Properties of polar and apolar cells from the 16-cell mouse morula

The surface properties of newly formed, isolated 1/16 mouse blastomeres have been analyzed over the 10-12 h period prior to their division to 2/32 cells. Two populations of cells are formed at the 8- to 16-cell transition and their surface phenotypes vary with their relative position within the morula. Outer cells are polar, relatively non-adhesive and relatively large; inner cells are apolar, adhesive and smaller. The surface phenotypes of both inner and outer 1/16 cells are stable during culture for 11 h in isolation. However, the surface phenotypes can be induced to change by culture in combination with a second 1/16 cell, in a manner that is dependent upon the identity of the second cell. Two aggregated polar cells never flatten completely against each other, and both cells retain a clearly defined polar phenotype for 11-12 h. In aggregates of two apolar cells, cell outlines are lost as a result of intercellular flattening and microvilli are displaced away from areas of cell contact. However, if the two apolar cells are subsequently separated an even distribution of microvilli is restored. In most aggregates of an apolar and a polar cell, the polar cell envelops the apolar cell completely. These results are discussed in the context of the normal fate and potential of each cell type within the morula.