1. Fertilization, Pregnancy and Birth

3. Fertilization Where? Fallopian tube
What? Union of sperm and secondary oocyte
Secondary oocyte completes development to become fertlized ovum (zygote)
Zygote receives 23 chromosomes from each sex cell

4. Fertilization occurs when there is a union between a sperm cell and a secondary oocyte in a fallopian tube
The fertilized oocyte completes development and becomes the fertlized ovum (zygote)

5. ~500 million sperm ejaculated during intercourse
million travel through cervix to uterus
Only a few hundred reach the fallopian tube
Several sperm attach to outer edge of ovulated oocyte, only one sperm fuses with it

7. Preventing Polyspermy
Contact and fusion causes ion channels to open in the eggs plasma membrane
This allows sodium ions to flow into the egg
This alters the membrane potential (depolarization)
Occurs within 1-3 seconds after sperm binds
Prevents additional sperm from fusing with the eggs plasma membrane

8. It takes between 3-5 days for the fertilized egg to travel the 10-12 cm of the fallopian tube
During this time it undergoes many cell divisions in a process called cleavage
Cleavage- cell division of a zygote, the number of cells increases without any change in the size of the zygote
As a result of cleavage, the cells of the zygote become progressively smaller

9. When the zygote reaches the uterus (6 days) it has developed into a fluid-filled structure called a blastocyst
Blastocyst
Consists of an outer sphere of cells from which the extraembryonic structures develop, and an inner cell mass, from which the embryo develops
Once in the uterus, the blastocyst becomes attached to the wall of the endometrium
This process is referred to as implantation

10. 4 days after fertilization, zygote becomes an embryo
After the 8th week of pregnancy the zygote is referred to as a fetus
For the pregnancy to continue, menstruation cannot occur
A shedding of the endometrium would expel the embryo from the uterus

11. Changes to Accomodate Pregnancy
The problem:
Estrogen and progesterone must remain high to prevent menstruation
High levels of these hormones have a negative feedback effect on the secretion of gonadotropic hormones
LH levels must remain high to sustain the corpus luteum
If the corpus luteum deteriorates, the levels of estrogen and progesterone drop which stimulates uterine contractions and endometrium shedding

12. The blastocyst secretes hormones
The solution:
The blastocyst secretes hormones
The outer layer of the blastocyst gives rise to two structures
Chorion
Amnion

13. Chorion Amnion Produces human chorionic gonadotropic hormone (hCG)
This hormone maintains the corpus luteum for the first 3 months of pregnancy
Corpus luteum continues to produce progesterone and estrogen which maintains the endometrium
hCG levels in the urine indicates pregnancy
Amnion
Fluid filled extraembryonic structure

14. Amniotes
Embryos of birds, reptiles and mammals develop in a fluid-filled sac in a shell or the uterus
Organisms with these adaptations are called amniotes

15. During amniote development, 4 extraembryonic membranes form around the embryo
chorion :functions in gas exchange
amnion: encloses the amniotic fluid
yolk sac :encloses the yolk
allantois: disposes of waste products and contributes to gas exchange

17. Between the amnion and the fetus is the amniotic cavity
It is a fluid filled sac that insulates the embryo, and later the fetus, protecting it from infection, dehydration, impact and changes in temperature

18. The extraembryonic coelum is a fluid filled space between the amnion and chorion
By the second week of the pregnancy, the yolk sac forms beneath the embryo (there is no yolk)
This is the site of early red blood cell formation
Later contributes to the primitive digestive tract

19. Cells from the embryo and endometrium combine to form the placenta
It allows for the exchange of materials between mother and embryo
At four months it starts to produce estrogen and progesterone
The secretion of progesterone from the placenta prevents ovulation (can’t get pregnant while pregnant)

20. The placenta is richly supplied with blood vessels
Projections called chorionic villi ensure that there are a large number of blood vessels from the fetus that are exposed to maternal blood

21. The allontois provides umbilical blood vessels in the placenta
Unlike the chorion and amnion, the allantois does not envelop the fetus

22. The placenta provides an interface for exchange between mother and fetus
Nutrients and oxygen diffuse from the mother’s blood into the blood of the developing fetus
Wastes move from the fetus to the mother
The umbilical cord connects the embryo with the placenta

23. Embryonic Development
Morphogenesis is the development of an organism
Two stages: growth and differentiation

24. First Trimester
Period of pregnancy from conception to the end of the third month
Second week-cells of the blastula have reorganized into a flatten disk with two layers
Gastrulation is the process where the two layered structure develops into a three layered one known as a gastrula
Stage where embryo has three distinct layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer)

25. The amnion will grow and the yolk sac will shrink
The amnion will surround the embryo to form the amniotic sac
Folding of the layers creates a 3D baby
By week 9, embryo has
become a fetus

26. Second Trimester Third Trimester Period of rapid growth!
Organ systems mature
Movements become strong and more frequent
Baby turns head down due to gravity
Third Trimester
Muscle tone develops from moving
Eyelids begin opening

27. Sex Determination
1987 researchers located the sex determination gene on the Y chromosome
SRY (Sex-determining region of the Y chromosome) is the gene mostly responsible for the male phenotype in humans
In presence of SRY gene, in the 7th week the developing gonads become testis
Testis direct subsequent male development

28. Birth ~ 266 days after implantation uterine contractions start
Cervix thins and dilates
Amnion is forced through birth canal, rupturing, acting as a lubricant
Uterine contractions move the baby through the birth canal

30. Birth Relaxin Oxytocin
Secreted by the placenta prior to labour causes pelvic muscles and ligaments to loosen and cervix to soften
Oxytocin
Secreted from the pituitary causes uterine contractions

31. Lactation During pregnancy:
Elevated levels of estrogen and progesterone prepare the breast for milk production
Each breast
Contain 20 lobes of glandular tissue
Each supplied with a duct that carries fluid towards the nipple

32. Hormones controlling lactation
Prolactin
Hormone secreted from the pituitary gland
Stimulates the glands in breast to produce fluids
Estrogen and progesterone drop after birth, lowering prolactin, but increases prolactin activity because the progesterone-induced inhibition is relieved
Initially produces colostrum; substance resembling milk but contains no milk fat
Few days after birth prolactin stimulates the production of breast milk

33. Milk produced in glandular tissue must be forced into the milk ducts
Suckling from newborn stimulates nerve endings in the areola
Sensory nerves carry information to the pituitary gland, causing the relase of oxytocin
Oxytocin causes weak smooth muscle contractions in the breast forcing milk into the ducts