This transcript provides a detailed pathological examination of a skin lesion characterized by acral hyperkeratosis and a dermis replaced by fibrotic tissue and a proliferation of bland spindle cells arranged in fascicles. The key diagnostic feature identified are the numerous intracytoplasmic, eosinophilic "inclusion bodies," composed of tangled actin fibers. The entity is diagnosed as Infantile Digital Fibroma (IDF), also known as Inclusion Body Fibromatosis, typically found as a dome-shaped papule on the digits of infants. The discussion concludes by highlighting a potential diagnostic pitfall where extravasated erythrocytes can mimic the characteristic inclusion bodies.
Pathology of Infantile Digital Fibroma (Inclusion Body Fibromatosis)
0:30 Histological Context: The slide shows features consistent with acral skin (hyperkeratosis) with the dermis appearing extensively fibrotic and replaced by collagen.
0:58 Cellular Morphology: The lesion is composed of eosinophilic, pink bundles and fascicles of bland spindle cells, which are fibroblasts or myofibroblasts, and can extend into the subcutis.
1:32 Gross Appearance: The lesion is a classic dome-shaped papule affecting the acral skin epidermis.
1:57 Key Diagnostic Feature (Inclusion Bodies): The crucial finding is the presence of small, pink, intracytoplasmic "balls" or inclusion bodies, which are tangled up actin fibers. These are best seen on high-power H&E and are numerous once recognized.
2:30 Nomenclature: The condition is known by two interchangeable names: Infantile Digital Fibroma (IDF) and Inclusion Body Fibromatosis.
2:49 Clinical Relevance: As suggested by the name, this lesion occurs on the digits (fingers/toes) of babies/infants.
2:54 Prognosis: The lesion is benign, though it has a tendency to recur locally.
3:04 Diagnostic Challenge: A potential pitfall is that extravasated erythrocytes (red blood cells) can occasionally mimic the appearance of the pink actin inclusion bodies, requiring careful differentiation.