How the zebrafish gets its stripes found

Scientists, including Indian-origin researchers, have discovered how zebrafish, a small fresh water fish, forms a strikingly beautiful pattern of blue stripes alternating with golden stripes.
Three major pigment cell types, black cells, reflective silvery cells, and yellow cells emerge during growth in the skin of the tiny juvenile fish and arrange as a multi-layered mosaic to compose the characteristic colour pattern.
While it was known that all three cell types have to interact to form proper stripes, the embryonic origin of the pigment cells that develop the stripes of the adult fish has remained a mystery up to now.
Scientists of the Max Planck Institute for Developmental Biology in Tubingen, Germany have now discovered how these cells arise and behave to form the 'zebra' pattern.

Their work may help to understand the development and evolution of the great diversity of striking patterns in the animal world.
New research shows that the yellow cells undergo dramatic changes in cell shape to tint the stripe pattern of zebrafish.

"We were surprised to observe such cell behaviours, as these were totally unexpected from what we knew about colour pattern formation," said Prateek Mahalwar, first author of the study.
Individual juvenile fish carrying fluorescently labelled pigment cell precursors were imaged every day for up to three weeks to chart out the cellular behaviours.
This enabled the scientists to trace the multiplication, migration and spreading of individual cells and their progeny over the entire patterning process of stripe formation in the living and growing animal.

"We had to develop a very gentle procedure to be able to observe individual fish repeatedly over long periods of time. So we used a state of the art microscope which allowed us to reduce the adverse effects of fluorescence illumination to a minimum," said Ajeet Singh, first author of another study.
Surprisingly, the analysis revealed that the three cell types reach the skin by completely different routes: A pluripotent cell population situated at the dorsal side of the embryo gives rise to larval yellow cells, which cover the skin of the embryo. These cells begin to multiply at the onset of metamorphosis when the fish is about two to three weeks old.

However, the black and silvery cells come from a small set of stem cells associated with nerve nodes located close to the spinal cord in each segment.
The black cells reach the skin migrating along the segmental nerves to appear in the stripe region, whereas the silvery cells pass through the longitudinal cleft that separates the musculature and then multiply and spread in the skin.
The research was published in the journal Science.