1. Pregnancy and Human Development Part A28

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 24 hours
Sperm is viable 72 hours (longer if cervical mucas is optimal)
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. 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 a reaction
Enzymes are released near the oocyte
Hundreds of sperm release their enzymes to digest the zona pellucida. Only one sperm makes it in, but hundreds need to release the enzymes that break the outer coating ogf the oocyte.

6. Acrosomal Reaction and Sperm Penetration
Figure 28.2a

7. 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

8. 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

9. Events Immediately Following Sperm Penetration
Figure 28.3

10. Preembryonic Development
The first cleavage produces two daughter cells called blastomeres
By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)

11. 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

12. The trophoblasts then proliferate and form two distinct layers
Implantation
Begins six to seven days after ovulation when the trophoblasts (outer layer of ovum) 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

13. 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

14. 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 (immature placenta) - 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

15. Hormonal Changes During Pregnancy
Figure 28.6

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

17. 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

18. Placentation
Figure 28.7a-c

19. Placentation
Figure 28.7d

20. Placentation
Figure 28.7f

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Primary Germ Layers
Figure 28.8a-e

28. Primary Germ Layers
Figure 28.8e-h

30. Effects of Pregnancy: Anatomical Changes
Chadwick’s sign – the vagina develops a purplish hue
Breasts enlarge and their areolae darken
The uterus expands, occupying most of the abdominal cavity
Lordosis is common due to the change of the body’s center of gravity
Relaxin causes pelvic ligaments and the pubic symphysis to relax
Typical weight gain is about 29 pounds

31. Parturition: Initiation of Labor
Estrogen reaches a peak during the last weeks of pregnancy causing myometrial weakness and irritability
Weak Braxton Hicks contractions may take place
As birth nears, oxytocin and prostaglandins cause uterine contractions
Emotional and physical stress:
Activates the hypothalamus
Sets up a positive feedback mechanism, releasing more oxytocin

32. Stages of Labor: Dilation Stage
From the onset of labor until the cervix is fully dilated (10 cm)
Initial contractions are 15–30 minutes apart and 10–30 seconds in duration
The cervix effaces and dilates
The amnion ruptures, releasing amniotic fluid (breaking of the water)
Engagement occurs as the infant’s head enters the true pelvis