1. Development

2. Differentiation and development
Gradual modification of physical and physiological characteristics
Differentiation
The creation of different types of cells

3. Stages of development
Prenatal development
Embryological
Changes occurring the first two months after fertilization
Fetal
Begins at the start of the ninth week and continues until birth
Postnatal development
Commences at birth and continues to maturity

4. Fertilization (conception)
Occurs in the uterine tubes
Within a day of ovulation
Spermatozoa cannot fertilize an ovum until after capacitation

5. Fertilization

6. The Oocyte at Ovulation
Oocyte is in meiosis II
Surrounded by the corona radiate
Spermatozoa release hyaluronidase and acrosin
Enzymes required to penetrate corona radiate
Single spermatozoan contacts oocyte, fertilization begins
Oocyte activation
-Oocyte completes meiosis II
-Functionally mature ovum
-Female pronucleus and male pronucleus fuse (amphimixis)
-Polyspermy prevented by membrane depolarization and cortical reaction

7. Embryonic and Fetal Periods
Gestation periods
Three trimesters
First trimester
Second trimester
Third trimester

8. The First Trimester
Cleavage
Zygote becomes a preembryo then a blastocyst
Implantation
Blastocyst burrows into uterine endometrium
Placentation
Blood vessels form around blastocyst and placenta develops
Embryogenesis
Formation of a viable embryo

9. Cleavage and Blastocyst Formation

10. Cleavage and blastocyst formation
A series of cell divisions that subdivides the cytoplasm of the zygote
outer layer of cells- Trophoblast
Inner cell mass – cluster of cells at one end of blastocyst

11. Implantation
Occurs about 7 days after fertilization
Trophoblast enlarges and spreads
Maternal blood flows through open lacunae
Gastrulation
Embryonic disc composed of germ layers
Endoderm
Mesoderm
Ectoderm

12. Stages in Implantation

13. The Inner Cell Mass and Gastrulation

14. Germ layers
Gastrulation
By day 12 surface cells move toward the primitive streak
A third germ layer forms
The three germ layers are:
Ectoderm – superficial cells that did not migrate
Endoderm – cells facing the blastocoele
Mesoderm – migrating cells between ectoderm and endoderm

15. Extraembryonic Membranes
Four extraembryonic membranes:
Yolk sac
Amnion
Allantois
Chorion

16. Extraembryonic Membranes and Placenta Formation

17. Extraembryonic Membranes and Placenta Formation

18. Extraembryonic Membranes and Placenta Formation

19. Embryo Anatomy
Yolk sac
Important site of blood cell formation
Amnion
Encloses fluid that surrounds and cushions developing embryo
Allantois
Eventually becomes bladder
Chorion

20. A Three-Dimensional View of Placental Structure

21. Placentation
Chorionic villi extend into maternal tissue
Forms intricate branching network for maternal blood
Umbilical cord connects fetus to placenta

22. Hormones of the placenta
Trophoblast secretes hormones to maintain pregnancy
HCG
Estrogens
Progesterone
Placental prolactin
relaxin

23. Second and Third Trimesters
Second trimester
Organ systems increase in complexity
Third trimester
Many organ systems become fully functional
Fetus undergoes largest weight change
At end of gestation fetus and uterus push maternal organs out of position

24. The Second and Third Trimesters