## 3 Results  
_Photon microscopy examination of hematoxylin-esoin (HE)_ stained sections using the Olympus BX40 optical microscope with attached Olympus E330 camera identified a number of aspects suggestive of the diagnosis of keratoacanthoma--epidermis with the presence of horny plugs formed by keratin lamellae between which polymorphonuclear cells were detected. The epidermis showed significant acanthosis, papillomavirus, rare horny cysts, and pseudoepitheliomatous aspects and, in the superficial dermis, an abundant polymorphous inflammatory infiltrate was identified (Figure 1).  
Complementary to the histopathological examination, _electron microscopic examination_ of keratoacanthoma lesions with a mild-to-moderate degree of dysplasia revealed the following ultrastructural details (Figure 2):  
* _at the cytoplasmic level_--there are numerous types of vacuoles with heterogeneous appearance among which vesicles specific to classical endocytosis but also to fluid phase micropinocytosis can be found. In all keratinocyte layers, nuclei are enlarged in volume and irregularly shaped by undulation and edentulousness of the nuclear envelope. In contrast to basal layers, in suprabasal layers the number of mitochondria, responsible for cellular energetics, progressively increases and the matrix shows a smooth, vacuolated appearance through reduction and disorganization of mitochondrial ridges, with negative influence on cellular metabolic processes. Organelles specialised in cell synthesis and secretion such as ribosomes, the rough endoplasmic reticulum (RER) and the Golgi apparatus are hypertrophied, especially in perivascular macrophages;
* _at the cytoskeletal level_--tonofilaments are reduced in number and grouped into short clusters located at a distance from the plasmalemma;
* _at the membrane level_--cell-cell adhesion junctional complexes are disorganized inducing acantholysis and widening of intercellular spaces. At the basal pole level, both basement membrane discontinuity and hemidesmosome disruption are identified, with damage to the extracellular matrix.  
Figure 1: Keratoacanthoma. HE (40\(\times\)) (**a**) cannthosis; (**b**) horny plugs, pseudoepitheliomatous aspects.  
Compared to the lesions described above, in keratoacanthomas associated with a moderate-severe degree of dysplasia there is both an increase in the frequency and intensity of the ultrastructural changes mentioned above and additional details such as (Figure 3 and Table 1):  
* _at the cytoplasmic level_--the keratinocyte nuclei of all epidermal layers were strongly edentated by the cytoplasmic invaginations associated with nuclear pore expansion. Enlargement of the perinuclear cisterna imprints the vacuolar appearance of the nuclear envelope. Characteristic of tumour cells, these nuclei have multiple, bulky, peripherally arranged nucleoli. Ribosomes and mitochondria have a degenerative appearance;
* _at the cytoskeletal level_--much thickened tonofilaments are arranged perinuclearly, in a large electron-dense clusters, leading to cytoplasmic membrane invagination and destructuring of desmosomes and hemidesmosomes;
* _at the membrane level_--discontinuities are more frequent and more extensive.  
Figure 2: TEM. Keratoacanthoma with mild-moderate dysplasia: (**a**) presence of nuclei increased in volume; (**b**) undulating nuclear membrane; and (**c**) discontinuities and invaginations in the plasmalemma.