1 / 37

Pregnancy and Human Development

28. P A R T A. Pregnancy and Human Development. 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. From Egg to Embryo.

dextra
Download Presentation

Pregnancy and Human Development

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. 28 P A R T A Pregnancy and Human Development

  2. 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

  3. From Egg to Embryo • 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

  4. Relative Size of Human Conceptus Figure 28.1

  5. 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

  6. 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

  7. 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

  8. 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

  9. Figure 28.2a

  10. 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

  11. 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

  12. Events Immediately Following Sperm Penetration Figure 28.3

  13. 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)

  14. 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

  15. Cleavage: From Zygote to Blastocyst Figure 28.4

  16. 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

  17. Implantation • The implanted blastocyst is covered over by endometrial cells • Implantation is completed by the fourteenth day after ovulation

  18. Implantation of the Blastocyst Figure 28.5a

  19. Implantation of the Blastocyst Figure 28.5b

  20. 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

  21. Hormonal Changes During Pregnancy Figure 28.6

  22. Placentation • Formation of the placenta from: • Embryonic trophoblastic tissues • Maternal endometrial tissues

  23. 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

  24. 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

  25. Placentation • 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

  26. Placentation Figure 28.7a–c

  27. Placentation Figure 28.7d

  28. Placentation Figure 28.7f

  29. 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

  30. 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

  31. 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

  32. 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

  33. Gastrulation • During the 3rd 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

  34. 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

  35. 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

  36. Primary Germ Layers Figure 28.8a–e

  37. Primary Germ Layers Figure 28.8e–h

More Related