As a student of chiropody back in the 80’s I remember studying “Neale’s Common Foot Disorders” as a core text. Looking over the pages I remember the section covering warts and how it discussed the presence of black dots within a lesion representing “microthrombi within the blood vessels”. Anatomically all blood vessels are located in the dermis. It also implied that their sudden appearance or increase in numbers may suggest regression was about to occur. Looking at the most recent edition of the book, this statement is still present (1). This is also documented in dermatology texts (2).
As with most things you tend to accept the given explanation, and so often it goes unchallenged but a forthcoming paper from the Journal of the American Academy of Dermatology (3) has questioned just this. In their study they looked at 18 sections of palmoplantar warts and stained them using haematoxylin/eosin (HE) and none of them demonstrated any microthrombi in any of the examined sections. To test this further they employed the CD61 antibody which is also sensitive to platelets but still no evidence of microthrombi.
There was a major question hanging over this study in that it was not confirmed that any of the excised wart sections had microthrombi visually present to begin with, so it could be down to chance that they selected thrombi-free warts although as the authors point out as between 62-78% of warts are reported to have black dots. So, instead the study team looked into the epidermis, where there are no blood vessels and stained sections of this using HE stains again and benzidine. This time it was positive in 11/18 lesions (65%) of HE stains and 13/18 (72%) of the benzidine stained sections. Consequently, the authors suggest that black dots are not microthrombi as they are located in the intracorneal skin of the epidermis where there are no vessels, but in fact may represent intra-corneal haemorrhage.
The finding concurs with the clinical observation known to podiatrists that simple removal of the overlying epidermis, removes with it all the black dots typically seen in a lesion, suggesting they are in fact located in the avascular epidermis.
References
1. Springett K, Johnson M. The skin and nails in podiatry. In: Frowen P, O'Donnell M, Lorimer D, Burrow JG, editors. Neales Disorders of the Foot. London: Elsevier; 2006. p. 19-45.
2. Griffiths CE, Barker J, Chalmers R, Bleiker T, Creamer D, editors. Rooks's Textbook of Dermatology. 9th ed. London: Wiley; 2016.
3. Fried I, Kasper RS, Hegyi I, Kempf W. Black dots in palmoplantar warts –challenging a concept: A histopathologic study. J Am Acad Dermatol. 2018.