ABSTRACT
The word morula derives from the root morus, the Latin word for ‘mulberry’. It was so named in the early days of embryological investigation because of its berry-like shape and appearance, especially notable in amphibians1. The human preembryo is said to have become a morula at approximately the 8-cell stage of development, when the process of compaction commences2. In the human, up to 16 discrete blastomeres may be observed in growing preembryos, but the existence of such high cell numbers without evidence of compaction is now recognized as detrimental to further development (Figure 3.1). Normal compaction results in the formation of an outer layer of cells that become the trophectoderm of the blastocyst, while inner cells give rise to the inner cell mass (insideoutside theory) (Figure 3.2). The embryonic genome becomes functionally active during, or just before, the morula stage.
