Color enhanced transmission electron micrograph of a basal cell from the epidermis of the ammocoete of Petromyzon fluviatilis. At this high magnification, keratin filaments appear as punctate of elongate profiles depending on their orientation in the section. They are uniform in size, approximately 9 nm thick, and may have a beaded appearance along their length as though composed of repeating subunits. Among the filaments microtubules (at arrows) are occasionally seen.