A unique form of endoplasmic reticulum in endocardial endothelia of the desert iguana

A unique and elaborate network of endoplasmic reticulum has been found in the endocardial endothelia of the desert iguana. The organelle consists of branched and anastomosed tubules which lack ribosomal particles. The tubules vary greatly in size and may be divided categorically into two groups of primary and secondary tubules, with average diameters of 750 A and 250 A, respectively. The tubular system appears to be arranged in a regular pattern. In some planes of section, most of the secondary tubules are seen to twist around one another in ropelike formations. In other planes of section, groups of one primary and two secondary tubules cut at different angles are arranged in definite rows. The most orderly configuration is observed in the endothelial cells resting upon noncontractile regions and appears distorted upon contraction of the underlying myocardia. In all circumstances, the amount of endoplasmic reticulum is unusual for endothelial cells because it occupies their entire cytoplasm. The tubules and membraneous surface of the endoplasmic reticulum are so abundant as to suggest some special physiological or biological activity although the true function of this interesting organelle is unknown.