Cleavage - First Week

The cleavage include rapid cell divisions and during these cell divisions the size of the embryo is no increasing. It is still the sized of an egg cell. The egg cell has nutrients and the embryo needs to rely on these nutrients during the first fast cell divisions. Normally cell division (mitosis) includes several steps and it lasts about 16 hours but during cleavage the cells skip most of the steps and and there is no transcription of the genes, thus the cell division lasts only 30 minutes (in mice). After the first few cell divisions the embryo is called morula.

At this stage the cells become compacted when the blastomeres maximize the cell contacts. The cells become polarized and compaction yields inner and outer cells. Some of the Inner Cell Mass (ICM) form gap junctions and these cells give rise to the embryonic tissues.

The outer cells become trophoblast and give rise to extra embryonic membranes. The trophoblast also secretes fluids into the morula and forms blastocoel in the late cleavage. The developing embryo becomes blastocyst (hollow ball of cells).

After the formation of blastocyst, the embryo is ready to go to gastrulation to form multilayered embryo.

Here's the time window for these events:

• Day 1: First cell division
• Day 2: Second cell division (4-cell stage)
  
• Day 3: 6-12 cell stage
• Day 4: 16-32 cell stage and the embryo becomes morula
• Day 5: Embryo forms blastocyst
• Days 6-7: Embryo implants into the uterus wall