Comparative Biology of Red Blood Cells

The emergence of specialized cells that were produced to perform efficient gas exchange in the organism dates back to earlier than 500 million years ago when clams, lampreys and hagfishes appeared. Since then, red blood cells have evolved together with their owners to fit the habitat. They provide enough oxygen for the champions in running fast, flying high, diving deep and hibernating long. They participate in control of vascular tone and scavenge free radicals. Shapes, structures and properties of red blood cells differ strikingly for various species. Most of red cells are complex nucleated cells capable of protein synthesis and, as most other cells, rely on oxygen levels in blood for generating ATP by the mitochondria. Calcium signaling is very important for all red blood cells, but vary depending on the presence of endoplasmic reticulum and other organelles. Rheological properties of red blood cells depend on the ability to control water content and cell volume. Here as well, heterogeneity is striking not only between the species, but also within one same species, such as for high K+ and low potassium sheep red cells. A broad variety of shapes is associated with the differences in membrane protein and lipid composition. The number of hemoglobin types may vary from one to more than 8, allowing for fast adaptations to the acute changes in the environmental oxygen levels. Red blood cells of multiple species are equipped with the mechanisms resisting infection and invasion of them with Plasmodium, Babesia and other pathogens. Similar to human cells that, when heterozygous for hemoglobin S gene, may reduce the incidence of malaria infection, some fishes, deer and cat species may resist invasion of the parasites with reversible aggregation of hemoglobin.

Erythropoietic activity, turnover rate and life span of red blood cells fit the life cycle of the animals. These cells may stay in the circulation for 25 days in mice to 170 days in horses.

Red cells of different species are equipped with various sensors making these cells mechano-sensitive, responsive to the changes in glucose metabolism, redox state and a number of cytokines and hormones.

Fascinating variety in red cells in species other than humans and rodent models remains underexplored. This collection of articles aims to cover the modern state of our knowledge on comparative red blood cell biology. We hope to use these articles in the future when putting together comparative section chapters of a book on red cells in the future.