Emergence of a geometric pattern of cell fates from tissue-scale mechanics in the Drosophila eye

Pattern formation of biological structures involves the arrangement of
different types of cells in an ordered spatial configuration. In this
study, we investigate the mechanism of patterning the Drosophila eye into
a precise triangular grid of photoreceptor clusters called ommatidia.
Previous studies had led to a long-standing biochemical model whereby a
reaction-diffusion process is templated by recently formed ommatidia to
propagate a molecular prepattern across the eye epithelium. Here, we find
that the templating mechanism is instead, mechanical in origin; newly born
columns of ommatidia serve as a template to spatially pattern cell flows
that move the cells in the epithelium into position to form each new
column of ommatidia. Cell flow is generated by a pressure gradient that is
caused by a narrow zone of cell dilation precisely positioned behind the
growing wavefront of ommatidia. The newly formed lattice grid of ommatidia
cells are immobile, deflecting and focusing the flow of other cells. Thus,
the self-organization of a regular pattern of cell fates in an epithelium
is mechanically driven.