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Human Embryo at the 8 Cell Stage

Eight-cell embryo. Coloured scanning electron micrograph (SEM) of a human embryo at the eight cell stage, three days after fertilisation. Known as a morula, this is a cluster of eight large rounded cells called blastomeres. Smaller spherical structures (centre right and left) will degenerate. The surface of each cell is covered in microvilli. This embryo is at the early stage of transformation into a human composed of millions of cells. Here it is dividing to form a hollow ball of cells (the blastocyst). At this eight- cell stage the morula has not yet implanted in the uterus (womb). Magnification: x900 at 6x7cm size. Mag.: x2,500 at 8x10”; x1080 at 4x5” size.

The “smaller spherical structures” are called polar bodies, and are the result of unequal cell division — moving more stuff into one of the dividing cells before the pinching off of two new cells.

You may be more familiar with polar bodies from oogenesis, in which (some) female animals produce one enormous egg from two rounds of cell division. Different type of division, same idea.

Polar bodies aren’t counted when determining the size of the morula, which is why you sometimes see a morula described as having a number of cells that isn’t a square of 2, for example, a 12-cell morula. If these balls of cells divided geometrically, you’d only see morulas of 2, 4, 8, 16, 32 , etc., cells.

(Source: sciencephoto.com)