1. Basal epidermal cells and their junction with the dermis, as revealed in thin sections of osmium-fixed human and rodent skin, were studied with the electron microscope. Phosphotungstic acid staining was occasionally used to increase the electron density of membranous and filamentous structures.

2. Along the dermo-epidermal junction in all skin specimens there is a sub-microscopic (∼350 A thick) membrane following the basal contours of the epidermal cells, but separated from them by an ∼300 A space. No epidermal or dermal filaments can be seen to cross it and except in embryonic skin it has no associated band of amorphous material. It is called the "dermal membrane" to distinguish it from the thicker membrane and associated material commonly called the "basement membrane."

3. In adult human skin the basal cell membrane facing the dermal membrane is continuous with, or adjacent to regularly spaced groups of small dense rodlets at which tonofilaments are attached and appear to terminate. Less dense spherical granules are also found in the cellular ectoplasm and are unattached to filaments.

4. The connective tissue fibers in the upper dermis were narrower (≧300 A) than, but displayed the same period (≧350 A) as the collagen fibers in the deeper dermis. The related fibers in embryonic human skin were even narrower (150 to 250 A). In accordance with the views of others that these are "young" collagen with great affinity for polysaccharide, they are called collagen fibers.

5. The same cytoplasmic components are found in all basal epidermal cells: mitochondria, many filaments, many submicroscopic particulates, and only very occasional vesicles of the endoplasmic reticulum. Adult human cells possess pigment granules and intercellular bridges, in addition. Before keratinization is evident, no intercellular bridges and little or no cytoplasmic filaments are visible. The scarcity of vesicles of the endoplasmic reticulum and the prevalence of submicroscopic particulates (80 to 150 A) distributed at random through the cytoplasm, support the view (19) that basophilia and cytoplasmic nucleoprotein are associated with the particulates.

6. Cytoplasmic filaments are <100 A wide and are directed toward the dermo-epidermal junction and to intercellular bridges (when they occur). Because of their obvious identity as the submicroscopic constituents of tonofibrils, they were called "tonofilaments" and believed to represent the keratin-like fibrous protein "epidermin" (29). Bundles of them appear identical with Herxheimer fibers when, particularly in thick skin, they extend from the outer nuclear membrane to the granules spaced along the dermo-epidermal junction.

7. In human skin, adjacent basal cells are separated by simple cell membranes and connected by intercellular bridges. The filaments of one cell are attached to dense elongate granules in these bridges which are separated by a narrow less electron-dense space from a matching granule to which the filaments of the neighboring cell are attached. No filaments have been observed to cross the space in these bridges. In between the bridges adjacent cell membranes may separate from each other leaving an intercellular space. Although occasional basal cells possess a large perinuclear area devoid of cytoplasmic filaments, none following the classical histological description of "clear" cells were noted.

8. Pigment granules are extremely electron-dense with irregular angular outline. Their smallest diameter was significantly greater in Negro than in white skin while the lengths also appeared greater in the former skin type.

With the Technical Assistance of Sarah Lichtenberg