Optical far-field microscopy such as confocal fluorescence microscopy is a very popular technique for investigating the living cell. Unfortunately, its spatial resolution is limited to around 200 nm, impeding the imaging of small molecular assemblies. Recent decades have seen the development of optical nanoscopy, optical far-field microscopy with a spatial resolution down to molecular scales. STED microscopy was the first of such nanoscopy techniques. Despite the fact, that it in principle only requires the addition of a strong STED laser to a conventional microscope, STED nanoscopy was for a long time considered as a very complex technique, impossible to be applicable as a turn-key technique in everyday biological research. However, recent years has seen important improvements of the STED nanoscopy approach which have significantly simplified the setup. These developments mainly followed from optimization of fluorescent labels, laser technology and optical simplifications. As a result, STED microscopy setups have got more compact and have been realized on commercial instruments, allowing access to lessexperienced users in open imaging facilities. Here, we give a brief overview of the recent improvements in STED microscopy that made these important developments possible.

The frequency of autosomal dominant inherited palmoplantar keratoderma (HPPK) in the northernmost county of Sweden (Norrbotten) is 0.55%. Histopathological examination of 91 biopsies from patients with the dominant form of HPPK revealed no case of epidermolytic PPK. This finding is in contrast to the results of a re-examination of descendants of the original family published by Thost which showed the characteristic features of epidermolytic PPK, and re-evaluation of biopsies from other families has shown that it is the most frequent type. The existence of PPK type Unna-Thost in relation to epidermolytic PPK and to HPPK of the northernmost county of Sweden will be discussed. At the same time a revision of designation of this type is proposed. A dermo-epidermal mononuclear cell infiltrate belongs to the classical description of PPK Unna-Thost. It was shown that this cell infiltrate occurs significantly more often in patients with HPPK and dermatophytosis. Relapsing vesicular eruptions along the hyperkeratotic border are a clinical sign of the severity of dermatophyte infections. Such spongiotic vesicles together with a mononuclear cell infiltrate should be considered as eczematous reaction to dermatophytosis.

The lateral diffusion coefficient of a ligand which repeatedly dissociates and rebinds to sites on a planar surface is described. An analytical expression showing the manner in which the diffusion coefficient depends on the propensity for rebinding after dissociation, as the molecule moves through the solution and across the surface, is derived. The diffusion coefficient is time dependent and ranges from the surface diffusion coefficient (at time zero when a tagged molecule is placed on the surface) to the solution diffusion coefficient (at time infinity). The time-dependent diffusion coefficient depends only on the intrinsic dissociation rate and a "rebinding parameter", which in turn depends on a group of constants including the intrinsic association and dissociation rates, the density of unoccupied surface binding sites, and the diffusion coefficient in solution.

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