Lecture Outline Conception Periods of Prenatal Development Period of the zygote Period of the embryo Period of the fetu

1. 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 60-70 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. 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