Fig. 4.

Transmission electron micrograph (TEM) of a 20 μm diameter tissue cyst from the brain of sea otter #1 (SO1). A) Within the tissue cyst are large numbers of metrocytes, including some that are dividing, indicated by the presence of two nuclear profiles within a single metrocyte (arrows). Prominent surface villi are apparent along the outer cyst wall (bar = 2.5 μm). B) Higher magnification view of the outer wall of the same tissue cyst (enlarged area indicated by box and small arrowheads at the lower left corner of A). Note the slight basal constriction of surface villi, with an increase in diameter at the midpoint, then narrowing to a tapered tip. The villous surface has a prominent, electron-dense outer layer with a “hobnailed” or beaded appearance (arrowhead). Within each microvillus, two or more fine, linear, electron dense, filamentous structures (microtubules - arrow) extend longitudinally from the villous tip to the outer portion of the granular layer that comprises the inner cyst wall. Villi are approximately 1 μm long by 0.2 μm wide (bar = 0.25 μm).