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

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