Fine structure of epithelial thymus cysts in dogs.

Epithelial thymus cysts in dogs were investigated by electron microscopy. Epithelial cells from one and the same cyst showed morphological differences. Regular columnar or cuboidal epithelial cells predominated. These had an elongated nucleus, poor in chromatin but with abundant cytoplasmic organelles, e.g. mitochondria, and a Golgi region with sacs and vacuoles. Other cells possesed a narrower, indented nucleus richer in chromatin, a cytoplasm with numerous tonofilaments and cell membranes showing interdigitations with other cell membranes. Some cysts displayed a pseudostratified epithelium. In these cells the same organelles were present as were seen in the cuboidal cells. Abundant cilia and microvilli were present in the lumenal cells. All these different morphological characteristics were considered to be different functional aspects of the same kind of thymus epithelial cell. The cells had a structure indicating a secretory-activity.