BACKGROUND: The initial pathology in hidradenitis suppurativa (HS)/acne inversa takes place in the folliculopilosebaceous unit (FPSU) and its surrounding tissue. The process involves follicular hyperkeratosis, inflammation and perifolliculitis. Identification of the exact origin of inflammation may shed new light on the pathogenesis and aetiology of the disease.

OBJECTIVES: To study the morphology of the basement membrane zone (BMZ) in patients with HS.

METHODS: In total, 65 operative specimens from 20 patients diagnosed with HS were cut stepwise. Within each specimen, the focus was set on heavily involved HS regions (centre) and clinically uninvolved regions (border). All specimens were stained with periodic acid-Schiff (PAS) to visualize the epithelial support structures of the FPSU (i.e. the BMZ), the sinus tracts (STs) and the interfollicular basement membrane (BM). The intensity of BMZ PAS staining was graded from 0 to 4+.

RESULTS: Compared with the axillary skin of human controls, the sebofollicular junction in patients with HS was found to be almost devoid of PAS-positive material (grade 0/1+) in both the border and centre lesions of HS, whereas STs and BMs showed uniformly grade 2-3+ positivity irrespective of any inflammation present. The distribution of inflammatory cells around the sebofollicular junction occurred predominantly in areas of BMZ thinning.

CONCLUSIONS: The BMZ PAS positivity of clinically uninvolved FPSUs of patients with HS appears to be wispy or not present at all. It is speculated that this may explain the apparent fragility of the sebofollicular junction. There is an increased concentration of inflammatory cells adjacent to these areas, while inflammatory cells are scarce in areas where the PAS-positive material is intact. It is hypothesized that the PAS gap identifies (i) areas susceptible to leakage, trauma and rupture, leading to release of materials that trigger inflammatory mediators, and (ii) the seeding of the dermis with free-living stem cells generating benign but invasive epithelialized sinuses, spreading horizontally in and below the dermis.