1. Pregnancy and Human Development28
P A R T A
Pregnancy and Human Development

2. Gestation period – from the last menstrual period until birth
From Egg to Embryo
Pregnancy – events that occur from fertilization until the infant is born
Gestation period – from the last menstrual period until birth

3. Preembryo –from fertilization (zygote) until it is two weeks old
From Egg to Embryo
Preembryo –from fertilization (zygote) until it is two weeks old
Embryo – zygote implanted in uterine lining during the third through the eighth week
Fetus –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:
Leakage
Low pH of vagina
Failure to reach cervix
Destroyed by phagocytosis/lost in cavity
Failure to reach tubes
Sperm must undergo capacitation before they can penetrate the oocyte

7. Figure 28.2a

8. Completion of Meiosis II and Fertilization
Upon entry of sperm, the secondary oocyte:
Completes meiosis II
Casts out the second polar body
Fertilization – when the pronuclei come together

9. Events Immediately Following Sperm Penetration
Figure 28.3

10. Cleavage and Implantation
Cleavage – rapid mitosis of the zygote following fertilization.

11. 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)

12. Preembryonic Development
Blastocyst – a fluid-filled hollow sphere composed of:
1. A single flattened layer of cells called trophoblasts
2. An inner cell mass
Trophoblasts take part in placenta formation
The inner cell mass becomes the embryonic disc

13. Cleavage: From Zygote to Blastocyst
Figure 28.4

14. Implantation
6-7 day
trophoblast implants into the endometrium – high levels of estrogen and progesterone

15. blastocyst is covered over by endometrial cells
Implantation
blastocyst is covered over by endometrial cells
completed by the fourteenth day after ovulation

16. Implantation of the Blastocyst
Figure 28.5a

17. Implantation of the Blastocyst
Figure 28.5b

18. corpus luteum -secrete estrogen and progesterone
Implantation
corpus luteum -secrete estrogen and progesterone
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

19. Hormonal Changes During Pregnancy
Figure 28.6

20. Formation of the placenta from: Embryonic trophoblastic tissues
Placentation
Formation of the placenta from:
Embryonic trophoblastic tissues
Maternal endometrial tissues
These form the chorion

21. Placentation
The placenta is fully formed and functional by the end of the third month

22. Placentation
Embryonic placental barriers allow for passage of nutrients but the blood does not inter-mix
The placenta also secretes other hormones – human placental lactogen, human chorionic thyrotropin, and relaxin

23. Placentation
Figure 28.7a–c

24. Placentation
Figure 28.7d

25. Placentation
Figure 28.7f

26. Amnion, yolk sac, allantois, chorion
Germ Layers
Inner cell mass produces 4 membranes during the first 2-3 weeks of development
Amnion, yolk sac, allantois, chorion

27. Amnion – a transparent membrane filled with amniotic fluid
Embryonic Membranes
Amnion – 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

28. Yolk sac – a sac on the ventral surface of the embryo
Embryonic Membranes
Yolk sac – 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

29. Allantois – a small outpocketing at the caudal end of the yolk sac
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

30. The primary germ layers are ectoderm, mesoderm, and endoderm
Gastrulation
During the 3rd week, the two-layered embryonic disc becomes a three-layered embryo – gastrulation
Primitive streak – raised dorsal groove that establishes the longitudinal axis of the embryo
The primary germ layers are ectoderm, mesoderm, and endoderm

31. Notochord – rod of mesodermal cells that serves as axial support
Gastrulation
Notochord – rod of mesodermal cells that serves as axial support

32. Ectoderm – forms structures of the nervous system and skin epidermis
Primary Germ Layers
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