Biology · MU Sofia

38. Preimplantation mammalian embryo.

Traces human preimplantation development from fertilisation in the fallopian tube through the zygote, cleavage stages, morula and blastocyst, to hatching and implantation. It covers mammalian cleavage features, compaction, trophectoderm and inner cell mass formation, cavitation, cell potency and regulative development. Four mnemonics summarise the stage sequence, cleavage features, compaction and hatching, and a table lists the cleavage features.

Inside: 20 sections · 4 labelled pictures · 2 tables · 8 flashcards · 9 min

Topic 39 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 20 sections, about 1,107 words (9 min of reading), with 8 flashcards.

In this note

  1. Preimplantation mammalian embryo.
  2. Preimplantation Development in Humans
  3. Key Stages of Preimplantation Embryonic Development
  4. Characteristics of Mammalian Cleavage
  5. Unique Features of Preimplantation Development in Mammals
  6. Compaction and Cellular Polarization
  7. Differentiation of the Trophectoderm and Inner Cell Mass
  8. Role of the Trophectoderm
  9. Formation of the Blastocoel – Cavitation
  10. Structure and Composition of the Blastocyst
  11. Hatching of the Blastocyst
  12. Implantation Process
  13. Cell Potency During Early Development
  14. Regulative Development in Mammals
  15. Natural and Experimental Evidence for Regulative Development
  16. MNEMONICS
  17. 🧫 1. Stages of Preimplantation Development
  18. 🧬 2. Unique Mammalian Cleavage Features
  19. 🌟 3. Compaction and Trophectoderm vs Inner Cell Mass
  20. 🧪 4. Blastocyst Hatching and Implantation

Preimplantation Development in Humans

  • The preimplantation period begins at the moment of fertilization and continues until the embryo successfully implants into the uterine wall.
  • Fertilization occurs in the fallopian tube, where the sperm cell unites with the mature egg to form a single-cell zygote.
  • This zygote then undergoes a series of cell divisions known as cleavage while traveling from the oviduct towards the uterus.
  • By the time the embryo reaches the uterus, it has developed into a structure called the blastocyst, which is ready to implant into the endometrial lining.

Key Stages of Preimplantation Embryonic Development

  1. 1. Zygote Stage
  • Occurs about 12 to 24 hours after fertilization.
  • The single-cell zygote marks the start of a new human life.
  1. 2. Two-cell Stage
  • Appears approximately 30 hours after fertilization.
  • The first cleavage divides the zygote into two blastomeres.
  • Cell divisions follow in intervals of roughly 18 hours.
  1. 3. Four-cell Stage
  • Reached between 40 to 50 hours post-fertilization.
  • Continued cleavage increases cell number without increasing total embryonic size.
  1. 4. Eight-cell Stage
  • Achieved by around 60 hours.
  • Blastomeres are still distinct and visible at this stage.
  1. 5. Morula Stage
  • Occurs at approximately four days post-fertilization with 12 to 16 cells.
  • At this stage, compaction begins, where cells flatten against each other and begin forming tight junctions.
  • This transformation leads to a ball of closely packed cells known as the morula.
  1. 6. Blastocyst Stage
  • Forms by the fifth day after fertilization.
  • A fluid-filled cavity called the blastocoel develops within the embryo.
  • The embryo becomes a blastocyst, which is ready to hatch from the zona pellucida and implant in the uterus.

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