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

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

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

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

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

Conception and Implantation

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

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

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

The Placenta and Umbilical Cord

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

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

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