Rouleaux (singular is rouleau) are stacks of red blood cells (RBCs)
which form because of the unique discoid shape of the cells in
vertebrate body. The flat surface of the discoid RBCs give them a
large surface area to make contact and stick to each other; thus,
forming a rouleau. They occur when the plasma protein
concentration is high, and because of them the ESR (erythrocyte
sedimentation rate) is also increased. This is a non-specific indicator
of the presence of disease.
Conditions which cause rouleaux formation include infections,
inflammatory and connective tissue disorders, and cancers. It also
occurs in diabetes mellitus and is one of the causative factors for
microvascular occlusion in diabetic retinopathy.
The presence of acute phase proteins, particularly fibrinogen,
interacts with sialic acid on the surface of RBC and allows the
formation of rouleau. Anaemia, by altering the ratio of RBC to
plasma, increases rouleaux formation and accelerates sedimentation.
Rouleaux formation is retarded by albumin proteins.
Rouleaux formations are also adopted by spermatozoa as a means of
cooperation between genetically similar gametocytes so as to improve
reproductive success through enhanced motility and, therefore,
fertilization capacity, e.g. in the guinea pig.
According to Smoluchowski, the kinetics of aggregation of colloids is
based on the assumption that each particle is surrounded by a ‘sphere
influence’. Single spherical particles, which undergo Brownian
motion collide and lead to sticking of particles. As aggregation
proceeds, the average diffusion constant of the aggregate population
decreases. The aggregation of red blood cells progresses in the same
manner except that cells are biconcave rather than spherical.