An ultrastructural study of the development of the dermal iridophores and structural pigmentation in <i>Poecilia reticulata</i> (peters)

Abstract

Three general stages of iridophore development were found in Poecilia reticulata that correspond to the development of structural pigmentation. The first stage was prevalent in fish embryos about to hatch to young fish 4 months old. Dermal cells containing elements of endoplasmic reticulum and a Golgi apparatus developed into iridophores. The endoplasmic reticulum early in iridophore development became a few sparse cisternae, and the Golgi apparatus elaborated long rectangular vacuoles with two membranes. From 5 to 15 vacuoles were arranged in parallel stacks in each developing iridophore. Crystals of guanine were deposited within the inner compartment of each vacuole. At this stage of development, the young fish had only a few dermal iridophores next to the lateral muscle. Fish 4 to 6 months old had a more advanced type of iridophore development including several layers of iridophore cells in the dermis. The innermost iridophores near the muscle had many mature crystal-containing vacuoles (iridosomes). Each cell had upt to three stacks of 10-20 iridosomes with their long axis oriented at a slight oblique angle to the surface of the fish. The outer layers of iridophores resembled the immature developing cells found in very young fish. The third developmental stage was found in sexually functional adults. All dermal iridophores contained 2-3 groups of 10-20 mature iridosomes. In mature iridophores, the Golgi apparatus was not found in the cytoplasm. The thickness of the guanine crystals (70 nm) and cytoplasmic intervals (90 nm) results in a constructive interference reflection of 496 nm (blue-green). This iridescence increased concomitantly with the increase in iridophore cells in the dermis and the maturation of their iridosomes.