The blastocoele is an fluid filled intercellular cavity which increases its volume during blastocyst expansion. This process eventually leads to hatching, allowing thus the blastocyst to enter in contact with the uterine endometrium and to implant. Three stages can be described: 1) the blastocoele volume is less than 50% of the volume, 2) the blastocoele is greater than 50% but the blastocyst has not expanded yet, 3) the blastocoele is greater than the initial volume and the zona pellucida has started thinning.
Inner cell mass
The inner cell mass (ICM) is a mass of cells inside the blastocoele which will eventually give rise to the definitive structures of the fetus. The number of cells in the ICM and the degree of compaction of these cells changes during blastocyst development. The ICM can contain few or barely not visible cells, few distinguishable but uncompacted cells, may compacted cells.
Also known as trophoblast, the trophectoderm is a single layer of cells tightly associated cell at the periphery of the blastocyst. The trophectoderm may contain very few cells, few still identifiable cells, many cells forming a tight epithelium.