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Embryo Development

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Gametogenesis produces gametes containing only a single complement of genetic material. Fertilization restores the full diploid genetic complement. Human embryogenesis is that period of human development between fertilization/implantation and the fetal stage of growth which starts in the 9th week of gestation.
What are the processes and stages of human embryogenesis? How does a fetus develop from a one-cell fertilized egg? What factors in the immediate environment of the embryo are required to be present for normal embryo growth? What factors will disrupt this normal growth and cause problems?

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Solution Summary

Embryogenesis, or the development of the embryo from the blastocyst stage and implantation to the fetal stage occurs in the first 8 weeks of human development. Gametogenesis is the production of sperm and ova through meiosis (the production of gametes with a single complement of genetic material each) while embryogenesis occurs through mitosis and differentiation of cells into tissues and organs. A healthy embryo requires an adequate blood supply from the mother for proper growth. Folic acid is essential for neural tube growth during this stage while toxic substances such as alcohol and environmental estrogens will disrupt embryogenesis and could result in birth defects.

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I. Gametogenesis: formation of gametes, oocyte and sperm
II. Fertilization: sperm meets oocyte and form zygote
III. Embryogenesis: the phase of human development during the first eight weeks after implantation of the fertilized egg or zygote
IV. Fetal Growth: from eight weeks to birth (normally 9 months or 266 days
V. Birth

III. Human embryogenesis =
A. The process of development that occurs from fertilization up to the ninth week of gestation. Key point: at nine weeks, the developing human is called a "fetus" and not an embryo.

B. When fertilization occurs, a single cell is formed that contains a full complement of human genetic material (46 chromosomes). Half of the complement is paternal and half is maternal. This single cell is known as a zygote.

C. The zygote begins to replicate cells via mitosis, a process known as cleavage (day 2). Compaction and Differentiation then occur over the next 2 days resulting in the blastocyst on Day 5 after fertilization. Think of the blastocyst like a jelly-filled ball with another ball attached to one side of its inner layer. The outer layer is called a trophoblast, the inner ball is known as the inner cell mass (ICM), and the jelly-like internal remainder is the blastocoele. The ICM goes on to form the embryo and is also known at the embryoblast. At this stage, the blastocyst is composed of 70-100 cells.

1. The trophoblast
a. A layer of trophoblastic ...

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