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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 - PowerPoint PPT Presentation
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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