Covers the early divisions of the zygote known as cleavage, which produce blastomeres and reach the morula stage, along with the features of cleavage and the types seen in humans. It then explains compaction and the separation of inner and outer cell masses, followed by blastogenesis, the formation of the blastocyst and its structure, and the loss of the zona pellucida before implantation.
Topic 51 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 6 sections, about 394 words (3 min of reading), with 13 flashcards.
In this note
Topic 51 Segmentation, blastogenesis
Segmentation / Cleavage
Characteristics of Cleavage
Blastogenesis
First Week of Development
Note
Segmentation / Cleavage
Within 24 hours after fertilisation, the zygote initiates a rapid series of mitotic cell divisions called cleavage.
These divisions are not accompanied by cell growth, so they subdivide the large zygote into many smaller daughter cells called blastomeres.
Cleavage lasts approximately 3–4 days after fertilisation.
This process begins in the ampulla of the fallopian tube, shortly after fertilisation, when an influx of calcium ions activates the zygote and triggers metabolic processes such as protein synthesis.
The first mitotic division produces two identical cells, which divide again to form a 4-celled embryo. These divisions continue, resulting in a 32-cell stage called the morula (around day 4).
Characteristics of Cleavage
The total size of the embryo does not increase; it remains enclosed within the zona pellucida.
The number of cells increases, but individual cell size decreases.
The zygote is subdivided into multiple identical blastomeres, which serve as the building blocks of the developing embryo.
By 3 days, the embryo consists of 6–12 cells; by 4 days, 16–32 cells (morula stage).
Types of Cleavage in Humans
Total cleavage
Unequal cleavage
Microblastomeres
Macroblastomeres
Asynchronous cleavage
Compaction
Starting at the 8-cell stage, the originally round, loosely adherent blastomeres begin to flatten and adhere tightly to one another — a process called compaction.
Compaction creates an inside-outside polarity that maximises cell-to-cell contact and involves cytoskeletal reorganisation.
Two distinct cell populations arise:
Inner cell mass (embryoblast): gives rise to the tissues of the embryo proper.
Outer cell mass (trophoblast): contributes to the formation of the chorion and placenta.
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