completed by the fourteenth day after ovulation

17. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Implantation of the Blastocyst Figure 28.5a

18. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Implantation of the Blastocyst Figure 28.5b

19. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Implantation  Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG) secreted by the trophoblasts  hCG prompts the corpus luteum to continue to secrete progesterone and estrogen  Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus  Between the second and third month, the placenta:  Assumes the role of progesterone and estrogen production  Is providing nutrients and removing wastes

20. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Hormonal Changes During Pregnancy Figure 28.6

21. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation  Formation of the placenta from:  Embryonic trophoblastic tissues  Maternal endometrial tissues

22. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation  The chorion develops fingerlike villi, which:  Become vascularized  Extend to the embryo as umbilical arteries and veins  Lie immersed in maternal blood  Decidua basalis – part of the endometrium that lies between the chorionic villi and the stratum basalis

23. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation  Decidua capsularis – part of the endometrium surrounding the uterine cavity face of the implanted embryo  The placenta is fully formed and functional by the end of the third month  Embryonic placental barriers include:  The chorionic villi  The endothelium of embryonic capillaries  The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin

24. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation Figure 28.7a-c

25. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation Figure 28.7d

26. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Placentation Figure 28.7f

27. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Germ Layers  The blastocyst develops into a gastrula with three primary germ layers: ectoderm, endoderm, and mesoderm  Before becoming three-layered, the inner cell mass subdivides into the upper epiblast and lower hypoblast  These layers form two of the four embryonic membranes

28. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Embryonic Membranes  Amnion – epiblast cells form a transparent membrane filled with amniotic fluid  Provides a buoyant environment that protects the embryo  Helps maintain a constant homeostatic temperature  Amniotic fluid comes from maternal blood, and later, fetal urine

29. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Embryonic Membranes  Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo  Forms part of the digestive tube  Produces earliest blood cells and vessels  Is the source of primordial germ cells

30. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Embryonic Membranes  Allantois – a small outpocketing at the caudal end of the yolk sac  Structural base for the umbilical cord  Becomes part of the urinary bladder  Chorion – helps form the placenta  Encloses the embryonic body and all other membranes

31. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Gastrulation  During the 3 rd week, the two-layered embryonic disc becomes a three-layered embryo  The primary germ layers are ectoderm, mesoderm, and endoderm  Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo

32. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Gastrulation  As cells begin to migrate:  The first cells that enter the groove form the endoderm  The cells that follow push laterally between the cells forming the mesoderm  The cells that remain on the embryo’s dorsal surface form the ectoderm  Notochord – rod of mesodermal cells that serves as axial support

33. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Primary Germ Layers  Serve as primitive tissues from which all body organs will derive  Ectoderm – forms structures of the nervous system and skin epidermis  Endoderm – forms epithelial linings of the digestive, respiratory, and urogenital systems  Mesoderm – forms all other tissues  Endoderm and ectoderm are securely joined and are considered epithelia

34. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Primary Germ Layers Figure 28.8a-e

35. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Primary Germ Layers Figure 28.8e-h