Figure 1: In vivo healthy skin. Images of the epidermis are acquired by means of the multiphoton laser microscope DermaInspect (JenLab GmbH, Jena, Germany) at a 760 nm excitation wavelength. (a) Stratum corneum, 0 μm depth: strong autofluorescence coming from keratin and corneocytes, hexagonal-shaped flat cells. (b) Stratum granulosum, 20 μm depth: large keratinocytes showing strong cytoplasmatic autofluorescence due to the presence of keratohyalin granules, NADPH, and keratin; the nuclei appear dark because they contain less fluorophores than the cytoplasm. (c) Stratum spinosum, 30 μm depth: smaller keratinocytes and increased cellular density. (d) Stratum basale, 40 μm depth: the keratinocytes are small, polygonal-shaped, and with dark nuclei. (e) Dermal papillae, 55 μm depth: they are visible beyond the dermal-epidermal junction as oval-shaped, dark areas with a fluorescent contour, corresponding to a transversal section of the rete ridges surrounded by fluorescent basal cells. (f) Dermis, 85 μm depth, 800 nm excitation wavelength: bright autofluorescence coming from fibrous structures corresponding to collagen and elastin.