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27.6A: Fifth Through Eighth Weeks of Development

  • Page ID
    8283
  • Weeks five to eight of gestation develops the major organs, including the circulatory, nervous, and gastrointestinal systems.

     

    LEARNING OBJECTIVES

     

    Outline embryonic development between five and eight weeks

     

    KEY TAKEAWAYS

    Key Points

     

    • Being susceptible to the effects of teratogens is high during embryonic development.
    • At week five, the brain, spinal cord, vertebrae, heart, vasculature, and gastrointestinal tract begin to develop.
    • During weeks six to seven, the embryo grows from 4 mm in length to 9 mm and begins to curve into a C-shape. The fetal heart bulges, develops further, and begins to beat in a regular rhythm. Rudimentary blood begins to move through the blood vessels. The neural tube, which forms the brain, closes.
    • During weeks six and seven, the limb buds form. The eyes, mouth, and ear structures begin to form. The initial differentiation of the tissues that will become the spleen, gallbladder, pancreas, liver, kidneys, stomach, and lungs occurs.
    • By week eight of gestation, the lungs begin to form, as well as the lymphatic system. The main development of the external genitalia begins, and the brain continues to develop. The arms and legs have grown longer, and the foot and hand areas can be clearly distinguished.

     

    Key Terms

     

    • chorionic membrane: One of the membranes that exist during pregnancy between the developing fetus and mother. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to fetal blood.
    • Gestational age: The time that has passed since the onset of the last menstruation.
    • Embryonic age: Measures the actual age of the embryo or fetus from the time of fertilization.
    • teratogens: An agent, such as a virus, a drug, or radiation, that causes a malformation of an embryo or fetus.

    Weeks five through eight of gestation are characterized by the development of the major organ systems, including the circulatory, nervous, lymphatic, and gastrointestinal systems. During this time, the embryo is extremely susceptible to the effects of teratogens.

    Gestational age is the time that has passed since the onset of the last menstruation, which occurs two weeks before the actual fertilization. Embryonic age measures the actual age of the embryo or fetus from the time of fertilization. Thus, the first week of embryonic age is already week three counting with gestational age. The number of the week (used here) is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the first week after fertilization.

     

    Week 5

     

    At week five, the brain, spinal cord, vertebrae, heart, vasculature, and gastrointestinal tract begin to develop.

     

    Week 6–7

    This is a scan of a human embryo at seven weeks—an embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age). The heart is the dark spot at the center of the image, bulging out of the embryo.

     

     

    Human embryo at seven weeks: An embryo from an ectopic pregnancy, still in the oviduct. This embryo is about five weeks old (or from the seventh week of menstrual age). The heart is the dark spot at the center of the image, bulging out of the embryo.

    During weeks six and seven, the embryo grows from four millimeters in length to nine millimeters and begins to curve into a C-shape. The fetal heart bulges, develops further, and begins to beat in a regular rhythm. Rudimentary blood begins to move through the main embryonic blood vessels, connecting to the yolk sac and the chorionic membrane of the placenta.

    The arm and leg buds, which will grow into the full limbs over the rest of development, become visible. The neural tube, which forms the brain, closes. The brain then develops into five areas and some cranial nerves are visible.

    The eyes, mouth, and ear structures begin to form. The initial differentiation of the tissues that will become the spleen, gallbladder, pancreas, liver, kidneys, stomach, and lungs occurs.

     

    Week 8

     

    By week eight of gestation, the embryo measures 13 millimeters in length. The lungs begin to form, as well as the lymphatic system. The main development of the external genitalia begins, and the brain continues to develop.

    The arms and legs have grown longer, and the foot and hand areas can be clearly distinguished. The hands and feet have fingers and toes, but may still be webbed.

    This is a color scan of an embryo from an ectopic pregnancy, located in the part of the uterus to which the fallopian tube is attached. The features are consistent with a developmental age of seven weeks (ninth week of menstrual age).

     

    Human embryo from an ectopic pregnancy: An embryo from an ectopic pregnancy, located in the part of the uterus to which the fallopian tube is attached. The features are consistent with a developmental age of seven weeks (the ninth week of menstrual age).

     

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