1. 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
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. Relative Size of Human Conceptus
Figure 28.1

4. 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. 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. 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. 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. Acrosomal Reaction and Sperm Penetration
Figure 28.2a

9. Only one sperm is allowed to penetrate the oocyte
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. 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. Events Immediately Following Sperm Penetration
Figure 28.3

12. 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. 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. Cleavage: From Zygote to Blastocyst
Degenerating zona pellucida
Inner cell mass
Blastocyst cavity
Blastocyst cavity
Trophoblast
(a) Zygote (fertilized egg)
(b) 4-cell stage 2 days
(c) Morula 3 days
(d) Early blastocyst 4 days
(e) Implanting blastocyst 6 days
Fertilization (sperm meets egg)
(a)
(b)
(c)
Uterine tube
Ovary
Oocyte (egg)
(d)
Ovulation
(e)
Uterus
Endometrium
Cavity of uterus
Figure 28.4

15. The trophoblasts then proliferate and form two distinct layers
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. The implanted blastocyst is covered over by endometrial cells
Implantation
The implanted blastocyst is covered over by endometrial cells
Implantation is completed by the fourteenth day after ovulation

17. Implantation of the Blastocyst
Figure 28.5a

18. Implantation of the Blastocyst
Figure 28.5b

19. Between the second and third month, the placenta:
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. Hormonal Changes During Pregnancy
Figure 28.6

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

22. The chorion develops fingerlike villi, which:
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. 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. Placentation
Figure 28.7a-c

25. Placentation
Figure 28.7d

26. Placentation
Figure 28.7f

27. These layers form two of the four embryonic membranes
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. 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. 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. 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

31. The primary germ layers are ectoderm, mesoderm, and endoderm
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

32. As cells begin to migrate:
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. Serve as primitive tissues from which all body organs will derive
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. Primary Germ Layers
Figure 28.8a-e

35. Primary Germ Layers
Figure 28.8e-h