Keratin as an aide-memoire

Two of the main challenges of early development are to increase the number of cells through repeated rounds of cell division, and to ensure that these cells assume distinct forms and functions at the right time and place to generate functional tissues and organs. [...]keratin filaments are inherited by daughter cells that retain an outward face. By using keratin filaments to stably mark the peripheral cortex, mammalian embryos (in which patterns of cell division differ widely between individuals) can ensure that cells fated to become trophectoderm are always formed in the outer layer of the cell cluster, irrespective of the orientation of divisions.