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Development of the Human Embryo. Developmental History of a Human. Stages of Development. 1. 2. 3. 4. 5. 6. 1. Fertilization. Fertilization. 3. 4. 5. 6. 2. Cleavage. 2. Cleavage. Begins ~ 12 hours post-fertilization Zygote divides into 2 cells (mitosis)

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Development of the human embryo l.jpg

Development of the Human Embryo



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Stages of Development

1.

2.

3.

4.

5.

6.



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Fertilization

3.

4.

5.

6.



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2. Cleavage

  • Begins ~ 12 hours post-fertilization

  • Zygote divides into 2 cells (mitosis)

  • 46 chromosomes in zygote = 46 chromosomes in both daughter cells

  • 2 cell into 4 cell stage (24 – 36 hours)

  • 4 cell into 8 cell stage (36 – 72 hours)

  • 16 cell stage -- Morula


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3. Morula

Morula


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3. Morula

  • ~ 16 cell stage

  • Develops ~ 72 hours (3 days) from fertilization

  • Morula enters the uterus ~ after 3 days in oviduct


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3. Morula

72 hours post-

fertilization

entering uterus



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4. Blastocyst

  • Morula, once entering the uterine cavity, floats freely

  • Morula begins to accumulate fluid and forms a cavity between its cells

  • Once cavity appears, it is now called a

    blastocyst.



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4. Blastocyst

  • Blastocyst has fluid-filled inner cavity

  • Evolves from morula on day 5


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Day 6

  • Three events happen



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Two Distinct Cell Types

Inner cell mass

trophoblasts


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Two Distinct Cell Types

  • Trophoblasts – will form the invading placenta

  • Inner cell mass cells – will form the embryo


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hCG is produced

  • hCG is human chorionic gonadotropin

  • It is produced by the trophoblasts starting on day 6

  • hCG is a hormone

  • hCG causes endometrium of uterus to grow and proliferate

  • hCG prevents the menstrual cycle from occuring

  • This is why a female misses her periods when she is pregnant


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Week 2

  • Implantation continues

  • Erosion of maternal blood

    vessels

  • Complete emersion into

    endometrium of uterus


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Weeks 3-4

  • Development of CV and nervous systems


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Weeks 5-8

  • Embryo will develop all structures that an adult has by the end of week 8


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Embryo/Fetus

  • Embryonic period is weeks 1-8

  • Fetal period begins on week 9 and goes until birth at 38 weeks.

  • Embryonic period is characterized by development of structures (organs).

  • Fetal period is characterized by growth of those structures.


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Fetal Period

9-12

13-16

17-20

21-38


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