26/06/2025
π Fertilization and Zygote Formation
β’ Location: Ampulla of the fallopian tube (oviduct), the widest part.
β’ Event: A single s***m penetrates the secondary oocyte, which is arrested in metaphase II.
β’ This triggers the oocyte to complete meiosis II.
β’ The result is a haploid o**m (1N) and a polar body.
β’ Fusion of s***m and o**m nuclei creates a diploid (2N, 46 chromosomes) zygote.
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π Cleavage (Mitotic Division) β Days 1 to 4
β’ Cleavage involves rapid mitotic divisions without overall growth in size, still enclosed within the zona pellucida.
Day 1: Zygote
β’ First mitotic division β 2-cell stage.
Day 2: 2-cell blastomere
β’ Each cell is called a blastomere.
β’ Still enclosed by the zona pellucida.
Day 3: 4-cell stage
β’ Continued cleavage β 4 equal blastomeres.
Day 4: Morula (16β32 cells)
β’ Solid ball of cells.
β’ Cells begin to compact, increasing contact and communication (tight junction formation).
β’ Still migrating down the fallopian tube.
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π Blastocyst Formation β Day 5
β’ Fluid enters the morula, forming a cavity (blastocoel) β Blastocyst.
β’ First differentiation of cells:
β’ Trophoblast (outer layer): Will form the placenta.
β’ Embryoblast (inner cell mass): Will form the embryo.
β’ The blastocyst hatches from the zona pellucida, enabling it to interact with the uterine lining.
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π± Implantation Begins β Day 6
β’ Location: Endometrium of the uterus.
β’ The trophoblast contacts the uterine epithelium and begins to differentiate:
β’ Cytotrophoblast: Inner layer of mononucleated cells.
β’ Syncytiotrophoblast: Outer, multinucleated layer that invades the endometrial stroma and secretes hCG to support the corpus luteum.
β’ The embryoblast will further develop into the bilaminar disc (epiblast and hypoblast).
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π¬ Additional Key Structures in the Diagram:
β’ Zona Pellucida: A glycoprotein shell that prevents polys***my and supports the oocyte/early embryo.
β’ Corona Radiata: Granulosa cells surrounding the oocyte, providing nutrients.
β’ O***y: Releases the oocyte during ovulation.
β’ Uterine Tube (Fallopian Tube): Pathway for the oocyte and site of fertilization.
β’ Uterus: Final site of implantation and embryonic development.
We summarize the picture below πβοΈ.
