Further observation on the fine structure of a colloid cyst of the third ventricle.

The fine structure of a surgically excised colloid cyst is described. The cyst was lined by ciliated and nonciliated columnar cells. The nonciliated cells contained secretory material and had a surface coating. In addition, a third type of smaller cell was seen wedged between the columnar cells and abutting on the basement membrane. These cells contained abundant free ribosomes and tonofilaments and displayed well developed desmosomes and half desmosomes. The cyst lining closely resembled the upper respiratory epithelium rather than neuroepithelium, thus supporting the contention that colloid cysts are derived from an endodermal source.