What Is a Blastocyst? Days 3 to 6

In numbers

• MeasuredMorula on day 4; blastocyst on day 5
• MeasuredAbout 60 cells on day 5, 125 on day 7
• EstimateExpanded, about 0.1–0.3 mm across

Every number is labelled measured, an estimate or drawn: how we know.

The science

Around the 8-cell stage the blastomeres compact: they flatten against each other and seal their contacts with tight junctions, and the embryo's own genes have switched on. By day 4 it is a morula of 16–32 cells (Latin morus, a mulberry). Pumps in the outer cells draw fluid inward to open the blastocoel, and the cells split into two lineages: the trophectoderm, an outer epithelium that gives rise to the placenta, and the inner cell mass, which gives rise to the embryo itself. The blastocyst swells, which thins the zona pellucida, and breaks out of it on about day 5–7 (hatching), helped by enzymes and by swelling and collapsing again and again. Human blastocysts grown in the laboratory had about 60 cells on day 5, 85 on day 6 and 125 on day 7.

In Endo-View

This is chapter 8 of the journey to life, an interactive 3D simulation at true scale: watch it, slow it down or run it at real speed, and read every number with its label.

Watch it in 3D