Red blood cells are the body’s true shape shifters, perhaps the most malleable of all cell types, transforming – among many other forms -- into compressed discs capable of going through capillaries with diameters smaller than the blood cell itself. While studying how blood clots contract John W. Weisel, Ph.D., professor of Cell and Developmental Biology at the Perelman School of Medicine, University of Pennsylvania, and colleagues, discovered a new geometry that red blood cells assume, when compressed during clot formation.

Although red blood cells were first visualized in the mid-17th century and studied extensively since then, this new study, published online ahead of print in the journal Blood, describes a previously unknown shape and potential new function for red blood cells. The Penn team found that red blood cells can be compressed into many-sided, closely-packed polyhedral structures – instead of their free-flowing bi-concave, disc shape.

What’s more, contrary to expectations, the aggregates of fibrin and platelets that make up highly contracted clots lie primarily on the clot exterior, with the red blood cells crowded within the clot interior, although the contents of clots are more homogeneous before contraction takes place.

Contracted clots may form an impermeable seal and help prevent vascular obstruction, but confer resistance to penetration by drugs that break down fibrin, the structural component of clots, a common treatment option for heart attacks and strokes.

“The first time we saw this, we thought: ‘This can’t be biological,’” recalls Weisel. The team first saw the polyhedron-shaped red blood cells when studying the clot-contraction process using a novel magnetic resonance technology, with co-authors from T2 Biosystems, along with co-author Douglas Cines, M.D., director of the Coagulation Laboratory and professor of Pathology and Laboratory Medicine at Penn. They observed a signal that indicated tightly packed red blood cells. 

Click here to view the full release.