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Questions and Answers
What is the result of the male and female pronuclei uniting during fertilization?
What is the term for the series of mitotic cell divisions that occur in the zygote as it passes along the uterine tube?
What is the structure that forms in the morula, converting it into a blastocyst?
What is the function of the trophoblast?
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What is the process by which the zona pellucida is shed?
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What is the layer that forms on the deep surface of the embryoblast by the end of the first week?
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What is the result of the syncytiotrophoblast invading the endometrial epithelium and underlying connective tissue?
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How many days after fertilization does the blastocyst adhere to the endometrial epithelium?
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What is the primary function of the syncytiotrophoblast during implantation?
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What occurs on day 10 of implantation?
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What is the result of the adaptation of endometrial tissues in preparation for implantation?
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What is the function of the prechordal plate?
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What is the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc?
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What is the fate of the primary umbilical vesicle?
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What is the result of the fusion of adjacent lacunae?
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What appears at the beginning of the third week as a thickening of the epiblast at the caudal end of the embryonic disc?
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Study Notes
Fertilization and Early Development
- Fertilization is complete when the male and female pronuclei unite and the maternal and paternal chromosomes intermingle during metaphase of the first mitotic division of the zygote.
- The zygote undergoes cleavage into a number of smaller cells called blastomeres as it passes along the uterine tube toward the uterus.
- Approximately 3 days after fertilization, a ball of 12 or more blastomeres, called a morula, enters the uterus.
- A cavity forms in the morula, converting it into a blastocyst consisting of the embryoblast, a blastocystic cavity, and the trophoblast.
Implantation
- The zona pellucida is shed, and the trophoblast adjacent to the embryoblast attaches to the endometrial epithelium 4-5 days after fertilization.
- The trophoblast at the embryonic pole differentiates into two layers: an outer syncytiotrophoblast and an inner cytotrophoblast.
- The syncytiotrophoblast invades the endometrial epithelium and underlying connective tissue, and a cuboidal layer of hypoblast forms on the deep surface of the embryoblast.
- By the end of the first week, the blastocyst is superficially implanted in the endometrium.
Implantation Timeline
- Day 5: The zona pellucida degenerates due to enzymatic lysis.
- Day 6: The blastocyst adheres to the endometrial epithelium.
- Day 7: The trophoblast differentiates into two layers: syncytiotrophoblast and cytotrophoblast.
- Day 8: The syncytiotrophoblast erodes endometrial tissues, and the blastocyst begins to embed in the endometrium.
- Day 9: Blood-filled lacunae appear in the syncytiotrophoblast.
- Day 10: The blastocyst sinks beneath the endometrial epithelium, and the defect is filled by a closing plug.
- Day 10-11: Lacunar networks form by fusion of adjacent lacunae.
2nd Week
- Rapid proliferation and differentiation of the trophoblast occur as the blastocyst completes implantation in the uterine endometrium.
- The endometrial changes resulting from the adaptation of these tissues in preparation for implantation are known as the decidual reaction.
- The primary umbilical vesicle (yolk sac) forms, and extraembryonic mesoderm develops.
- The extraembryonic coelom (cavity) forms from spaces that develop in the extraembryonic mesoderm.
- The amniotic cavity appears as a space between the cytotrophoblast and embryoblast.
2nd Week Development
- The embryoblast differentiates into a bilaminar embryonic disc consisting of epiblast and hypoblast.
- The prechordal plate develops as a localized thickening of the hypoblast, indicating the future cranial region of the embryo.
3rd Week
- The bilaminar embryonic disc is converted into a trilaminar embryonic disc during gastrulation.
- The primitive streak appears at the beginning of the third week as a thickening of the epiblast at the caudal end of the embryonic disc.
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Description
Learn about the process of fertilization, from sperm entry to zygote formation, and the subsequent stages of cell division and development. Discover how the male and female pronuclei unite and the chromosomes intermingle during the first mitotic division.
