1. Periods of Prenatal Development
Lecture Outline
Conception
Periods of Prenatal Development
Period of the zygote
Period of the embryo
Period of the fetus

2. Conception
Ovulation: Release of ovum (egg) from one of a woman’s two ovaries
Occurs approximately every 28 days
Egg moves through one of the fallopian tubes toward the uterus
If sexual intercourse occurs near ovulation, conception can occur
A sperm must penetrate the outer membrane of the egg
Each sperm and egg cell has only 23 chromosomes
When merged, the resulting cell has 46 chromosomes
Conception usually occurs in the fallopian tube
Fertilized egg is called a zygote

3. Period of the Zygote (or Germinal Period)
Lasts about 2 weeks, from conception to implantation
Zygote undergoes mitosis (cell duplication) as it travels down the fallopian tube to the uterus
By approximately the 4th day after conception, the zygote has become a blastocyst
Fluid-filled sphere of about cells

4. Two parts of the blastocyst:
Inner cell mass: Cells on the inside of the blastocyst
Will become the embryo
Trophoblasts: Cells on the outside of the blastocyst
Will develop into tissues that protect and nourish the embryo

5. Implantation of the blastocyst into the uterine wall occurs approximately between 7 and 9 days post-conception
Support structures begin to develop from the trophoblasts after implantation

6. Conception and Implantation

7. Amnion: Membrane that encloses the embryo in amniotic fluid
Support Structures:
Amnion: Membrane that encloses the embryo in amniotic fluid
Cushions organism from injury
Helps to keep temperature constant

8. Chorion: Membrane surrounding the amnion
Develops by the end of the second week post-conception
Tiny blood vessels (villi) emerge from the chorion and burrow into the uterine wall
Placenta begins to develop from these blood vessels

9. Placenta is fed by blood vessels from the mother and from the embryo
Placenta is connected to the embryo by the umbilical cord
Placenta is semi-permeable
Some substances can pass through, but not all
Prevents blood of mother and embryo from mixing directly
Placenta allows nutrients and oxygen to reach the organism and waste products and carbon dioxide to be carried away

10. The Placenta and Umbilical Cord

11. Period of the Embryo
Lasts from the 3rd through the 8th week of pregnancy (about 6 weeks)

12. Inner cell mass differentiates into three layers (about 3rd week post-conception):
Ectoderm --> nervous system, outer layer of skin, nails, teeth, ears, eyes
Mesoderm-->muscles, skeleton, circulatory system, inner layers of skin
Endoderm-->digestive system, lungs, urinary tract, glands

13. Ectoderm folds over to form a neural tube (primitive spinal cord)
Top of the neural tube swells to form a brain (about 3.5 weeks post-conception)
External body structures (e.g., arms, legs) and internal organs (e.g., heart) begin to develop
Rapid brain development occurs