Fig. 4. (A-D) Scanning electron microscopy of the skin surface in Arnt^/ (A,C,D) and control (B) mouse newborns. The surface of Arnt^/ epidermis is flat and taut (A) whereas control epidermis is extensively folded, forming longitudinal ridges (B). At high magnification, corneocytes in the control epidermis have a perfectly smooth surface (not shown), whereas in Arnt^/ skin surface of outermost cells is rough, irregular, and often perforated (C,D). Surface cells in Arnt^/ epidermis often contain nuclei (C, arrow) suggestive of parakeratosis. (E-H) Transmission electron microscopy identified fragments of cytoplasmic organelles in the upper corny layer of Arnt^/ epidermis (F, white arrowheads). Corneocytes in Arnt^/ epidermis are much thicker and are tightly packed together (compare E with G). Whereas in normal epidermis (H) corneosomes are degraded in the lower part of corny layer (arrows), in Arnt^/ skin (F), corneosomes are intact even in the outermost portion of the corny layer (arrows) thus providing strong bonds between corneocytes. Black arrowheads in F and H indicate cornified envelope.